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rfc:rfc1123

Network Working Group Internet Engineering Task Force Request for Comments: 1123 R. Braden, Editor

                                                          October 1989
     Requirements for Internet Hosts -- Application and Support

Status of This Memo

 This RFC is an official specification for the Internet community.  It
 incorporates by reference, amends, corrects, and supplements the
 primary protocol standards documents relating to hosts.  Distribution
 of this document is unlimited.

Summary

 This RFC is one of a pair that defines and discusses the requirements
 for Internet host software.  This RFC covers the application and
 support protocols; its companion RFC-1122 covers the communication
 protocol layers: link layer, IP layer, and transport layer.
                         Table of Contents
 1.  INTRODUCTION ...............................................    5
    1.1  The Internet Architecture ..............................    6
    1.2  General Considerations .................................    6
       1.2.1  Continuing Internet Evolution .....................    6
       1.2.2  Robustness Principle ..............................    7
       1.2.3  Error Logging .....................................    8
       1.2.4  Configuration .....................................    8
    1.3  Reading this Document ..................................   10
       1.3.1  Organization ......................................   10
       1.3.2  Requirements ......................................   10
       1.3.3  Terminology .......................................   11
    1.4  Acknowledgments ........................................   12
 2.  GENERAL ISSUES .............................................   13
    2.1  Host Names and Numbers .................................   13
    2.2  Using Domain Name Service ..............................   13
    2.3  Applications on Multihomed hosts .......................   14
    2.4  Type-of-Service ........................................   14
    2.5  GENERAL APPLICATION REQUIREMENTS SUMMARY ...............   15

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 3.  REMOTE LOGIN -- TELNET PROTOCOL ............................   16
    3.1  INTRODUCTION ...........................................   16
    3.2  PROTOCOL WALK-THROUGH ..................................   16
       3.2.1  Option Negotiation ................................   16
       3.2.2  Telnet Go-Ahead Function ..........................   16
       3.2.3  Control Functions .................................   17
       3.2.4  Telnet "Synch" Signal .............................   18
       3.2.5  NVT Printer and Keyboard ..........................   19
       3.2.6  Telnet Command Structure ..........................   20
       3.2.7  Telnet Binary Option ..............................   20
       3.2.8  Telnet Terminal-Type Option .......................   20
    3.3  SPECIFIC ISSUES ........................................   21
       3.3.1  Telnet End-of-Line Convention .....................   21
       3.3.2  Data Entry Terminals ..............................   23
       3.3.3  Option Requirements ...............................   24
       3.3.4  Option Initiation .................................   24
       3.3.5  Telnet Linemode Option ............................   25
    3.4  TELNET/USER INTERFACE ..................................   25
       3.4.1  Character Set Transparency ........................   25
       3.4.2  Telnet Commands ...................................   26
       3.4.3  TCP Connection Errors .............................   26
       3.4.4  Non-Default Telnet Contact Port ...................   26
       3.4.5  Flushing Output ...................................   26
    3.5.  TELNET REQUIREMENTS SUMMARY ...........................   27
 4.  FILE TRANSFER ..............................................   29
    4.1  FILE TRANSFER PROTOCOL -- FTP ..........................   29
       4.1.1  INTRODUCTION ......................................   29
       4.1.2.  PROTOCOL WALK-THROUGH ............................   29
          4.1.2.1  LOCAL Type ...................................   29
          4.1.2.2  Telnet Format Control ........................   30
          4.1.2.3  Page Structure ...............................   30
          4.1.2.4  Data Structure Transformations ...............   30
          4.1.2.5  Data Connection Management ...................   31
          4.1.2.6  PASV Command .................................   31
          4.1.2.7  LIST and NLST Commands .......................   31
          4.1.2.8  SITE Command .................................   32
          4.1.2.9  STOU Command .................................   32
          4.1.2.10  Telnet End-of-line Code .....................   32
          4.1.2.11  FTP Replies .................................   33
          4.1.2.12  Connections .................................   34
          4.1.2.13  Minimum Implementation; RFC-959 Section .....   34
       4.1.3  SPECIFIC ISSUES ...................................   35
          4.1.3.1  Non-standard Command Verbs ...................   35
          4.1.3.2  Idle Timeout .................................   36
          4.1.3.3  Concurrency of Data and Control ..............   36
          4.1.3.4  FTP Restart Mechanism ........................   36
       4.1.4  FTP/USER INTERFACE ................................   39

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RFC1123 INTRODUCTION October 1989

          4.1.4.1  Pathname Specification .......................   39
          4.1.4.2  "QUOTE" Command ..............................   40
          4.1.4.3  Displaying Replies to User ...................   40
          4.1.4.4  Maintaining Synchronization ..................   40
       4.1.5   FTP REQUIREMENTS SUMMARY .........................   41
    4.2  TRIVIAL FILE TRANSFER PROTOCOL -- TFTP .................   44
       4.2.1  INTRODUCTION ......................................   44
       4.2.2  PROTOCOL WALK-THROUGH .............................   44
          4.2.2.1  Transfer Modes ...............................   44
          4.2.2.2  UDP Header ...................................   44
       4.2.3  SPECIFIC ISSUES ...................................   44
          4.2.3.1  Sorcerer's Apprentice Syndrome ...............   44
          4.2.3.2  Timeout Algorithms ...........................   46
          4.2.3.3  Extensions ...................................   46
          4.2.3.4  Access Control ...............................   46
          4.2.3.5  Broadcast Request ............................   46
       4.2.4  TFTP REQUIREMENTS SUMMARY .........................   47
 5.  ELECTRONIC MAIL -- SMTP and RFC-822 ........................   48
    5.1  INTRODUCTION ...........................................   48
    5.2  PROTOCOL WALK-THROUGH ..................................   48
       5.2.1  The SMTP Model ....................................   48
       5.2.2  Canonicalization ..................................   49
       5.2.3  VRFY and EXPN Commands ............................   50
       5.2.4  SEND, SOML, and SAML Commands .....................   50
       5.2.5  HELO Command ......................................   50
       5.2.6  Mail Relay ........................................   51
       5.2.7  RCPT Command ......................................   52
       5.2.8  DATA Command ......................................   53
       5.2.9  Command Syntax ....................................   54
       5.2.10  SMTP Replies .....................................   54
       5.2.11  Transparency .....................................   55
       5.2.12  WKS Use in MX Processing .........................   55
       5.2.13  RFC-822 Message Specification ....................   55
       5.2.14  RFC-822 Date and Time Specification ..............   55
       5.2.15  RFC-822 Syntax Change ............................   56
       5.2.16  RFC-822  Local-part ..............................   56
       5.2.17  Domain Literals ..................................   57
       5.2.18  Common Address Formatting Errors .................   58
       5.2.19  Explicit Source Routes ...........................   58
    5.3  SPECIFIC ISSUES ........................................   59
       5.3.1  SMTP Queueing Strategies ..........................   59
          5.3.1.1 Sending Strategy ..............................   59
          5.3.1.2  Receiving strategy ...........................   61
       5.3.2  Timeouts in SMTP ..................................   61
       5.3.3  Reliable Mail Receipt .............................   63
       5.3.4  Reliable Mail Transmission ........................   63
       5.3.5  Domain Name Support ...............................   65

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       5.3.6  Mailing Lists and Aliases .........................   65
       5.3.7  Mail Gatewaying ...................................   66
       5.3.8  Maximum Message Size ..............................   68
    5.4  SMTP REQUIREMENTS SUMMARY ..............................   69
 6. SUPPORT SERVICES ............................................   72
    6.1 DOMAIN NAME TRANSLATION .................................   72
       6.1.1 INTRODUCTION .......................................   72
       6.1.2  PROTOCOL WALK-THROUGH .............................   72
          6.1.2.1  Resource Records with Zero TTL ...............   73
          6.1.2.2  QCLASS Values ................................   73
          6.1.2.3  Unused Fields ................................   73
          6.1.2.4  Compression ..................................   73
          6.1.2.5  Misusing Configuration Info ..................   73
       6.1.3  SPECIFIC ISSUES ...................................   74
          6.1.3.1  Resolver Implementation ......................   74
          6.1.3.2  Transport Protocols ..........................   75
          6.1.3.3  Efficient Resource Usage .....................   77
          6.1.3.4  Multihomed Hosts .............................   78
          6.1.3.5  Extensibility ................................   79
          6.1.3.6  Status of RR Types ...........................   79
          6.1.3.7  Robustness ...................................   80
          6.1.3.8  Local Host Table .............................   80
       6.1.4  DNS USER INTERFACE ................................   81
          6.1.4.1  DNS Administration ...........................   81
          6.1.4.2  DNS User Interface ...........................   81
          6.1.4.3 Interface Abbreviation Facilities .............   82
       6.1.5  DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY ...........   84
    6.2  HOST INITIALIZATION ....................................   87
       6.2.1  INTRODUCTION ......................................   87
       6.2.2  REQUIREMENTS ......................................   87
          6.2.2.1  Dynamic Configuration ........................   87
          6.2.2.2  Loading Phase ................................   89
    6.3  REMOTE MANAGEMENT ......................................   90
       6.3.1  INTRODUCTION ......................................   90
       6.3.2  PROTOCOL WALK-THROUGH .............................   90
       6.3.3  MANAGEMENT REQUIREMENTS SUMMARY ...................   92
 7.  REFERENCES .................................................   93

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RFC1123 INTRODUCTION October 1989

1. INTRODUCTION

 This document is one of a pair that defines and discusses the
 requirements for host system implementations of the Internet protocol
 suite.  This RFC covers the applications layer and support protocols.
 Its companion RFC, "Requirements for Internet Hosts -- Communications
 Layers" [INTRO:1] covers the lower layer protocols: transport layer,
 IP layer, and link layer.
 These documents are intended to provide guidance for vendors,
 implementors, and users of Internet communication software.  They
 represent the consensus of a large body of technical experience and
 wisdom, contributed by members of the Internet research and vendor
 communities.
 This RFC enumerates standard protocols that a host connected to the
 Internet must use, and it incorporates by reference the RFCs and
 other documents describing the current specifications for these
 protocols.  It corrects errors in the referenced documents and adds
 additional discussion and guidance for an implementor.
 For each protocol, this document also contains an explicit set of
 requirements, recommendations, and options.  The reader must
 understand that the list of requirements in this document is
 incomplete by itself; the complete set of requirements for an
 Internet host is primarily defined in the standard protocol
 specification documents, with the corrections, amendments, and
 supplements contained in this RFC.
 A good-faith implementation of the protocols that was produced after
 careful reading of the RFC's and with some interaction with the
 Internet technical community, and that followed good communications
 software engineering practices, should differ from the requirements
 of this document in only minor ways.  Thus, in many cases, the
 "requirements" in this RFC are already stated or implied in the
 standard protocol documents, so that their inclusion here is, in a
 sense, redundant.  However, they were included because some past
 implementation has made the wrong choice, causing problems of
 interoperability, performance, and/or robustness.
 This document includes discussion and explanation of many of the
 requirements and recommendations.  A simple list of requirements
 would be dangerous, because:
 o    Some required features are more important than others, and some
      features are optional.
 o    There may be valid reasons why particular vendor products that

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RFC1123 INTRODUCTION October 1989

      are designed for restricted contexts might choose to use
      different specifications.
 However, the specifications of this document must be followed to meet
 the general goal of arbitrary host interoperation across the
 diversity and complexity of the Internet system.  Although most
 current implementations fail to meet these requirements in various
 ways, some minor and some major, this specification is the ideal
 towards which we need to move.
 These requirements are based on the current level of Internet
 architecture.  This document will be updated as required to provide
 additional clarifications or to include additional information in
 those areas in which specifications are still evolving.
 This introductory section begins with general advice to host software
 vendors, and then gives some guidance on reading the rest of the
 document.  Section 2 contains general requirements that may be
 applicable to all application and support protocols.  Sections 3, 4,
 and 5 contain the requirements on protocols for the three major
 applications: Telnet, file transfer, and electronic mail,
 respectively. Section 6 covers the support applications: the domain
 name system, system initialization, and management.  Finally, all
 references will be found in Section 7.
 1.1  The Internet Architecture
    For a brief introduction to the Internet architecture from a host
    viewpoint, see Section 1.1 of [INTRO:1].  That section also
    contains recommended references for general background on the
    Internet architecture.
 1.2  General Considerations
    There are two important lessons that vendors of Internet host
    software have learned and which a new vendor should consider
    seriously.
    1.2.1  Continuing Internet Evolution
       The enormous growth of the Internet has revealed problems of
       management and scaling in a large datagram-based packet
       communication system.  These problems are being addressed, and
       as a result there will be continuing evolution of the
       specifications described in this document.  These changes will
       be carefully planned and controlled, since there is extensive
       participation in this planning by the vendors and by the
       organizations responsible for operations of the networks.

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       Development, evolution, and revision are characteristic of
       computer network protocols today, and this situation will
       persist for some years.  A vendor who develops computer
       communication software for the Internet protocol suite (or any
       other protocol suite!) and then fails to maintain and update
       that software for changing specifications is going to leave a
       trail of unhappy customers.  The Internet is a large
       communication network, and the users are in constant contact
       through it.  Experience has shown that knowledge of
       deficiencies in vendor software propagates quickly through the
       Internet technical community.
    1.2.2  Robustness Principle
       At every layer of the protocols, there is a general rule whose
       application can lead to enormous benefits in robustness and
       interoperability:
              "Be liberal in what you accept, and
               conservative in what you send"
       Software should be written to deal with every conceivable
       error, no matter how unlikely; sooner or later a packet will
       come in with that particular combination of errors and
       attributes, and unless the software is prepared, chaos can
       ensue.  In general, it is best to assume that the network is
       filled with malevolent entities that will send in packets
       designed to have the worst possible effect.  This assumption
       will lead to suitable protective design, although the most
       serious problems in the Internet have been caused by
       unenvisaged mechanisms triggered by low-probability events;
       mere human malice would never have taken so devious a course!
       Adaptability to change must be designed into all levels of
       Internet host software.  As a simple example, consider a
       protocol specification that contains an enumeration of values
       for a particular header field -- e.g., a type field, a port
       number, or an error code; this enumeration must be assumed to
       be incomplete.  Thus, if a protocol specification defines four
       possible error codes, the software must not break when a fifth
       code shows up.  An undefined code might be logged (see below),
       but it must not cause a failure.
       The second part of the principle is almost as important:
       software on other hosts may contain deficiencies that make it
       unwise to exploit legal but obscure protocol features.  It is
       unwise to stray far from the obvious and simple, lest untoward
       effects result elsewhere.  A corollary of this is "watch out

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RFC1123 INTRODUCTION October 1989

       for misbehaving hosts"; host software should be prepared, not
       just to survive other misbehaving hosts, but also to cooperate
       to limit the amount of disruption such hosts can cause to the
       shared communication facility.
    1.2.3  Error Logging
       The Internet includes a great variety of host and gateway
       systems, each implementing many protocols and protocol layers,
       and some of these contain bugs and mis-features in their
       Internet protocol software.  As a result of complexity,
       diversity, and distribution of function, the diagnosis of user
       problems is often very difficult.
       Problem diagnosis will be aided if host implementations include
       a carefully designed facility for logging erroneous or
       "strange" protocol events.  It is important to include as much
       diagnostic information as possible when an error is logged.  In
       particular, it is often useful to record the header(s) of a
       packet that caused an error.  However, care must be taken to
       ensure that error logging does not consume prohibitive amounts
       of resources or otherwise interfere with the operation of the
       host.
       There is a tendency for abnormal but harmless protocol events
       to overflow error logging files; this can be avoided by using a
       "circular" log, or by enabling logging only while diagnosing a
       known failure.  It may be useful to filter and count duplicate
       successive messages.  One strategy that seems to work well is:
       (1) always count abnormalities and make such counts accessible
       through the management protocol (see Section 6.3); and (2)
       allow the logging of a great variety of events to be
       selectively enabled.  For example, it might useful to be able
       to "log everything" or to "log everything for host X".
       Note that different managements may have differing policies
       about the amount of error logging that they want normally
       enabled in a host.  Some will say, "if it doesn't hurt me, I
       don't want to know about it", while others will want to take a
       more watchful and aggressive attitude about detecting and
       removing protocol abnormalities.
    1.2.4  Configuration
       It would be ideal if a host implementation of the Internet
       protocol suite could be entirely self-configuring.  This would
       allow the whole suite to be implemented in ROM or cast into
       silicon, it would simplify diskless workstations, and it would

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RFC1123 INTRODUCTION October 1989

       be an immense boon to harried LAN administrators as well as
       system vendors.  We have not reached this ideal; in fact, we
       are not even close.
       At many points in this document, you will find a requirement
       that a parameter be a configurable option.  There are several
       different reasons behind such requirements.  In a few cases,
       there is current uncertainty or disagreement about the best
       value, and it may be necessary to update the recommended value
       in the future.  In other cases, the value really depends on
       external factors -- e.g., the size of the host and the
       distribution of its communication load, or the speeds and
       topology of nearby networks -- and self-tuning algorithms are
       unavailable and may be insufficient.  In some cases,
       configurability is needed because of administrative
       requirements.
       Finally, some configuration options are required to communicate
       with obsolete or incorrect implementations of the protocols,
       distributed without sources, that unfortunately persist in many
       parts of the Internet.  To make correct systems coexist with
       these faulty systems, administrators often have to "mis-
       configure" the correct systems.  This problem will correct
       itself gradually as the faulty systems are retired, but it
       cannot be ignored by vendors.
       When we say that a parameter must be configurable, we do not
       intend to require that its value be explicitly read from a
       configuration file at every boot time.  We recommend that
       implementors set up a default for each parameter, so a
       configuration file is only necessary to override those defaults
       that are inappropriate in a particular installation.  Thus, the
       configurability requirement is an assurance that it will be
       POSSIBLE to override the default when necessary, even in a
       binary-only or ROM-based product.
       This document requires a particular value for such defaults in
       some cases.  The choice of default is a sensitive issue when
       the configuration item controls the accommodation to existing
       faulty systems.  If the Internet is to converge successfully to
       complete interoperability, the default values built into
       implementations must implement the official protocol, not
       "mis-configurations" to accommodate faulty implementations.
       Although marketing considerations have led some vendors to
       choose mis-configuration defaults, we urge vendors to choose
       defaults that will conform to the standard.
       Finally, we note that a vendor needs to provide adequate

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RFC1123 INTRODUCTION October 1989

       documentation on all configuration parameters, their limits and
       effects.
 1.3  Reading this Document
    1.3.1  Organization
       In general, each major section is organized into the following
       subsections:
       (1)  Introduction
       (2)  Protocol Walk-Through -- considers the protocol
            specification documents section-by-section, correcting
            errors, stating requirements that may be ambiguous or
            ill-defined, and providing further clarification or
            explanation.
       (3)  Specific Issues -- discusses protocol design and
            implementation issues that were not included in the walk-
            through.
       (4)  Interfaces -- discusses the service interface to the next
            higher layer.
       (5)  Summary -- contains a summary of the requirements of the
            section.
       Under many of the individual topics in this document, there is
       parenthetical material labeled "DISCUSSION" or
       "IMPLEMENTATION".  This material is intended to give
       clarification and explanation of the preceding requirements
       text.  It also includes some suggestions on possible future
       directions or developments.  The implementation material
       contains suggested approaches that an implementor may want to
       consider.
       The summary sections are intended to be guides and indexes to
       the text, but are necessarily cryptic and incomplete.  The
       summaries should never be used or referenced separately from
       the complete RFC.
    1.3.2  Requirements
       In this document, the words that are used to define the
       significance of each particular requirement are capitalized.
       These words are:

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RFC1123 INTRODUCTION October 1989

  • "MUST"
            This word or the adjective "REQUIRED" means that the item
            is an absolute requirement of the specification.
  • "SHOULD"
            This word or the adjective "RECOMMENDED" means that there
            may exist valid reasons in particular circumstances to
            ignore this item, but the full implications should be
            understood and the case carefully weighed before choosing
            a different course.
  • "MAY"
            This word or the adjective "OPTIONAL" means that this item
            is truly optional.  One vendor may choose to include the
            item because a particular marketplace requires it or
            because it enhances the product, for example; another
            vendor may omit the same item.
       An implementation is not compliant if it fails to satisfy one
       or more of the MUST requirements for the protocols it
       implements.  An implementation that satisfies all the MUST and
       all the SHOULD requirements for its protocols is said to be
       "unconditionally compliant"; one that satisfies all the MUST
       requirements but not all the SHOULD requirements for its
       protocols is said to be "conditionally compliant".
    1.3.3  Terminology
       This document uses the following technical terms:
       Segment
            A segment is the unit of end-to-end transmission in the
            TCP protocol.  A segment consists of a TCP header followed
            by application data.  A segment is transmitted by
            encapsulation in an IP datagram.
       Message
            This term is used by some application layer protocols
            (particularly SMTP) for an application data unit.
       Datagram
            A [UDP] datagram is the unit of end-to-end transmission in
            the UDP protocol.

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       Multihomed
            A host is said to be multihomed if it has multiple IP
            addresses to connected networks.
 1.4  Acknowledgments
    This document incorporates contributions and comments from a large
    group of Internet protocol experts, including representatives of
    university and research labs, vendors, and government agencies.
    It was assembled primarily by the Host Requirements Working Group
    of the Internet Engineering Task Force (IETF).
    The Editor would especially like to acknowledge the tireless
    dedication of the following people, who attended many long
    meetings and generated 3 million bytes of electronic mail over the
    past 18 months in pursuit of this document: Philip Almquist, Dave
    Borman (Cray Research), Noel Chiappa, Dave Crocker (DEC), Steve
    Deering (Stanford), Mike Karels (Berkeley), Phil Karn (Bellcore),
    John Lekashman (NASA), Charles Lynn (BBN), Keith McCloghrie (TWG),
    Paul Mockapetris (ISI), Thomas Narten (Purdue), Craig Partridge
    (BBN), Drew Perkins (CMU), and James Van Bokkelen (FTP Software).
    In addition, the following people made major contributions to the
    effort: Bill Barns (Mitre), Steve Bellovin (AT&T), Mike Brescia
    (BBN), Ed Cain (DCA), Annette DeSchon (ISI), Martin Gross (DCA),
    Phill Gross (NRI), Charles Hedrick (Rutgers), Van Jacobson (LBL),
    John Klensin (MIT), Mark Lottor (SRI), Milo Medin (NASA), Bill
    Melohn (Sun Microsystems), Greg Minshall (Kinetics), Jeff Mogul
    (DEC), John Mullen (CMC), Jon Postel (ISI), John Romkey (Epilogue
    Technology), and Mike StJohns (DCA).  The following also made
    significant contributions to particular areas: Eric Allman
    (Berkeley), Rob Austein (MIT), Art Berggreen (ACC), Keith Bostic
    (Berkeley), Vint Cerf (NRI), Wayne Hathaway (NASA), Matt Korn
    (IBM), Erik Naggum (Naggum Software, Norway), Robert Ullmann
    (Prime Computer), David Waitzman (BBN), Frank Wancho (USA), Arun
    Welch (Ohio State), Bill Westfield (Cisco), and Rayan Zachariassen
    (Toronto).
    We are grateful to all, including any contributors who may have
    been inadvertently omitted from this list.

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RFC1123 APPLICATIONS LAYER – GENERAL October 1989

2. GENERAL ISSUES

 This section contains general requirements that may be applicable to
 all application-layer protocols.
 2.1  Host Names and Numbers
    The syntax of a legal Internet host name was specified in RFC-952
    [DNS:4].  One aspect of host name syntax is hereby changed: the
    restriction on the first character is relaxed to allow either a
    letter or a digit.  Host software MUST support this more liberal
    syntax.
    Host software MUST handle host names of up to 63 characters and
    SHOULD handle host names of up to 255 characters.
    Whenever a user inputs the identity of an Internet host, it SHOULD
    be possible to enter either (1) a host domain name or (2) an IP
    address in dotted-decimal ("#.#.#.#") form.  The host SHOULD check
    the string syntactically for a dotted-decimal number before
    looking it up in the Domain Name System.
    DISCUSSION:
         This last requirement is not intended to specify the complete
         syntactic form for entering a dotted-decimal host number;
         that is considered to be a user-interface issue.  For
         example, a dotted-decimal number must be enclosed within
         "[ ]" brackets for SMTP mail (see Section 5.2.17).  This
         notation could be made universal within a host system,
         simplifying the syntactic checking for a dotted-decimal
         number.
         If a dotted-decimal number can be entered without such
         identifying delimiters, then a full syntactic check must be
         made, because a segment of a host domain name is now allowed
         to begin with a digit and could legally be entirely numeric
         (see Section 6.1.2.4).  However, a valid host name can never
         have the dotted-decimal form #.#.#.#, since at least the
         highest-level component label will be alphabetic.
 2.2  Using Domain Name Service
    Host domain names MUST be translated to IP addresses as described
    in Section 6.1.
    Applications using domain name services MUST be able to cope with
    soft error conditions.  Applications MUST wait a reasonable
    interval between successive retries due to a soft error, and MUST

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RFC1123 APPLICATIONS LAYER – GENERAL October 1989

    allow for the possibility that network problems may deny service
    for hours or even days.
    An application SHOULD NOT rely on the ability to locate a WKS
    record containing an accurate listing of all services at a
    particular host address, since the WKS RR type is not often used
    by Internet sites.  To confirm that a service is present, simply
    attempt to use it.
 2.3  Applications on Multihomed hosts
    When the remote host is multihomed, the name-to-address
    translation will return a list of alternative IP addresses.  As
    specified in Section 6.1.3.4, this list should be in order of
    decreasing preference.  Application protocol implementations
    SHOULD be prepared to try multiple addresses from the list until
    success is obtained.  More specific requirements for SMTP are
    given in Section 5.3.4.
    When the local host is multihomed, a UDP-based request/response
    application SHOULD send the response with an IP source address
    that is the same as the specific destination address of the UDP
    request datagram.  The "specific destination address" is defined
    in the "IP Addressing" section of the companion RFC [INTRO:1].
    Similarly, a server application that opens multiple TCP
    connections to the same client SHOULD use the same local IP
    address for all.
 2.4  Type-of-Service
    Applications MUST select appropriate TOS values when they invoke
    transport layer services, and these values MUST be configurable.
    Note that a TOS value contains 5 bits, of which only the most-
    significant 3 bits are currently defined; the other two bits MUST
    be zero.
    DISCUSSION:
         As gateway algorithms are developed to implement Type-of-
         Service, the recommended values for various application
         protocols may change.  In addition, it is likely that
         particular combinations of users and Internet paths will want
         non-standard TOS values.  For these reasons, the TOS values
         must be configurable.
         See the latest version of the "Assigned Numbers" RFC
         [INTRO:5] for the recommended TOS values for the major
         application protocols.

Internet Engineering Task Force [Page 14]

RFC1123 APPLICATIONS LAYER – GENERAL October 1989

 2.5  GENERAL APPLICATION REQUIREMENTS SUMMARY
                                             |          | | | |S| |
                                             |          | | | |H| |F
                                             |          | | | |O|M|o
                                             |          | |S| |U|U|o
                                             |          | |H| |L|S|t
                                             |          |M|O| |D|T|n
                                             |          |U|U|M| | |o
                                             |          |S|L|A|N|N|t
                                             |          |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ———————————————–|———-|-|-|-|-|-|–

                                             |          | | | | | |

User interfaces: | | | | | | |

Allow host name to begin with digit          |2.1       |x| | | | |
Host names of up to 635 characters           |2.1       |x| | | | |
Host names of up to 255 characters           |2.1       | |x| | | |
Support dotted-decimal host numbers          |2.1       | |x| | | |
Check syntactically for dotted-dec first     |2.1       | |x| | | |
                                             |          | | | | | |

Map domain names per Section 6.1 |2.2 |x| | | | | Cope with soft DNS errors |2.2 |x| | | | |

 Reasonable interval between retries         |2.2       |x| | | | |
 Allow for long outages                      |2.2       |x| | | | |

Expect WKS records to be available |2.2 | | | |x| |

                                             |          | | | | | |

Try multiple addr's for remote multihomed host |2.3 | |x| | | | UDP reply src addr is specific dest of request |2.3 | |x| | | | Use same IP addr for related TCP connections |2.3 | |x| | | | Specify appropriate TOS values |2.4 |x| | | | |

TOS values configurable                      |2.4       |x| | | | |
Unused TOS bits zero                         |2.4       |x| | | | |
                                             |          | | | | | |
                                             |          | | | | | |

Internet Engineering Task Force [Page 15]

RFC1123 REMOTE LOGIN – TELNET October 1989

3. REMOTE LOGIN – TELNET PROTOCOL

 3.1  INTRODUCTION
    Telnet is the standard Internet application protocol for remote
    login.  It provides the encoding rules to link a user's
    keyboard/display on a client ("user") system with a command
    interpreter on a remote server system.  A subset of the Telnet
    protocol is also incorporated within other application protocols,
    e.g., FTP and SMTP.
    Telnet uses a single TCP connection, and its normal data stream
    ("Network Virtual Terminal" or "NVT" mode) is 7-bit ASCII with
    escape sequences to embed control functions.  Telnet also allows
    the negotiation of many optional modes and functions.
    The primary Telnet specification is to be found in RFC-854
    [TELNET:1], while the options are defined in many other RFCs; see
    Section 7 for references.
 3.2  PROTOCOL WALK-THROUGH
    3.2.1  Option Negotiation: RFC-854, pp. 2-3
       Every Telnet implementation MUST include option negotiation and
       subnegotiation machinery [TELNET:2].
       A host MUST carefully follow the rules of RFC-854 to avoid
       option-negotiation loops.  A host MUST refuse (i.e, reply
       WONT/DONT to a DO/WILL) an unsupported option.  Option
       negotiation SHOULD continue to function (even if all requests
       are refused) throughout the lifetime of a Telnet connection.
       If all option negotiations fail, a Telnet implementation MUST
       default to, and support, an NVT.
       DISCUSSION:
            Even though more sophisticated "terminals" and supporting
            option negotiations are becoming the norm, all
            implementations must be prepared to support an NVT for any
            user-server communication.
    3.2.2  Telnet Go-Ahead Function: RFC-854, p. 5, and RFC-858
       On a host that never sends the Telnet command Go Ahead (GA),
       the Telnet Server MUST attempt to negotiate the Suppress Go
       Ahead option (i.e., send "WILL Suppress Go Ahead").  A User or
       Server Telnet MUST always accept negotiation of the Suppress Go

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RFC1123 REMOTE LOGIN – TELNET October 1989

       Ahead option.
       When it is driving a full-duplex terminal for which GA has no
       meaning, a User Telnet implementation MAY ignore GA commands.
       DISCUSSION:
            Half-duplex ("locked-keyboard") line-at-a-time terminals
            for which the Go-Ahead mechanism was designed have largely
            disappeared from the scene.  It turned out to be difficult
            to implement sending the Go-Ahead signal in many operating
            systems, even some systems that support native half-duplex
            terminals.  The difficulty is typically that the Telnet
            server code does not have access to information about
            whether the user process is blocked awaiting input from
            the Telnet connection, i.e., it cannot reliably determine
            when to send a GA command.  Therefore, most Telnet Server
            hosts do not send GA commands.
            The effect of the rules in this section is to allow either
            end of a Telnet connection to veto the use of GA commands.
            There is a class of half-duplex terminals that is still
            commercially important: "data entry terminals," which
            interact in a full-screen manner.  However, supporting
            data entry terminals using the Telnet protocol does not
            require the Go Ahead signal; see Section 3.3.2.
    3.2.3  Control Functions: RFC-854, pp. 7-8
       The list of Telnet commands has been extended to include EOR
       (End-of-Record), with code 239 [TELNET:9].
       Both User and Server Telnets MAY support the control functions
       EOR, EC, EL, and Break, and MUST support AO, AYT, DM, IP, NOP,
       SB, and SE.
       A host MUST be able to receive and ignore any Telnet control
       functions that it does not support.
       DISCUSSION:
            Note that a Server Telnet is required to support the
            Telnet IP (Interrupt Process) function, even if the server
            host has an equivalent in-stream function (e.g., Control-C
            in many systems).  The Telnet IP function may be stronger
            than an in-stream interrupt command, because of the out-
            of-band effect of TCP urgent data.
            The EOR control function may be used to delimit the

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RFC1123 REMOTE LOGIN – TELNET October 1989

            stream.  An important application is data entry terminal
            support (see Section 3.3.2).  There was concern that since
            EOR had not been defined in RFC-854, a host that was not
            prepared to correctly ignore unknown Telnet commands might
            crash if it received an EOR.  To protect such hosts, the
            End-of-Record option [TELNET:9] was introduced; however, a
            properly implemented Telnet program will not require this
            protection.
    3.2.4  Telnet "Synch" Signal: RFC-854, pp. 8-10
       When it receives "urgent" TCP data, a User or Server Telnet
       MUST discard all data except Telnet commands until the DM (and
       end of urgent) is reached.
       When it sends Telnet IP (Interrupt Process), a User Telnet
       SHOULD follow it by the Telnet "Synch" sequence, i.e., send as
       TCP urgent data the sequence "IAC IP IAC DM".  The TCP urgent
       pointer points to the DM octet.
       When it receives a Telnet IP command, a Server Telnet MAY send
       a Telnet "Synch" sequence back to the user, to flush the output
       stream.  The choice ought to be consistent with the way the
       server operating system behaves when a local user interrupts a
       process.
       When it receives a Telnet AO command, a Server Telnet MUST send
       a Telnet "Synch" sequence back to the user, to flush the output
       stream.
       A User Telnet SHOULD have the capability of flushing output
       when it sends a Telnet IP; see also Section 3.4.5.
       DISCUSSION:
            There are three possible ways for a User Telnet to flush
            the stream of server output data:
            (1)  Send AO after IP.
                 This will cause the server host to send a "flush-
                 buffered-output" signal to its operating system.
                 However, the AO may not take effect locally, i.e.,
                 stop terminal output at the User Telnet end, until
                 the Server Telnet has received and processed the AO
                 and has sent back a "Synch".
            (2)  Send DO TIMING-MARK [TELNET:7] after IP, and discard
                 all output locally until a WILL/WONT TIMING-MARK is

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RFC1123 REMOTE LOGIN – TELNET October 1989

                 received from the Server Telnet.
                 Since the DO TIMING-MARK will be processed after the
                 IP at the server, the reply to it should be in the
                 right place in the output data stream.  However, the
                 TIMING-MARK will not send a "flush buffered output"
                 signal to the server operating system.  Whether or
                 not this is needed is dependent upon the server
                 system.
            (3)  Do both.
            The best method is not entirely clear, since it must
            accommodate a number of existing server hosts that do not
            follow the Telnet standards in various ways.  The safest
            approach is probably to provide a user-controllable option
            to select (1), (2), or (3).
    3.2.5  NVT Printer and Keyboard: RFC-854, p. 11
       In NVT mode, a Telnet SHOULD NOT send characters with the
       high-order bit 1, and MUST NOT send it as a parity bit.
       Implementations that pass the high-order bit to applications
       SHOULD negotiate binary mode (see Section 3.2.6).
       DISCUSSION:
            Implementors should be aware that a strict reading of
            RFC-854 allows a client or server expecting NVT ASCII to
            ignore characters with the high-order bit set.  In
            general, binary mode is expected to be used for
            transmission of an extended (beyond 7-bit) character set
            with Telnet.
            However, there exist applications that really need an 8-
            bit NVT mode, which is currently not defined, and these
            existing applications do set the high-order bit during
            part or all of the life of a Telnet connection.  Note that
            binary mode is not the same as 8-bit NVT mode, since
            binary mode turns off end-of-line processing.  For this
            reason, the requirements on the high-order bit are stated
            as SHOULD, not MUST.
            RFC-854 defines a minimal set of properties of a "network
            virtual terminal" or NVT; this is not meant to preclude
            additional features in a real terminal.  A Telnet
            connection is fully transparent to all 7-bit ASCII
            characters, including arbitrary ASCII control characters.

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RFC1123 REMOTE LOGIN – TELNET October 1989

            For example, a terminal might support full-screen commands
            coded as ASCII escape sequences; a Telnet implementation
            would pass these sequences as uninterpreted data.  Thus,
            an NVT should not be conceived as a terminal type of a
            highly-restricted device.
    3.2.6  Telnet Command Structure: RFC-854, p. 13
       Since options may appear at any point in the data stream, a
       Telnet escape character (known as IAC, with the value 255) to
       be sent as data MUST be doubled.
    3.2.7  Telnet Binary Option: RFC-856
       When the Binary option has been successfully negotiated,
       arbitrary 8-bit characters are allowed.  However, the data
       stream MUST still be scanned for IAC characters, any embedded
       Telnet commands MUST be obeyed, and data bytes equal to IAC
       MUST be doubled.  Other character processing (e.g., replacing
       CR by CR NUL or by CR LF) MUST NOT be done.  In particular,
       there is no end-of-line convention (see Section 3.3.1) in
       binary mode.
       DISCUSSION:
            The Binary option is normally negotiated in both
            directions, to change the Telnet connection from NVT mode
            to "binary mode".
            The sequence IAC EOR can be used to delimit blocks of data
            within a binary-mode Telnet stream.
    3.2.8  Telnet Terminal-Type Option: RFC-1091
       The Terminal-Type option MUST use the terminal type names
       officially defined in the Assigned Numbers RFC [INTRO:5], when
       they are available for the particular terminal.  However, the
       receiver of a Terminal-Type option MUST accept any name.
       DISCUSSION:
            RFC-1091 [TELNET:10] updates an earlier version of the
            Terminal-Type option defined in RFC-930.  The earlier
            version allowed a server host capable of supporting
            multiple terminal types to learn the type of a particular
            client's terminal, assuming that each physical terminal
            had an intrinsic type.  However, today a "terminal" is
            often really a terminal emulator program running in a PC,
            perhaps capable of emulating a range of terminal types.
            Therefore, RFC-1091 extends the specification to allow a

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RFC1123 REMOTE LOGIN – TELNET October 1989

            more general terminal-type negotiation between User and
            Server Telnets.
 3.3  SPECIFIC ISSUES
    3.3.1  Telnet End-of-Line Convention
       The Telnet protocol defines the sequence CR LF to mean "end-
       of-line".  For terminal input, this corresponds to a command-
       completion or "end-of-line" key being pressed on a user
       terminal; on an ASCII terminal, this is the CR key, but it may
       also be labelled "Return" or "Enter".
       When a Server Telnet receives the Telnet end-of-line sequence
       CR LF as input from a remote terminal, the effect MUST be the
       same as if the user had pressed the "end-of-line" key on a
       local terminal.  On server hosts that use ASCII, in particular,
       receipt of the Telnet sequence CR LF must cause the same effect
       as a local user pressing the CR key on a local terminal.  Thus,
       CR LF and CR NUL MUST have the same effect on an ASCII server
       host when received as input over a Telnet connection.
       A User Telnet MUST be able to send any of the forms: CR LF, CR
       NUL, and LF.  A User Telnet on an ASCII host SHOULD have a
       user-controllable mode to send either CR LF or CR NUL when the
       user presses the "end-of-line" key, and CR LF SHOULD be the
       default.
       The Telnet end-of-line sequence CR LF MUST be used to send
       Telnet data that is not terminal-to-computer (e.g., for Server
       Telnet sending output, or the Telnet protocol incorporated
       another application protocol).
       DISCUSSION:
            To allow interoperability between arbitrary Telnet clients
            and servers, the Telnet protocol defined a standard
            representation for a line terminator.  Since the ASCII
            character set includes no explicit end-of-line character,
            systems have chosen various representations, e.g., CR, LF,
            and the sequence CR LF.  The Telnet protocol chose the CR
            LF sequence as the standard for network transmission.
            Unfortunately, the Telnet protocol specification in RFC-
            854 [TELNET:1] has turned out to be somewhat ambiguous on
            what character(s) should be sent from client to server for
            the "end-of-line" key.  The result has been a massive and
            continuing interoperability headache, made worse by
            various faulty implementations of both User and Server

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RFC1123 REMOTE LOGIN – TELNET October 1989

            Telnets.
            Although the Telnet protocol is based on a perfectly
            symmetric model, in a remote login session the role of the
            user at a terminal differs from the role of the server
            host.  For example, RFC-854 defines the meaning of CR, LF,
            and CR LF as output from the server, but does not specify
            what the User Telnet should send when the user presses the
            "end-of-line" key on the terminal; this turns out to be
            the point at issue.
            When a user presses the "end-of-line" key, some User
            Telnet implementations send CR LF, while others send CR
            NUL (based on a different interpretation of the same
            sentence in RFC-854).  These will be equivalent for a
            correctly-implemented ASCII server host, as discussed
            above.  For other servers, a mode in the User Telnet is
            needed.
            The existence of User Telnets that send only CR NUL when
            CR is pressed creates a dilemma for non-ASCII hosts: they
            can either treat CR NUL as equivalent to CR LF in input,
            thus precluding the possibility of entering a "bare" CR,
            or else lose complete interworking.
            Suppose a user on host A uses Telnet to log into a server
            host B, and then execute B's User Telnet program to log
            into server host C.  It is desirable for the Server/User
            Telnet combination on B to be as transparent as possible,
            i.e., to appear as if A were connected directly to C.  In
            particular, correct implementation will make B transparent
            to Telnet end-of-line sequences, except that CR LF may be
            translated to CR NUL or vice versa.
       IMPLEMENTATION:
            To understand Telnet end-of-line issues, one must have at
            least a general model of the relationship of Telnet to the
            local operating system.  The Server Telnet process is
            typically coupled into the terminal driver software of the
            operating system as a pseudo-terminal.  A Telnet end-of-
            line sequence received by the Server Telnet must have the
            same effect as pressing the end-of-line key on a real
            locally-connected terminal.
            Operating systems that support interactive character-at-
            a-time applications (e.g., editors) typically have two
            internal modes for their terminal I/O: a formatted mode,
            in which local conventions for end-of-line and other

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RFC1123 REMOTE LOGIN – TELNET October 1989

            formatting rules have been applied to the data stream, and
            a "raw" mode, in which the application has direct access
            to every character as it was entered.  A Server Telnet
            must be implemented in such a way that these modes have
            the same effect for remote as for local terminals.  For
            example, suppose a CR LF or CR NUL is received by the
            Server Telnet on an ASCII host.  In raw mode, a CR
            character is passed to the application; in formatted mode,
            the local system's end-of-line convention is used.
    3.3.2  Data Entry Terminals
       DISCUSSION:
            In addition to the line-oriented and character-oriented
            ASCII terminals for which Telnet was designed, there are
            several families of video display terminals that are
            sometimes known as "data entry terminals" or DETs.  The
            IBM 3270 family is a well-known example.
            Two Internet protocols have been designed to support
            generic DETs: SUPDUP [TELNET:16, TELNET:17], and the DET
            option [TELNET:18, TELNET:19].  The DET option drives a
            data entry terminal over a Telnet connection using (sub-)
            negotiation.  SUPDUP is a completely separate terminal
            protocol, which can be entered from Telnet by negotiation.
            Although both SUPDUP and the DET option have been used
            successfully in particular environments, neither has
            gained general acceptance or wide implementation.
            A different approach to DET interaction has been developed
            for supporting the IBM 3270 family through Telnet,
            although the same approach would be applicable to any DET.
            The idea is to enter a "native DET" mode, in which the
            native DET input/output stream is sent as binary data.
            The Telnet EOR command is used to delimit logical records
            (e.g., "screens") within this binary stream.
       IMPLEMENTATION:
            The rules for entering and leaving native DET mode are as
            follows:
            o    The Server uses the Terminal-Type option [TELNET:10]
                 to learn that the client is a DET.
            o    It is conventional, but not required, that both ends
                 negotiate the EOR option [TELNET:9].
            o    Both ends negotiate the Binary option [TELNET:3] to

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RFC1123 REMOTE LOGIN – TELNET October 1989

                 enter native DET mode.
            o    When either end negotiates out of binary mode, the
                 other end does too, and the mode then reverts to
                 normal NVT.
    3.3.3  Option Requirements
       Every Telnet implementation MUST support the Binary option
       [TELNET:3] and the Suppress Go Ahead option [TELNET:5], and
       SHOULD support the Echo [TELNET:4], Status [TELNET:6], End-of-
       Record [TELNET:9], and Extended Options List [TELNET:8]
       options.
       A User or Server Telnet SHOULD support the Window Size Option
       [TELNET:12] if the local operating system provides the
       corresponding capability.
       DISCUSSION:
            Note that the End-of-Record option only signifies that a
            Telnet can receive a Telnet EOR without crashing;
            therefore, every Telnet ought to be willing to accept
            negotiation of the End-of-Record option.  See also the
            discussion in Section 3.2.3.
    3.3.4  Option Initiation
       When the Telnet protocol is used in a client/server situation,
       the server SHOULD initiate negotiation of the terminal
       interaction mode it expects.
       DISCUSSION:
            The Telnet protocol was defined to be perfectly
            symmetrical, but its application is generally asymmetric.
            Remote login has been known to fail because NEITHER side
            initiated negotiation of the required non-default terminal
            modes.  It is generally the server that determines the
            preferred mode, so the server needs to initiate the
            negotiation; since the negotiation is symmetric, the user
            can also initiate it.
       A client (User Telnet) SHOULD provide a means for users to
       enable and disable the initiation of option negotiation.
       DISCUSSION:
            A user sometimes needs to connect to an application
            service (e.g., FTP or SMTP) that uses Telnet for its

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RFC1123 REMOTE LOGIN – TELNET October 1989

            control stream but does not support Telnet options.  User
            Telnet may be used for this purpose if initiation of
            option negotiation is  disabled.
    3.3.5  Telnet Linemode Option
       DISCUSSION:
            An important new Telnet option, LINEMODE [TELNET:12], has
            been proposed.  The LINEMODE option provides a standard
            way for a User Telnet and a Server Telnet to agree that
            the client rather than the server will perform terminal
            character processing.  When the client has prepared a
            complete line of text, it will send it to the server in
            (usually) one TCP packet.  This option will greatly
            decrease the packet cost of Telnet sessions and will also
            give much better user response over congested or long-
            delay networks.
            The LINEMODE option allows dynamic switching between local
            and remote character processing.  For example, the Telnet
            connection will automatically negotiate into single-
            character mode while a full screen editor is running, and
            then return to linemode when the editor is finished.
            We expect that when this RFC is released, hosts should
            implement the client side of this option, and may
            implement the server side of this option.  To properly
            implement the server side, the server needs to be able to
            tell the local system not to do any input character
            processing, but to remember its current terminal state and
            notify the Server Telnet process whenever the state
            changes.  This will allow password echoing and full screen
            editors to be handled properly, for example.
 3.4  TELNET/USER INTERFACE
    3.4.1  Character Set Transparency
       User Telnet implementations SHOULD be able to send or receive
       any 7-bit ASCII character.  Where possible, any special
       character interpretations by the user host's operating system
       SHOULD be bypassed so that these characters can conveniently be
       sent and received on the connection.
       Some character value MUST be reserved as "escape to command
       mode"; conventionally, doubling this character allows it to be
       entered as data.  The specific character used SHOULD be user
       selectable.

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RFC1123 REMOTE LOGIN – TELNET October 1989

       On binary-mode connections, a User Telnet program MAY provide
       an escape mechanism for entering arbitrary 8-bit values, if the
       host operating system doesn't allow them to be entered directly
       from the keyboard.
       IMPLEMENTATION:
            The transparency issues are less pressing on servers, but
            implementors should take care in dealing with issues like:
            masking off parity bits (sent by an older, non-conforming
            client) before they reach programs that expect only NVT
            ASCII, and properly handling programs that request 8-bit
            data streams.
    3.4.2  Telnet Commands
       A User Telnet program MUST provide a user the capability of
       entering any of the Telnet control functions IP, AO, or AYT,
       and SHOULD provide the capability of entering EC, EL, and
       Break.
    3.4.3  TCP Connection Errors
       A User Telnet program SHOULD report to the user any TCP errors
       that are reported by the transport layer (see "TCP/Application
       Layer Interface" section in [INTRO:1]).
    3.4.4  Non-Default Telnet Contact Port
       A User Telnet program SHOULD allow the user to optionally
       specify a non-standard contact port number at the Server Telnet
       host.
    3.4.5  Flushing Output
       A User Telnet program SHOULD provide the user the ability to
       specify whether or not output should be flushed when an IP is
       sent; see Section 3.2.4.
       For any output flushing scheme that causes the User Telnet to
       flush output locally until a Telnet signal is received from the
       Server, there SHOULD be a way for the user to manually restore
       normal output, in case the Server fails to send the expected
       signal.

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RFC1123 REMOTE LOGIN – TELNET October 1989

 3.5.  TELNET REQUIREMENTS SUMMARY
                                               |        | | | |S| |
                                               |        | | | |H| |F
                                               |        | | | |O|M|o
                                               |        | |S| |U|U|o
                                               |        | |H| |L|S|t
                                               |        |M|O| |D|T|n
                                               |        |U|U|M| | |o
                                               |        |S|L|A|N|N|t
                                               |        |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ————————————————-|——–|-|-|-|-|-|–

                                               |        | | | | | |

Option Negotiation |3.2.1 |x| | | | |

Avoid negotiation loops                        |3.2.1   |x| | | | |
Refuse unsupported options                     |3.2.1   |x| | | | |
Negotiation OK anytime on connection           |3.2.1   | |x| | | |
Default to NVT                                 |3.2.1   |x| | | | |
Send official name in Term-Type option         |3.2.8   |x| | | | |
Accept any name in Term-Type option            |3.2.8   |x| | | | |
Implement Binary, Suppress-GA options          |3.3.3   |x| | | | |
Echo, Status, EOL, Ext-Opt-List options        |3.3.3   | |x| | | |
Implement Window-Size option if appropriate    |3.3.3   | |x| | | |
Server initiate mode negotiations              |3.3.4   | |x| | | |
User can enable/disable init negotiations      |3.3.4   | |x| | | |
                                               |        | | | | | |

Go-Aheads | | | | | | |

Non-GA server negotiate SUPPRESS-GA option     |3.2.2   |x| | | | |
User or Server accept SUPPRESS-GA option       |3.2.2   |x| | | | |
User Telnet ignore GA's                        |3.2.2   | | |x| | |
                                               |        | | | | | |

Control Functions | | | | | | |

Support SE NOP DM IP AO AYT SB                 |3.2.3   |x| | | | |
Support EOR EC EL Break                        |3.2.3   | | |x| | |
Ignore unsupported control functions           |3.2.3   |x| | | | |
User, Server discard urgent data up to DM      |3.2.4   |x| | | | |
User Telnet send "Synch" after IP, AO, AYT     |3.2.4   | |x| | | |
Server Telnet reply Synch to IP                |3.2.4   | | |x| | |
Server Telnet reply Synch to AO                |3.2.4   |x| | | | |
User Telnet can flush output when send IP      |3.2.4   | |x| | | |
                                               |        | | | | | |

Encoding | | | | | | |

Send high-order bit in NVT mode                |3.2.5   | | | |x| |
Send high-order bit as parity bit              |3.2.5   | | | | |x|
Negot. BINARY if pass high-ord. bit to applic  |3.2.5   | |x| | | |
Always double IAC data byte                    |3.2.6   |x| | | | |

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RFC1123 REMOTE LOGIN – TELNET October 1989

Double IAC data byte in binary mode            |3.2.7   |x| | | | |
Obey Telnet cmds in binary mode                |3.2.7   |x| | | | |
End-of-line, CR NUL in binary mode             |3.2.7   | | | | |x|
                                               |        | | | | | |

End-of-Line | | | | | | |

EOL at Server same as local end-of-line        |3.3.1   |x| | | | |
ASCII Server accept CR LF or CR NUL for EOL    |3.3.1   |x| | | | |
User Telnet able to send CR LF, CR NUL, or LF  |3.3.1   |x| | | | |
  ASCII user able to select CR LF/CR NUL       |3.3.1   | |x| | | |
  User Telnet default mode is CR LF            |3.3.1   | |x| | | |
Non-interactive uses CR LF for EOL             |3.3.1   |x| | | | |
                                               |        | | | | | |

User Telnet interface | | | | | | |

Input & output all 7-bit characters            |3.4.1   | |x| | | |
Bypass local op sys interpretation             |3.4.1   | |x| | | |
Escape character                               |3.4.1   |x| | | | |
   User-settable escape character              |3.4.1   | |x| | | |
Escape to enter 8-bit values                   |3.4.1   | | |x| | |
Can input IP, AO, AYT                          |3.4.2   |x| | | | |
Can input EC, EL, Break                        |3.4.2   | |x| | | |
Report TCP connection errors to user           |3.4.3   | |x| | | |
Optional non-default contact port              |3.4.4   | |x| | | |
Can spec: output flushed when IP sent          |3.4.5   | |x| | | |
Can manually restore output mode               |3.4.5   | |x| | | |
                                               |        | | | | | |

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RFC1123 FILE TRANSFER – FTP October 1989

4. FILE TRANSFER

 4.1  FILE TRANSFER PROTOCOL -- FTP
    4.1.1  INTRODUCTION
       The File Transfer Protocol FTP is the primary Internet standard
       for file transfer.  The current specification is contained in
       RFC-959 [FTP:1].
       FTP uses separate simultaneous TCP connections for control and
       for data transfer.  The FTP protocol includes many features,
       some of which are not commonly implemented.  However, for every
       feature in FTP, there exists at least one implementation.  The
       minimum implementation defined in RFC-959 was too small, so a
       somewhat larger minimum implementation is defined here.
       Internet users have been unnecessarily burdened for years by
       deficient FTP implementations.  Protocol implementors have
       suffered from the erroneous opinion that implementing FTP ought
       to be a small and trivial task.  This is wrong, because FTP has
       a user interface, because it has to deal (correctly) with the
       whole variety of communication and operating system errors that
       may occur, and because it has to handle the great diversity of
       real file systems in the world.
    4.1.2.  PROTOCOL WALK-THROUGH
       4.1.2.1  LOCAL Type: RFC-959 Section 3.1.1.4
          An FTP program MUST support TYPE I ("IMAGE" or binary type)
          as well as TYPE L 8 ("LOCAL" type with logical byte size 8).
          A machine whose memory is organized into m-bit words, where
          m is not a multiple of 8, MAY also support TYPE L m.
          DISCUSSION:
               The command "TYPE L 8" is often required to transfer
               binary data between a machine whose memory is organized
               into (e.g.) 36-bit words and a machine with an 8-bit
               byte organization.  For an 8-bit byte machine, TYPE L 8
               is equivalent to IMAGE.
               "TYPE L m" is sometimes specified to the FTP programs
               on two m-bit word machines to ensure the correct
               transfer of a native-mode binary file from one machine
               to the other.  However, this command should have the
               same effect on these machines as "TYPE I".

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       4.1.2.2  Telnet Format Control: RFC-959 Section 3.1.1.5.2
          A host that makes no distinction between TYPE N and TYPE T
          SHOULD implement TYPE T to be identical to TYPE N.
          DISCUSSION:
               This provision should ease interoperation with hosts
               that do make this distinction.
               Many hosts represent text files internally as strings
               of ASCII characters, using the embedded ASCII format
               effector characters (LF, BS, FF, ...) to control the
               format when a file is printed.  For such hosts, there
               is no distinction between "print" files and other
               files.  However, systems that use record structured
               files typically need a special format for printable
               files (e.g., ASA carriage control).   For the latter
               hosts, FTP allows a choice of TYPE N or TYPE T.
       4.1.2.3  Page Structure: RFC-959 Section 3.1.2.3 and Appendix I
          Implementation of page structure is NOT RECOMMENDED in
          general. However, if a host system does need to implement
          FTP for "random access" or "holey" files, it MUST use the
          defined page structure format rather than define a new
          private FTP format.
       4.1.2.4  Data Structure Transformations: RFC-959 Section 3.1.2
          An FTP transformation between record-structure and file-
          structure SHOULD be invertible, to the extent possible while
          making the result useful on the target host.
          DISCUSSION:
               RFC-959 required strict invertibility between record-
               structure and file-structure, but in practice,
               efficiency and convenience often preclude it.
               Therefore, the requirement is being relaxed.  There are
               two different objectives for transferring a file:
               processing it on the target host, or just storage.  For
               storage, strict invertibility is important.  For
               processing, the file created on the target host needs
               to be in the format expected by application programs on
               that host.
               As an example of the conflict, imagine a record-
               oriented operating system that requires some data files
               to have exactly 80 bytes in each record.  While STORing

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               a file on such a host, an FTP Server must be able to
               pad each line or record to 80 bytes; a later retrieval
               of such a file cannot be strictly invertible.
       4.1.2.5  Data Connection Management: RFC-959 Section 3.3
          A User-FTP that uses STREAM mode SHOULD send a PORT command
          to assign a non-default data port before each transfer
          command is issued.
          DISCUSSION:
               This is required because of the long delay after a TCP
               connection is closed until its socket pair can be
               reused, to allow multiple transfers during a single FTP
               session.  Sending a port command can avoided if a
               transfer mode other than stream is used, by leaving the
               data transfer connection open between transfers.
       4.1.2.6  PASV Command: RFC-959 Section 4.1.2
          A server-FTP MUST implement the PASV command.
          If multiple third-party transfers are to be executed during
          the same session, a new PASV command MUST be issued before
          each transfer command, to obtain a unique port pair.
          IMPLEMENTATION:
               The format of the 227 reply to a PASV command is not
               well standardized.  In particular, an FTP client cannot
               assume that the parentheses shown on page 40 of RFC-959
               will be present (and in fact, Figure 3 on page 43 omits
               them).  Therefore, a User-FTP program that interprets
               the PASV reply must scan the reply for the first digit
               of the host and port numbers.
               Note that the host number h1,h2,h3,h4 is the IP address
               of the server host that is sending the reply, and that
               p1,p2 is a non-default data transfer port that PASV has
               assigned.
       4.1.2.7  LIST and NLST Commands: RFC-959 Section 4.1.3
          The data returned by an NLST command MUST contain only a
          simple list of legal pathnames, such that the server can use
          them directly as the arguments of subsequent data transfer
          commands for the individual files.
          The data returned by a LIST or NLST command SHOULD use an

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          implied TYPE AN, unless the current type is EBCDIC, in which
          case an implied TYPE EN SHOULD be used.
          DISCUSSION:
               Many FTP clients support macro-commands that will get
               or put files matching a wildcard specification, using
               NLST to obtain a list of pathnames.  The expansion of
               "multiple-put" is local to the client, but "multiple-
               get" requires cooperation by the server.
               The implied type for LIST and NLST is designed to
               provide compatibility with existing User-FTPs, and in
               particular with multiple-get commands.
       4.1.2.8  SITE Command: RFC-959 Section 4.1.3
          A Server-FTP SHOULD use the SITE command for non-standard
          features, rather than invent new private commands or
          unstandardized extensions to existing commands.
       4.1.2.9  STOU Command: RFC-959 Section 4.1.3
          The STOU command stores into a uniquely named file.  When it
          receives an STOU command, a Server-FTP MUST return the
          actual file name in the "125 Transfer Starting" or the "150
          Opening Data Connection" message that precedes the transfer
          (the 250 reply code mentioned in RFC-959 is incorrect).  The
          exact format of these messages is hereby defined to be as
          follows:
              125 FILE: pppp
              150 FILE: pppp
          where pppp represents the unique pathname of the file that
          will be written.
       4.1.2.10  Telnet End-of-line Code: RFC-959, Page 34
          Implementors MUST NOT assume any correspondence between READ
          boundaries on the control connection and the Telnet EOL
          sequences (CR LF).
          DISCUSSION:
               Thus, a server-FTP (or User-FTP) must continue reading
               characters from the control connection until a complete
               Telnet EOL sequence is encountered, before processing
               the command (or response, respectively).  Conversely, a
               single READ from the control connection may include

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               more than one FTP command.
       4.1.2.11  FTP Replies: RFC-959 Section 4.2, Page 35
          A Server-FTP MUST send only correctly formatted replies on
          the control connection.  Note that RFC-959 (unlike earlier
          versions of the FTP spec) contains no provision for a
          "spontaneous" reply message.
          A Server-FTP SHOULD use the reply codes defined in RFC-959
          whenever they apply.  However, a server-FTP MAY use a
          different reply code when needed, as long as the general
          rules of Section 4.2 are followed. When the implementor has
          a choice between a 4xx and 5xx reply code, a Server-FTP
          SHOULD send a 4xx (temporary failure) code when there is any
          reasonable possibility that a failed FTP will succeed a few
          hours later.
          A User-FTP SHOULD generally use only the highest-order digit
          of a 3-digit reply code for making a procedural decision, to
          prevent difficulties when a Server-FTP uses non-standard
          reply codes.
          A User-FTP MUST be able to handle multi-line replies.  If
          the implementation imposes a limit on the number of lines
          and if this limit is exceeded, the User-FTP MUST recover,
          e.g., by ignoring the excess lines until the end of the
          multi-line reply is reached.
          A User-FTP SHOULD NOT interpret a 421 reply code ("Service
          not available, closing control connection") specially, but
          SHOULD detect closing of the control connection by the
          server.
          DISCUSSION:
               Server implementations that fail to strictly follow the
               reply rules often cause FTP user programs to hang.
               Note that RFC-959 resolved ambiguities in the reply
               rules found in earlier FTP specifications and must be
               followed.
               It is important to choose FTP reply codes that properly
               distinguish between temporary and permanent failures,
               to allow the successful use of file transfer client
               daemons.  These programs depend on the reply codes to
               decide whether or not to retry a failed transfer; using
               a permanent failure code (5xx) for a temporary error
               will cause these programs to give up unnecessarily.

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               When the meaning of a reply matches exactly the text
               shown in RFC-959, uniformity will be enhanced by using
               the RFC-959 text verbatim.  However, a Server-FTP
               implementor is encouraged to choose reply text that
               conveys specific system-dependent information, when
               appropriate.
       4.1.2.12  Connections: RFC-959 Section 5.2
          The words "and the port used" in the second paragraph of
          this section of RFC-959 are erroneous (historical), and they
          should be ignored.
          On a multihomed server host, the default data transfer port
          (L-1) MUST be associated with the same local IP address as
          the corresponding control connection to port L.
          A user-FTP MUST NOT send any Telnet controls other than
          SYNCH and IP on an FTP control connection. In particular, it
          MUST NOT attempt to negotiate Telnet options on the control
          connection.  However, a server-FTP MUST be capable of
          accepting and refusing Telnet negotiations (i.e., sending
          DONT/WONT).
          DISCUSSION:
               Although the RFC says: "Server- and User- processes
               should follow the conventions for the Telnet
               protocol...[on the control connection]", it is not the
               intent that Telnet option negotiation is to be
               employed.
       4.1.2.13  Minimum Implementation; RFC-959 Section 5.1
          The following commands and options MUST be supported by
          every server-FTP and user-FTP, except in cases where the
          underlying file system or operating system does not allow or
          support a particular command.
               Type: ASCII Non-print, IMAGE, LOCAL 8
               Mode: Stream
               Structure: File, Record*
               Commands:
                  USER, PASS, ACCT,
                  PORT, PASV,
                  TYPE, MODE, STRU,
                  RETR, STOR, APPE,
                  RNFR, RNTO, DELE,
                  CWD,  CDUP, RMD,  MKD,  PWD,

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                  LIST, NLST,
                  SYST, STAT,
                  HELP, NOOP, QUIT.
  • Record structure is REQUIRED only for hosts whose file

systems support record structure.

          DISCUSSION:
               Vendors are encouraged to implement a larger subset of
               the protocol.  For example, there are important
               robustness features in the protocol (e.g., Restart,
               ABOR, block mode) that would be an aid to some Internet
               users but are not widely implemented.
               A host that does not have record structures in its file
               system may still accept files with STRU R, recording
               the byte stream literally.
    4.1.3  SPECIFIC ISSUES
       4.1.3.1  Non-standard Command Verbs
          FTP allows "experimental" commands, whose names begin with
          "X".  If these commands are subsequently adopted as
          standards, there may still be existing implementations using
          the "X" form.  At present, this is true for the directory
          commands:
              RFC-959   "Experimental"
                MKD        XMKD
                RMD        XRMD
                PWD        XPWD
                CDUP       XCUP
                CWD        XCWD
          All FTP implementations SHOULD recognize both forms of these
          commands, by simply equating them with extra entries in the
          command lookup table.
          IMPLEMENTATION:
               A User-FTP can access a server that supports only the
               "X" forms by implementing a mode switch, or
               automatically using the following procedure: if the
               RFC-959 form of one of the above commands is rejected
               with a 500 or 502 response code, then try the
               experimental form; any other response would be passed
               to the user.

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       4.1.3.2  Idle Timeout
          A Server-FTP process SHOULD have an idle timeout, which will
          terminate the process and close the control connection if
          the server is inactive (i.e., no command or data transfer in
          progress) for a long period of time.  The idle timeout time
          SHOULD be configurable, and the default should be at least 5
          minutes.
          A client FTP process ("User-PI" in RFC-959) will need
          timeouts on responses only if it is invoked from a program.
          DISCUSSION:
               Without a timeout, a Server-FTP process may be left
               pending indefinitely if the corresponding client
               crashes without closing the control connection.
       4.1.3.3  Concurrency of Data and Control
          DISCUSSION:
               The intent of the designers of FTP was that a user
               should be able to send a STAT command at any time while
               data transfer was in progress and that the server-FTP
               would reply immediately with status -- e.g., the number
               of bytes transferred so far.  Similarly, an ABOR
               command should be possible at any time during a data
               transfer.
               Unfortunately, some small-machine operating systems
               make such concurrent programming difficult, and some
               other implementers seek minimal solutions, so some FTP
               implementations do not allow concurrent use of the data
               and control connections.  Even such a minimal server
               must be prepared to accept and defer a STAT or ABOR
               command that arrives during data transfer.
       4.1.3.4  FTP Restart Mechanism
          The description of the 110 reply on pp. 40-41 of RFC-959 is
          incorrect; the correct description is as follows.  A restart
          reply message, sent over the control connection from the
          receiving FTP to the User-FTP, has the format:
              110 MARK ssss = rrrr
          Here:
  • ssss is a text string that appeared in a Restart Marker

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               in the data stream and encodes a position in the
               sender's file system;
  • rrrr encodes the corresponding position in the

receiver's file system.

          The encoding, which is specific to a particular file system
          and network implementation, is always generated and
          interpreted by the same system, either sender or receiver.
          When an FTP that implements restart receives a Restart
          Marker in the data stream, it SHOULD force the data to that
          point to be written to stable storage before encoding the
          corresponding position rrrr.  An FTP sending Restart Markers
          MUST NOT assume that 110 replies will be returned
          synchronously with the data, i.e., it must not await a 110
          reply before sending more data.
          Two new reply codes are hereby defined for errors
          encountered in restarting a transfer:
            554 Requested action not taken: invalid REST parameter.
               A 554 reply may result from a FTP service command that
               follows a REST command.  The reply indicates that the
               existing file at the Server-FTP cannot be repositioned
               as specified in the REST.
            555 Requested action not taken: type or stru mismatch.
               A 555 reply may result from an APPE command or from any
               FTP service command following a REST command.  The
               reply indicates that there is some mismatch between the
               current transfer parameters (type and stru) and the
               attributes of the existing file.
          DISCUSSION:
               Note that the FTP Restart mechanism requires that Block
               or Compressed mode be used for data transfer, to allow
               the Restart Markers to be included within the data
               stream.  The frequency of Restart Markers can be low.
               Restart Markers mark a place in the data stream, but
               the receiver may be performing some transformation on
               the data as it is stored into stable storage.  In
               general, the receiver's encoding must include any state
               information necessary to restart this transformation at
               any point of the FTP data stream.  For example, in TYPE

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               A transfers, some receiver hosts transform CR LF
               sequences into a single LF character on disk.   If a
               Restart Marker happens to fall between CR and LF, the
               receiver must encode in rrrr that the transfer must be
               restarted in a "CR has been seen and discarded" state.
               Note that the Restart Marker is required to be encoded
               as a string of printable ASCII characters, regardless
               of the type of the data.
               RFC-959 says that restart information is to be returned
               "to the user".  This should not be taken literally.  In
               general, the User-FTP should save the restart
               information (ssss,rrrr) in stable storage, e.g., append
               it to a restart control file.  An empty restart control
               file should be created when the transfer first starts
               and deleted automatically when the transfer completes
               successfully.  It is suggested that this file have a
               name derived in an easily-identifiable manner from the
               name of the file being transferred and the remote host
               name; this is analogous to the means used by many text
               editors for naming "backup" files.
               There are three cases for FTP restart.
               (1)  User-to-Server Transfer
                    The User-FTP puts Restart Markers <ssss> at
                    convenient places in the data stream.  When the
                    Server-FTP receives a Marker, it writes all prior
                    data to disk, encodes its file system position and
                    transformation state as rrrr, and returns a "110
                    MARK ssss = rrrr" reply over the control
                    connection.  The User-FTP appends the pair
                    (ssss,rrrr) to its restart control file.
                    To restart the transfer, the User-FTP fetches the
                    last (ssss,rrrr) pair from the restart control
                    file, repositions its local file system and
                    transformation state using ssss, and sends the
                    command "REST rrrr" to the Server-FTP.
               (2)  Server-to-User Transfer
                    The Server-FTP puts Restart Markers <ssss> at
                    convenient places in the data stream.  When the
                    User-FTP receives a Marker, it writes all prior
                    data to disk, encodes its file system position and

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                    transformation state as rrrr, and appends the pair
                    (rrrr,ssss) to its restart control file.
                    To restart the transfer, the User-FTP fetches the
                    last (rrrr,ssss) pair from the restart control
                    file, repositions its local file system and
                    transformation state using rrrr, and sends the
                    command "REST ssss" to the Server-FTP.
               (3)  Server-to-Server ("Third-Party") Transfer
                    The sending Server-FTP puts Restart Markers <ssss>
                    at convenient places in the data stream.  When it
                    receives a Marker, the receiving Server-FTP writes
                    all prior data to disk, encodes its file system
                    position and transformation state as rrrr, and
                    sends a "110 MARK ssss = rrrr" reply over the
                    control connection to the User.  The User-FTP
                    appends the pair (ssss,rrrr) to its restart
                    control file.
                    To restart the transfer, the User-FTP fetches the
                    last (ssss,rrrr) pair from the restart control
                    file, sends "REST ssss" to the sending Server-FTP,
                    and sends "REST rrrr" to the receiving Server-FTP.
    4.1.4  FTP/USER INTERFACE
       This section discusses the user interface for a User-FTP
       program.
       4.1.4.1  Pathname Specification
          Since FTP is intended for use in a heterogeneous
          environment, User-FTP implementations MUST support remote
          pathnames as arbitrary character strings, so that their form
          and content are not limited by the conventions of the local
          operating system.
          DISCUSSION:
               In particular, remote pathnames can be of arbitrary
               length, and all the printing ASCII characters as well
               as space (0x20) must be allowed.  RFC-959 allows a
               pathname to contain any 7-bit ASCII character except CR
               or LF.

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       4.1.4.2  "QUOTE" Command
          A User-FTP program MUST implement a "QUOTE" command that
          will pass an arbitrary character string to the server and
          display all resulting response messages to the user.
          To make the "QUOTE" command useful, a User-FTP SHOULD send
          transfer control commands to the server as the user enters
          them, rather than saving all the commands and sending them
          to the server only when a data transfer is started.
          DISCUSSION:
               The "QUOTE" command is essential to allow the user to
               access servers that require system-specific commands
               (e.g., SITE or ALLO), or to invoke new or optional
               features that are not implemented by the User-FTP.  For
               example, "QUOTE" may be used to specify "TYPE A T" to
               send a print file to hosts that require the
               distinction, even if the User-FTP does not recognize
               that TYPE.
       4.1.4.3  Displaying Replies to User
          A User-FTP SHOULD display to the user the full text of all
          error reply messages it receives.  It SHOULD have a
          "verbose" mode in which all commands it sends and the full
          text and reply codes it receives are displayed, for
          diagnosis of problems.
       4.1.4.4  Maintaining Synchronization
          The state machine in a User-FTP SHOULD be forgiving of
          missing and unexpected reply messages, in order to maintain
          command synchronization with the server.

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    4.1.5   FTP REQUIREMENTS SUMMARY
                                         |               | | | |S| |
                                         |               | | | |H| |F
                                         |               | | | |O|M|o
                                         |               | |S| |U|U|o
                                         |               | |H| |L|S|t
                                         |               |M|O| |D|T|n
                                         |               |U|U|M| | |o
                                         |               |S|L|A|N|N|t
                                         |               |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ——————————————-|—————|-|-|-|-|-|– Implement TYPE T if same as TYPE N |4.1.2.2 | |x| | | | File/Record transform invertible if poss. |4.1.2.4 | |x| | | | User-FTP send PORT cmd for stream mode |4.1.2.5 | |x| | | | Server-FTP implement PASV |4.1.2.6 |x| | | | |

PASV is per-transfer                     |4.1.2.6        |x| | | | |

NLST reply usable in RETR cmds |4.1.2.7 |x| | | | | Implied type for LIST and NLST |4.1.2.7 | |x| | | | SITE cmd for non-standard features |4.1.2.8 | |x| | | | STOU cmd return pathname as specified |4.1.2.9 |x| | | | | Use TCP READ boundaries on control conn. |4.1.2.10 | | | | |x|

                                         |               | | | | | |

Server-FTP send only correct reply format |4.1.2.11 |x| | | | | Server-FTP use defined reply code if poss. |4.1.2.11 | |x| | | |

New reply code following Section 4.2     |4.1.2.11       | | |x| | |

User-FTP use only high digit of reply |4.1.2.11 | |x| | | | User-FTP handle multi-line reply lines |4.1.2.11 |x| | | | | User-FTP handle 421 reply specially |4.1.2.11 | | | |x| |

                                         |               | | | | | |

Default data port same IP addr as ctl conn |4.1.2.12 |x| | | | | User-FTP send Telnet cmds exc. SYNCH, IP |4.1.2.12 | | | | |x| User-FTP negotiate Telnet options |4.1.2.12 | | | | |x| Server-FTP handle Telnet options |4.1.2.12 |x| | | | | Handle "Experimental" directory cmds |4.1.3.1 | |x| | | | Idle timeout in server-FTP |4.1.3.2 | |x| | | |

  Configurable idle timeout              |4.1.3.2        | |x| | | |

Receiver checkpoint data at Restart Marker |4.1.3.4 | |x| | | | Sender assume 110 replies are synchronous |4.1.3.4 | | | | |x|

                                         |               | | | | | |

Support TYPE: | | | | | | |

ASCII - Non-Print (AN)                   |4.1.2.13       |x| | | | |
ASCII - Telnet (AT) -- if same as AN     |4.1.2.2        | |x| | | |
ASCII - Carriage Control (AC)            |959 3.1.1.5.2  | | |x| | |
EBCDIC - (any form)                      |959 3.1.1.2    | | |x| | |
IMAGE                                    |4.1.2.1        |x| | | | |
LOCAL 8                                  |4.1.2.1        |x| | | | |

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LOCAL m                                  |4.1.2.1        | | |x| | |2
                                         |               | | | | | |

Support MODE: | | | | | | |

Stream                                   |4.1.2.13       |x| | | | |
Block                                    |959 3.4.2      | | |x| | |
                                         |               | | | | | |

Support STRUCTURE: | | | | | | |

File                                     |4.1.2.13       |x| | | | |
Record                                   |4.1.2.13       |x| | | | |3
Page                                     |4.1.2.3        | | | |x| |
                                         |               | | | | | |

Support commands: | | | | | | |

USER                                     |4.1.2.13       |x| | | | |
PASS                                     |4.1.2.13       |x| | | | |
ACCT                                     |4.1.2.13       |x| | | | |
CWD                                      |4.1.2.13       |x| | | | |
CDUP                                     |4.1.2.13       |x| | | | |
SMNT                                     |959 5.3.1      | | |x| | |
REIN                                     |959 5.3.1      | | |x| | |
QUIT                                     |4.1.2.13       |x| | | | |
                                         |               | | | | | |
PORT                                     |4.1.2.13       |x| | | | |
PASV                                     |4.1.2.6        |x| | | | |
TYPE                                     |4.1.2.13       |x| | | | |1
STRU                                     |4.1.2.13       |x| | | | |1
MODE                                     |4.1.2.13       |x| | | | |1
                                         |               | | | | | |
RETR                                     |4.1.2.13       |x| | | | |
STOR                                     |4.1.2.13       |x| | | | |
STOU                                     |959 5.3.1      | | |x| | |
APPE                                     |4.1.2.13       |x| | | | |
ALLO                                     |959 5.3.1      | | |x| | |
REST                                     |959 5.3.1      | | |x| | |
RNFR                                     |4.1.2.13       |x| | | | |
RNTO                                     |4.1.2.13       |x| | | | |
ABOR                                     |959 5.3.1      | | |x| | |
DELE                                     |4.1.2.13       |x| | | | |
RMD                                      |4.1.2.13       |x| | | | |
MKD                                      |4.1.2.13       |x| | | | |
PWD                                      |4.1.2.13       |x| | | | |
LIST                                     |4.1.2.13       |x| | | | |
NLST                                     |4.1.2.13       |x| | | | |
SITE                                     |4.1.2.8        | | |x| | |
STAT                                     |4.1.2.13       |x| | | | |
SYST                                     |4.1.2.13       |x| | | | |
HELP                                     |4.1.2.13       |x| | | | |
NOOP                                     |4.1.2.13       |x| | | | |
                                         |               | | | | | |

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User Interface: | | | | | | |

Arbitrary pathnames                      |4.1.4.1        |x| | | | |
Implement "QUOTE" command                |4.1.4.2        |x| | | | |
Transfer control commands immediately    |4.1.4.2        | |x| | | |
Display error messages to user           |4.1.4.3        | |x| | | |
  Verbose mode                           |4.1.4.3        | |x| | | |
Maintain synchronization with server     |4.1.4.4        | |x| | | |

Footnotes:

(1) For the values shown earlier.

(2) Here m is number of bits in a memory word.

(3) Required for host with record-structured file system, optional

   otherwise.

Internet Engineering Task Force [Page 43]

RFC1123 FILE TRANSFER – TFTP October 1989

 4.2  TRIVIAL FILE TRANSFER PROTOCOL -- TFTP
    4.2.1  INTRODUCTION
       The Trivial File Transfer Protocol TFTP is defined in RFC-783
       [TFTP:1].
       TFTP provides its own reliable delivery with UDP as its
       transport protocol, using a simple stop-and-wait acknowledgment
       system.  Since TFTP has an effective window of only one 512
       octet segment, it can provide good performance only over paths
       that have a small delay*bandwidth product.  The TFTP file
       interface is very simple, providing no access control or
       security.
       TFTP's most important application is bootstrapping a host over
       a local network, since it is simple and small enough to be
       easily implemented in EPROM [BOOT:1, BOOT:2].  Vendors are
       urged to support TFTP for booting.
    4.2.2  PROTOCOL WALK-THROUGH
       The TFTP specification [TFTP:1] is written in an open style,
       and does not fully specify many parts of the protocol.
       4.2.2.1  Transfer Modes: RFC-783, Page 3
          The transfer mode "mail" SHOULD NOT be supported.
       4.2.2.2  UDP Header: RFC-783, Page 17
          The Length field of a UDP header is incorrectly defined; it
          includes the UDP header length (8).
    4.2.3  SPECIFIC ISSUES
       4.2.3.1  Sorcerer's Apprentice Syndrome
          There is a serious bug, known as the "Sorcerer's Apprentice
          Syndrome," in the protocol specification.  While it does not
          cause incorrect operation of the transfer (the file will
          always be transferred correctly if the transfer completes),
          this bug may cause excessive retransmission, which may cause
          the transfer to time out.
          Implementations MUST contain the fix for this problem: the
          sender (i.e., the side originating the DATA packets) must
          never resend the current DATA packet on receipt of a

Internet Engineering Task Force [Page 44]

RFC1123 FILE TRANSFER – TFTP October 1989

          duplicate ACK.
          DISCUSSION:
               The bug is caused by the protocol rule that either
               side, on receiving an old duplicate datagram, may
               resend the current datagram.  If a packet is delayed in
               the network but later successfully delivered after
               either side has timed out and retransmitted a packet, a
               duplicate copy of the response may be generated.  If
               the other side responds to this duplicate with a
               duplicate of its own, then every datagram will be sent
               in duplicate for the remainder of the transfer (unless
               a datagram is lost, breaking the repetition).  Worse
               yet, since the delay is often caused by congestion,
               this duplicate transmission will usually causes more
               congestion, leading to more delayed packets, etc.
               The following example may help to clarify this problem.
                   TFTP A                  TFTP B
               (1)  Receive ACK X-1
                    Send DATA X
               (2)                          Receive DATA X
                                            Send ACK X
                      (ACK X is delayed in network,
                       and  A times out):
               (3)  Retransmit DATA X
               (4)                          Receive DATA X again
                                            Send ACK X again
               (5)  Receive (delayed) ACK X
                    Send DATA X+1
               (6)                          Receive DATA X+1
                                            Send ACK X+1
               (7)  Receive ACK X again
                    Send DATA X+1 again
               (8)                          Receive DATA X+1 again
                                            Send ACK X+1 again
               (9)  Receive ACK X+1
                    Send DATA X+2
               (10)                         Receive DATA X+2
                                            Send ACK X+3
               (11) Receive ACK X+1 again
                    Send DATA X+2 again
               (12)                         Receive DATA X+2 again
                                            Send ACK X+3 again

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               Notice that once the delayed ACK arrives, the protocol
               settles down to duplicate all further packets
               (sequences 5-8 and 9-12).  The problem is caused not by
               either side timing out, but by both sides
               retransmitting the current packet when they receive a
               duplicate.
               The fix is to break the retransmission loop, as
               indicated above.  This is analogous to the behavior of
               TCP.  It is then possible to remove the retransmission
               timer on the receiver, since the resent ACK will never
               cause any action; this is a useful simplification where
               TFTP is used in a bootstrap program.  It is OK to allow
               the timer to remain, and it may be helpful if the
               retransmitted ACK replaces one that was genuinely lost
               in the network.  The sender still requires a retransmit
               timer, of course.
       4.2.3.2  Timeout Algorithms
          A TFTP implementation MUST use an adaptive timeout.
          IMPLEMENTATION:
               TCP retransmission algorithms provide a useful base to
               work from.  At least an exponential backoff of
               retransmission timeout is necessary.
       4.2.3.3  Extensions
          A variety of non-standard extensions have been made to TFTP,
          including additional transfer modes and a secure operation
          mode (with passwords).  None of these have been
          standardized.
       4.2.3.4  Access Control
          A server TFTP implementation SHOULD include some
          configurable access control over what pathnames are allowed
          in TFTP operations.
       4.2.3.5  Broadcast Request
          A TFTP request directed to a broadcast address SHOULD be
          silently ignored.
          DISCUSSION:
               Due to the weak access control capability of TFTP,
               directed broadcasts of TFTP requests to random networks

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               could create a significant security hole.
    4.2.4  TFTP REQUIREMENTS SUMMARY
                                               |        | | | |S| |
                                               |        | | | |H| |F
                                               |        | | | |O|M|o
                                               |        | |S| |U|U|o
                                               |        | |H| |L|S|t
                                               |        |M|O| |D|T|n
                                               |        |U|U|M| | |o
                                               |        |S|L|A|N|N|t
                                               |        |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ————————————————-|——–|-|-|-|-|-|– Fix Sorcerer's Apprentice Syndrome |4.2.3.1 |x| | | | | Transfer modes: | | | | | | |

netascii                                       |RFC-783 |x| | | | |
octet                                          |RFC-783 |x| | | | |
mail                                           |4.2.2.1 | | | |x| |
extensions                                     |4.2.3.3 | | |x| | |

Use adaptive timeout |4.2.3.2 |x| | | | | Configurable access control |4.2.3.4 | |x| | | | Silently ignore broadcast request |4.2.3.5 | |x| | | | ————————————————-|——–|-|-|-|-|-|– ————————————————-|——–|-|-|-|-|-|–

Internet Engineering Task Force [Page 47]

RFC1123 MAIL – SMTP & RFC-822 October 1989

5. ELECTRONIC MAIL – SMTP and RFC-822

 5.1  INTRODUCTION
    In the TCP/IP protocol suite, electronic mail in a format
    specified in RFC-822 [SMTP:2] is transmitted using the Simple Mail
    Transfer Protocol (SMTP) defined in RFC-821 [SMTP:1].
    While SMTP has remained unchanged over the years, the Internet
    community has made several changes in the way SMTP is used.  In
    particular, the conversion to the Domain Name System (DNS) has
    caused changes in address formats and in mail routing.  In this
    section, we assume familiarity with the concepts and terminology
    of the DNS, whose requirements are given in Section 6.1.
    RFC-822 specifies the Internet standard format for electronic mail
    messages.  RFC-822 supercedes an older standard, RFC-733, that may
    still be in use in a few places, although it is obsolete.  The two
    formats are sometimes referred to simply by number ("822" and
    "733").
    RFC-822 is used in some non-Internet mail environments with
    different mail transfer protocols than SMTP, and SMTP has also
    been adapted for use in some non-Internet environments.  Note that
    this document presents the rules for the use of SMTP and RFC-822
    for the Internet environment only; other mail environments that
    use these protocols may be expected to have their own rules.
 5.2  PROTOCOL WALK-THROUGH
    This section covers both RFC-821 and RFC-822.
    The SMTP specification in RFC-821 is clear and contains numerous
    examples, so implementors should not find it difficult to
    understand.  This section simply updates or annotates portions of
    RFC-821 to conform with current usage.
    RFC-822 is a long and dense document, defining a rich syntax.
    Unfortunately, incomplete or defective implementations of RFC-822
    are common.  In fact, nearly all of the many formats of RFC-822
    are actually used, so an implementation generally needs to
    recognize and correctly interpret all of the RFC-822 syntax.
    5.2.1  The SMTP Model: RFC-821 Section 2
       DISCUSSION:
            Mail is sent by a series of request/response transactions
            between a client, the "sender-SMTP," and a server, the

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            "receiver-SMTP".  These transactions pass (1) the message
            proper, which is composed of header and body, and (2) SMTP
            source and destination addresses, referred to as the
            "envelope".
            The SMTP programs are analogous to Message Transfer Agents
            (MTAs) of X.400.  There will be another level of protocol
            software, closer to the end user, that is responsible for
            composing and analyzing RFC-822 message headers; this
            component is known as the "User Agent" in X.400, and we
            use that term in this document.  There is a clear logical
            distinction between the User Agent and the SMTP
            implementation, since they operate on different levels of
            protocol.  Note, however, that this distinction is may not
            be exactly reflected the structure of typical
            implementations of Internet mail.  Often there is a
            program known as the "mailer" that implements SMTP and
            also some of the User Agent functions; the rest of the
            User Agent functions are included in a user interface used
            for entering and reading mail.
            The SMTP envelope is constructed at the originating site,
            typically by the User Agent when the message is first
            queued for the Sender-SMTP program.  The envelope
            addresses may be derived from information in the message
            header, supplied by the user interface (e.g., to implement
            a bcc: request), or derived from local configuration
            information (e.g., expansion of a mailing list).  The SMTP
            envelope cannot in general be re-derived from the header
            at a later stage in message delivery, so the envelope is
            transmitted separately from the message itself using the
            MAIL and RCPT commands of SMTP.
            The text of RFC-821 suggests that mail is to be delivered
            to an individual user at a host.  With the advent of the
            domain system and of mail routing using mail-exchange (MX)
            resource records, implementors should now think of
            delivering mail to a user at a domain, which may or may
            not be a particular host.  This DOES NOT change the fact
            that SMTP is a host-to-host mail exchange protocol.
    5.2.2  Canonicalization: RFC-821 Section 3.1
       The domain names that a Sender-SMTP sends in MAIL and RCPT
       commands MUST have been  "canonicalized," i.e., they must be
       fully-qualified principal names or domain literals, not
       nicknames or domain abbreviations.  A canonicalized name either
       identifies a host directly or is an MX name; it cannot be a

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       CNAME.
    5.2.3  VRFY and EXPN Commands: RFC-821 Section 3.3
       A receiver-SMTP MUST implement VRFY and SHOULD implement EXPN
       (this requirement overrides RFC-821).  However, there MAY be
       configuration information to disable VRFY and EXPN in a
       particular installation; this might even allow EXPN to be
       disabled for selected lists.
       A new reply code is defined for the VRFY command:
            252 Cannot VRFY user (e.g., info is not local), but will
                take message for this user and attempt delivery.
       DISCUSSION:
            SMTP users and administrators make regular use of these
            commands for diagnosing mail delivery problems.  With the
            increasing use of multi-level mailing list expansion
            (sometimes more than two levels), EXPN has been
            increasingly important for diagnosing inadvertent mail
            loops.  On the other hand,  some feel that EXPN represents
            a significant privacy, and perhaps even a security,
            exposure.
    5.2.4  SEND, SOML, and SAML Commands: RFC-821 Section 3.4
       An SMTP MAY implement the commands to send a message to a
       user's terminal: SEND, SOML, and SAML.
       DISCUSSION:
            It has been suggested that the use of mail relaying
            through an MX record is inconsistent with the intent of
            SEND to deliver a message immediately and directly to a
            user's terminal.  However, an SMTP receiver that is unable
            to write directly to the user terminal can return a "251
            User Not Local" reply to the RCPT following a SEND, to
            inform the originator of possibly deferred delivery.
    5.2.5  HELO Command: RFC-821 Section 3.5
       The sender-SMTP MUST ensure that the <domain> parameter in a
       HELO command is a valid principal host domain name for the
       client host.  As a result, the receiver-SMTP will not have to
       perform MX resolution on this name in order to validate the
       HELO parameter.
       The HELO receiver MAY verify that the HELO parameter really

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RFC1123 MAIL – SMTP & RFC-822 October 1989

       corresponds to the IP address of the sender.  However, the
       receiver MUST NOT refuse to accept a message, even if the
       sender's HELO command fails verification.
       DISCUSSION:
            Verifying the HELO parameter requires a domain name lookup
            and may therefore take considerable time.  An alternative
            tool for tracking bogus mail sources is suggested below
            (see "DATA Command").
            Note also that the HELO argument is still required to have
            valid <domain> syntax, since it will appear in a Received:
            line; otherwise, a 501 error is to be sent.
       IMPLEMENTATION:
            When HELO parameter validation fails, a suggested
            procedure is to insert a note about the unknown
            authenticity of the sender into the message header (e.g.,
            in the "Received:"  line).
    5.2.6  Mail Relay: RFC-821 Section 3.6
       We distinguish three types of mail (store-and-) forwarding:
       (1)  A simple forwarder or "mail exchanger" forwards a message
            using private knowledge about the recipient; see section
            3.2 of RFC-821.
       (2)  An SMTP mail "relay" forwards a message within an SMTP
            mail environment as the result of an explicit source route
            (as defined in section 3.6 of RFC-821).  The SMTP relay
            function uses the "@...:" form of source route from RFC-
            822 (see Section 5.2.19 below).
       (3)  A mail "gateway" passes a message between different
            environments.  The rules for mail gateways are discussed
            below in Section 5.3.7.
       An Internet host that is forwarding a message but is not a
       gateway to a different mail environment (i.e., it falls under
       (1) or (2)) SHOULD NOT alter any existing header fields,
       although the host will add an appropriate Received: line as
       required in Section 5.2.8.
       A Sender-SMTP SHOULD NOT send a RCPT TO: command containing an
       explicit source route using the "@...:" address form.  Thus,
       the relay function defined in section  3.6 of RFC-821 should
       not be used.

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RFC1123 MAIL – SMTP & RFC-822 October 1989

       DISCUSSION:
            The intent is to discourage all source routing and to
            abolish explicit source routing for mail delivery within
            the Internet environment.  Source-routing is unnecessary;
            the simple target address "user@domain" should always
            suffice.  This is the result of an explicit architectural
            decision to use universal naming rather than source
            routing for mail.  Thus, SMTP provides end-to-end
            connectivity, and the DNS provides globally-unique,
            location-independent names.  MX records handle the major
            case where source routing might otherwise be needed.
       A receiver-SMTP MUST accept the explicit source route syntax in
       the envelope, but it MAY implement the relay function as
       defined in section 3.6 of RFC-821.  If it does not implement
       the relay function, it SHOULD attempt to deliver the message
       directly to the host to the right of the right-most "@" sign.
       DISCUSSION:
            For example, suppose a host that does not implement the
            relay function receives a message with the SMTP command:
            "RCPT TO:<@ALPHA,@BETA:joe@GAMMA>", where ALPHA, BETA, and
            GAMMA represent domain names.  Rather than immediately
            refusing the message with a 550 error reply as suggested
            on page 20 of RFC-821, the host should try to forward the
            message to GAMMA directly, using: "RCPT TO:<joe@GAMMA>".
            Since this host does not support relaying, it is not
            required to update the reverse path.
            Some have suggested that source routing may be needed
            occasionally for manually routing mail around failures;
            however, the reality and importance of this need is
            controversial.  The use of explicit SMTP mail relaying for
            this purpose is discouraged, and in fact it may not be
            successful, as many host systems do not support it.  Some
            have used the "%-hack" (see Section 5.2.16) for this
            purpose.
    5.2.7  RCPT Command: RFC-821 Section 4.1.1
       A host that supports a receiver-SMTP MUST support the reserved
       mailbox "Postmaster".
       The receiver-SMTP MAY verify RCPT parameters as they arrive;
       however, RCPT responses MUST NOT be delayed beyond a reasonable
       time (see Section 5.3.2).
       Therefore, a "250 OK" response to a RCPT does not necessarily

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RFC1123 MAIL – SMTP & RFC-822 October 1989

       imply that the delivery address(es) are valid.  Errors found
       after message acceptance will be reported by mailing a
       notification message to an appropriate address (see Section
       5.3.3).
       DISCUSSION:
            The set of conditions under which a RCPT parameter can be
            validated immediately is an engineering design choice.
            Reporting destination mailbox errors to the Sender-SMTP
            before mail is transferred is generally desirable to save
            time and network bandwidth, but this advantage is lost if
            RCPT verification is lengthy.
            For example, the receiver can verify immediately any
            simple local reference, such as a single locally-
            registered mailbox.  On the other hand, the "reasonable
            time" limitation generally implies deferring verification
            of a mailing list until after the message has been
            transferred and accepted, since verifying a large mailing
            list can take a very long time.  An implementation might
            or might not choose to defer validation of addresses that
            are non-local and therefore require a DNS lookup.  If a
            DNS lookup is performed but a soft domain system error
            (e.g., timeout) occurs, validity must be assumed.
    5.2.8  DATA Command: RFC-821 Section 4.1.1
       Every receiver-SMTP (not just one that "accepts a message for
       relaying or for final delivery" [SMTP:1]) MUST insert a
       "Received:" line at the beginning of a message.  In this line,
       called a "time stamp line" in RFC-821:
  • The FROM field SHOULD contain both (1) the name of the

source host as presented in the HELO command and (2) a

            domain literal containing the IP address of the source,
            determined from the TCP connection.
  • The ID field MAY contain an "@" as suggested in RFC-822,

but this is not required.

  • The FOR field MAY contain a list of <path> entries when

multiple RCPT commands have been given.

       An Internet mail program MUST NOT change a Received: line that
       was previously added to the message header.

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RFC1123 MAIL – SMTP & RFC-822 October 1989

       DISCUSSION:
            Including both the source host and the IP source address
            in the Received: line may provide enough information for
            tracking illicit mail sources and eliminate a need to
            explicitly verify the HELO parameter.
            Received: lines are primarily intended for humans tracing
            mail routes, primarily of diagnosis of faults.  See also
            the discussion under 5.3.7.
       When the receiver-SMTP makes "final delivery" of a message,
       then it MUST pass the MAIL FROM: address from the SMTP envelope
       with the message, for use if an error notification message must
       be sent later (see Section 5.3.3).  There is an analogous
       requirement when gatewaying from the Internet into a different
       mail environment; see Section 5.3.7.
       DISCUSSION:
            Note that the final reply to the DATA command depends only
            upon the successful transfer and storage of the message.
            Any problem with the destination address(es) must either
            (1) have been reported in an SMTP error reply to the RCPT
            command(s), or (2) be reported in a later error message
            mailed to the originator.
       IMPLEMENTATION:
            The MAIL FROM: information may be passed as a parameter or
            in a Return-Path: line inserted at the beginning of the
            message.
    5.2.9  Command Syntax: RFC-821 Section 4.1.2
       The syntax shown in RFC-821 for the MAIL FROM: command omits
       the case of an empty path:  "MAIL FROM: <>" (see RFC-821 Page
       15).  An empty reverse path MUST be supported.
    5.2.10  SMTP Replies:  RFC-821 Section 4.2
       A receiver-SMTP SHOULD send only the reply codes listed in
       section 4.2.2 of RFC-821 or in this document.  A receiver-SMTP
       SHOULD use the text shown in examples in RFC-821 whenever
       appropriate.
       A sender-SMTP MUST determine its actions only by the reply
       code, not by the text (except for 251 and 551 replies); any
       text, including no text at all, must be acceptable.  The space
       (blank) following the reply code is considered part of the
       text.  Whenever possible, a sender-SMTP SHOULD test only the

Internet Engineering Task Force [Page 54]

RFC1123 MAIL – SMTP & RFC-822 October 1989

       first digit of the reply code, as specified in Appendix E of
       RFC-821.
       DISCUSSION:
            Interoperability problems have arisen with SMTP systems
            using reply codes that are not listed explicitly in RFC-
            821 Section 4.3 but are legal according to the theory of
            reply codes explained in Appendix E.
    5.2.11  Transparency: RFC-821 Section 4.5.2
       Implementors MUST be sure that their mail systems always add
       and delete periods to ensure message transparency.
    5.2.12  WKS Use in MX Processing: RFC-974, p. 5
       RFC-974 [SMTP:3] recommended that the domain system be queried
       for WKS ("Well-Known Service") records, to verify that each
       proposed mail target does support SMTP.  Later experience has
       shown that WKS is not widely supported, so the WKS step in MX
       processing SHOULD NOT be used.
    The following are notes on RFC-822, organized by section of that
    document.
    5.2.13  RFC-822 Message Specification: RFC-822 Section 4
       The syntax shown for the Return-path line omits the possibility
       of a null return path, which is used to prevent looping of
       error notifications (see Section 5.3.3).  The complete syntax
       is:
           return = "Return-path"  ":" route-addr
                  / "Return-path"  ":" "<" ">"
       The set of optional header fields is hereby expanded to include
       the Content-Type field defined in RFC-1049 [SMTP:7].  This
       field "allows mail reading systems to automatically identify
       the type of a structured message body and to process it for
       display accordingly".  [SMTP:7]  A User Agent MAY support this
       field.
    5.2.14  RFC-822 Date and Time Specification: RFC-822 Section 5
       The syntax for the date is hereby changed to:
          date = 1*2DIGIT month 2*4DIGIT

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       All mail software SHOULD use 4-digit years in dates, to ease
       the transition to the next century.
       There is a strong trend towards the use of numeric timezone
       indicators, and implementations SHOULD use numeric timezones
       instead of timezone names.  However, all implementations MUST
       accept either notation.  If timezone names are used, they MUST
       be exactly as defined in RFC-822.
       The military time zones are specified incorrectly in RFC-822:
       they count the wrong way from UT (the signs are reversed).  As
       a result, military time zones in RFC-822 headers carry no
       information.
       Finally, note that there is a typo in the definition of "zone"
       in the syntax summary of appendix D; the correct definition
       occurs in Section 3 of RFC-822.
    5.2.15  RFC-822 Syntax Change: RFC-822 Section 6.1
       The syntactic definition of "mailbox" in RFC-822 is hereby
       changed to:
          mailbox =  addr-spec            ; simple address
                  / [phrase] route-addr   ; name & addr-spec
       That is, the phrase preceding a route address is now OPTIONAL.
       This change makes the following header field legal, for
       example:
           From: <craig@nnsc.nsf.net>
    5.2.16  RFC-822  Local-part: RFC-822 Section 6.2
       The basic mailbox address specification has the form: "local-
       part@domain".  Here "local-part", sometimes called the "left-
       hand side" of the address, is domain-dependent.
       A host that is forwarding the message but is not the
       destination host implied by the right-hand side "domain" MUST
       NOT interpret or modify the "local-part" of the address.
       When mail is to be gatewayed from the Internet mail environment
       into a foreign mail environment (see Section 5.3.7), routing
       information for that foreign environment MAY be embedded within
       the "local-part" of the address.  The gateway will then
       interpret this local part appropriately for the foreign mail
       environment.

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       DISCUSSION:
            Although source routes are discouraged within the Internet
            (see Section 5.2.6), there are non-Internet mail
            environments whose delivery mechanisms do depend upon
            source routes.  Source routes for extra-Internet
            environments can generally be buried in the "local-part"
            of the address (see Section 5.2.16) while mail traverses
            the Internet.  When the mail reaches the appropriate
            Internet mail gateway, the gateway will interpret the
            local-part and build the necessary address or route for
            the target mail environment.
            For example, an Internet host might send mail to:
            "a!b!c!user@gateway-domain".  The complex local part
            "a!b!c!user" would be uninterpreted within the Internet
            domain, but could be parsed and understood by the
            specified mail gateway.
            An embedded source route is sometimes encoded in the
            "local-part" using "%" as a right-binding routing
            operator.  For example, in:
               user%domain%relay3%relay2@relay1
            the "%" convention implies that the mail is to be routed
            from "relay1" through "relay2", "relay3", and finally to
            "user" at "domain".  This is commonly known as the "%-
            hack".  It is suggested that "%" have lower precedence
            than any other routing operator (e.g., "!") hidden in the
            local-part; for example, "a!b%c" would be interpreted as
            "(a!b)%c".
            Only the target host (in this case, "relay1") is permitted
            to analyze the local-part "user%domain%relay3%relay2".
    5.2.17  Domain Literals: RFC-822 Section 6.2.3
       A mailer MUST be able to accept and parse an Internet domain
       literal whose content ("dtext"; see RFC-822) is a dotted-
       decimal host address.  This satisfies the requirement of
       Section 2.1 for the case of mail.
       An SMTP MUST accept and recognize a domain literal for any of
       its own IP addresses.

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    5.2.18  Common Address Formatting Errors: RFC-822 Section 6.1
       Errors in formatting or parsing 822 addresses are unfortunately
       common.  This section mentions only the most common errors.  A
       User Agent MUST accept all valid RFC-822 address formats, and
       MUST NOT generate illegal address syntax.
       o    A common error is to leave out the semicolon after a group
            identifier.
       o    Some systems fail to fully-qualify domain names in
            messages they generate.  The right-hand side of an "@"
            sign in a header address field MUST be a fully-qualified
            domain name.
            For example, some systems fail to fully-qualify the From:
            address; this prevents a "reply" command in the user
            interface from automatically constructing a return
            address.
            DISCUSSION:
                 Although RFC-822 allows the local use of abbreviated
                 domain names within a domain, the application of
                 RFC-822 in Internet mail does not allow this.  The
                 intent is that an Internet host must not send an SMTP
                 message header containing an abbreviated domain name
                 in an address field.  This allows the address fields
                 of the header to be passed without alteration across
                 the Internet, as required in Section 5.2.6.
       o    Some systems mis-parse multiple-hop explicit source routes
            such as:
                @relay1,@relay2,@relay3:user@domain.
       o    Some systems over-qualify domain names by adding a
            trailing dot to some or all domain names in addresses or
            message-ids.  This violates RFC-822 syntax.
    5.2.19  Explicit Source Routes: RFC-822 Section 6.2.7
       Internet host software SHOULD NOT create an RFC-822 header
       containing an address with an explicit source route, but MUST
       accept such headers for compatibility with earlier systems.
       DISCUSSION:

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            In an understatement, RFC-822 says "The use of explicit
            source routing is discouraged".  Many hosts implemented
            RFC-822 source routes incorrectly, so the syntax cannot be
            used unambiguously in practice.  Many users feel the
            syntax is ugly.  Explicit source routes are not needed in
            the mail envelope for delivery; see Section 5.2.6.  For
            all these reasons, explicit source routes using the RFC-
            822 notations are not to be used in Internet mail headers.
            As stated in Section 5.2.16, it is necessary to allow an
            explicit source route to be buried in the local-part of an
            address, e.g., using the "%-hack", in order to allow mail
            to be gatewayed into another environment in which explicit
            source routing is necessary.  The vigilant will observe
            that there is no way for a User Agent to detect and
            prevent the use of such implicit source routing when the
            destination is within the Internet.  We can only
            discourage source routing of any kind within the Internet,
            as unnecessary and undesirable.
 5.3  SPECIFIC ISSUES
    5.3.1  SMTP Queueing Strategies
       The common structure of a host SMTP implementation includes
       user mailboxes, one or more areas for queueing messages in
       transit, and one or more daemon processes for sending and
       receiving mail.  The exact structure will vary depending on the
       needs of the users on the host and the number and size of
       mailing lists supported by the host.  We describe several
       optimizations that have proved helpful, particularly for
       mailers supporting high traffic levels.
       Any queueing strategy MUST include:
       o    Timeouts on all activities.  See Section 5.3.2.
       o    Never sending error messages in response to error
            messages.
       5.3.1.1 Sending Strategy
          The general model of a sender-SMTP is one or more processes
          that periodically attempt to transmit outgoing mail.  In a
          typical system, the program that composes a message has some
          method for requesting immediate attention for a new piece of
          outgoing mail, while mail that cannot be transmitted

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          immediately MUST be queued and periodically retried by the
          sender.  A mail queue entry will include not only the
          message itself but also the envelope information.
          The sender MUST delay retrying a particular destination
          after one attempt has failed.  In general, the retry
          interval SHOULD be at least 30 minutes; however, more
          sophisticated and variable strategies will be beneficial
          when the sender-SMTP can determine the reason for non-
          delivery.
          Retries continue until the message is transmitted or the
          sender gives up; the give-up time generally needs to be at
          least 4-5 days.  The parameters to the retry algorithm MUST
          be configurable.
          A sender SHOULD keep a list of hosts it cannot reach and
          corresponding timeouts, rather than just retrying queued
          mail items.
          DISCUSSION:
               Experience suggests that failures are typically
               transient (the target system has crashed), favoring a
               policy of two connection attempts in the first hour the
               message is in the queue, and then backing off to once
               every two or three hours.
               The sender-SMTP can shorten the queueing delay by
               cooperation with the receiver-SMTP.  In particular, if
               mail is received from a particular address, it is good
               evidence that any mail queued for that host can now be
               sent.
               The strategy may be further modified as a result of
               multiple addresses per host (see Section 5.3.4), to
               optimize delivery time vs. resource usage.
               A sender-SMTP may have a large queue of messages for
               each unavailable destination host, and if it retried
               all these messages in every retry cycle, there would be
               excessive Internet overhead and the daemon would be
               blocked for a long period.  Note that an SMTP can
               generally determine that a delivery attempt has failed
               only after a timeout of a minute or more; a one minute
               timeout per connection will result in a very large
               delay if it is repeated for dozens or even hundreds of
               queued messages.

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          When the same message is to be delivered to several users on
          the same host, only one copy of the message SHOULD be
          transmitted.  That is, the sender-SMTP should use the
          command sequence: RCPT, RCPT,... RCPT, DATA instead of the
          sequence: RCPT, DATA, RCPT, DATA,... RCPT, DATA.
          Implementation of this efficiency feature is strongly urged.
          Similarly, the sender-SMTP MAY support multiple concurrent
          outgoing mail transactions to achieve timely delivery.
          However, some limit SHOULD be imposed to protect the host
          from devoting all its resources to mail.
          The use of the different addresses of a multihomed host is
          discussed below.
       5.3.1.2  Receiving strategy
          The receiver-SMTP SHOULD attempt to keep a pending listen on
          the SMTP port at all times.  This will require the support
          of multiple incoming TCP connections for SMTP.  Some limit
          MAY be imposed.
          IMPLEMENTATION:
               When the receiver-SMTP receives mail from a particular
               host address, it could notify the sender-SMTP to retry
               any mail pending for that host address.
    5.3.2  Timeouts in SMTP
       There are two approaches to timeouts in the sender-SMTP:  (a)
       limit the time for each SMTP command separately, or (b) limit
       the time for the entire SMTP dialogue for a single mail
       message.  A sender-SMTP SHOULD use option (a), per-command
       timeouts.  Timeouts SHOULD be easily reconfigurable, preferably
       without recompiling the SMTP code.
       DISCUSSION:
            Timeouts are an essential feature of an SMTP
            implementation.  If the timeouts are too long (or worse,
            there are no timeouts), Internet communication failures or
            software bugs in receiver-SMTP programs can tie up SMTP
            processes indefinitely.  If the timeouts are too short,
            resources will be wasted with attempts that time out part
            way through message delivery.
            If option (b) is used, the timeout has to be very large,
            e.g., an hour, to allow time to expand very large mailing
            lists.  The timeout may also need to increase linearly

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            with the size of the message, to account for the time to
            transmit a very large message.  A large fixed timeout
            leads to two problems:  a failure can still tie up the
            sender for a very long time, and very large messages may
            still spuriously time out (which is a wasteful failure!).
            Using the recommended option (a), a timer is set for each
            SMTP command and for each buffer of the data transfer.
            The latter means that the overall timeout is inherently
            proportional to the size of the message.
       Based on extensive experience with busy mail-relay hosts, the
       minimum per-command timeout values SHOULD be as follows:
       o    Initial 220 Message: 5 minutes
            A Sender-SMTP process needs to distinguish between a
            failed TCP connection and a delay in receiving the initial
            220 greeting message.  Many receiver-SMTPs will accept a
            TCP connection but delay delivery of the 220 message until
            their system load will permit more mail to be processed.
       o    MAIL Command: 5 minutes
       o    RCPT Command: 5 minutes
            A longer timeout would be required if processing of
            mailing lists and aliases were not deferred until after
            the message was accepted.
       o    DATA Initiation: 2 minutes
            This is while awaiting the "354 Start Input" reply to a
            DATA command.
       o    Data Block: 3 minutes
            This is while awaiting the completion of each TCP SEND
            call transmitting a chunk of data.
       o    DATA Termination: 10 minutes.
            This is while awaiting the "250 OK" reply. When the
            receiver gets the final period terminating the message
            data, it typically performs processing to deliver the
            message to a user mailbox.  A spurious timeout at this
            point would be very wasteful, since the message has been

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            successfully sent.
       A receiver-SMTP SHOULD have a timeout of at least 5 minutes
       while it is awaiting the next command from the sender.
    5.3.3  Reliable Mail Receipt
       When the receiver-SMTP accepts a piece of mail (by sending a
       "250 OK" message in response to DATA), it is accepting
       responsibility for delivering or relaying the message.  It must
       take this responsibility seriously, i.e., it MUST NOT lose the
       message for frivolous reasons, e.g., because the host later
       crashes or because of a predictable resource shortage.
       If there is a delivery failure after acceptance of a message,
       the receiver-SMTP MUST formulate and mail a notification
       message.  This notification MUST be sent using a null ("<>")
       reverse path in the envelope; see Section 3.6 of RFC-821.  The
       recipient of this notification SHOULD be the address from the
       envelope return path (or the Return-Path: line).  However, if
       this address is null ("<>"),  the receiver-SMTP MUST NOT send a
       notification.  If the address is an explicit source route, it
       SHOULD be stripped down to its final hop.
       DISCUSSION:
            For example, suppose that an error notification must be
            sent for a message that arrived with:
            "MAIL FROM:<@a,@b:user@d>".  The notification message
            should be sent to: "RCPT TO:<user@d>".
            Some delivery failures after the message is accepted by
            SMTP will be unavoidable.  For example, it may be
            impossible for the receiver-SMTP to validate all the
            delivery addresses in RCPT command(s) due to a "soft"
            domain system error or because the target is a mailing
            list (see earlier discussion of RCPT).
       To avoid receiving duplicate messages as the result of
       timeouts, a receiver-SMTP MUST seek to minimize the time
       required to respond to the final "." that ends a message
       transfer.  See RFC-1047 [SMTP:4] for a discussion of this
       problem.
    5.3.4  Reliable Mail Transmission
       To transmit a message, a sender-SMTP determines the IP address
       of the target host from the destination address in the
       envelope.  Specifically, it maps the string to the right of the

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       "@" sign into an IP address.  This mapping or the transfer
       itself may fail with a soft error, in which case the sender-
       SMTP will requeue the outgoing mail for a later retry, as
       required in Section 5.3.1.1.
       When it succeeds, the mapping can result in a list of
       alternative delivery addresses rather than a single address,
       because of (a) multiple MX records, (b) multihoming, or both.
       To provide reliable mail transmission, the sender-SMTP MUST be
       able to try (and retry) each of the addresses in this list in
       order, until a delivery attempt succeeds.  However, there MAY
       also be a configurable limit on the number of alternate
       addresses that can be tried.  In any case, a host SHOULD try at
       least two addresses.
       The following information is to be used to rank the host
       addresses:
       (1)  Multiple MX Records -- these contain a preference
            indication that should be used in sorting.  If there are
            multiple destinations with the same preference and there
            is no clear reason to favor one (e.g., by address
            preference), then the sender-SMTP SHOULD pick one at
            random to spread the load across multiple mail exchanges
            for a specific organization; note that this is a
            refinement of the procedure in [DNS:3].
       (2)  Multihomed host -- The destination host (perhaps taken
            from the preferred MX record) may be multihomed, in which
            case the domain name resolver will return a list of
            alternative IP addresses.  It is the responsibility of the
            domain name resolver interface (see Section 6.1.3.4 below)
            to have ordered this list by decreasing preference, and
            SMTP MUST try them in the order presented.
       DISCUSSION:
            Although the capability to try multiple alternative
            addresses is required, there may be circumstances where
            specific installations want to limit or disable the use of
            alternative addresses.  The question of whether a sender
            should attempt retries using the different addresses of a
            multihomed host has been controversial.  The main argument
            for using the multiple addresses is that it maximizes the
            probability of timely delivery, and indeed sometimes the
            probability of any delivery; the counter argument is that
            it may result in unnecessary resource use.
            Note that resource use is also strongly determined by the

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            sending strategy discussed in Section 5.3.1.
    5.3.5  Domain Name Support
       SMTP implementations MUST use the mechanism defined in Section
       6.1 for mapping between domain names and IP addresses.  This
       means that every Internet SMTP MUST include support for the
       Internet DNS.
       In particular, a sender-SMTP MUST support the MX record scheme
       [SMTP:3].  See also Section 7.4 of [DNS:2] for information on
       domain name support for SMTP.
    5.3.6  Mailing Lists and Aliases
       An SMTP-capable host SHOULD support both the alias and the list
       form of address expansion for multiple delivery.  When a
       message is delivered or forwarded to each address of an
       expanded list form, the return address in the envelope
       ("MAIL FROM:") MUST be changed to be the address of a person
       who administers the list, but the message header MUST be left
       unchanged; in particular, the "From" field of the message is
       unaffected.
       DISCUSSION:
            An important mail facility is a mechanism for multi-
            destination delivery of a single message, by transforming
            or "expanding" a pseudo-mailbox address into a list of
            destination mailbox addresses.  When a message is sent to
            such a pseudo-mailbox (sometimes called an "exploder"),
            copies are forwarded or redistributed to each mailbox in
            the expanded list.  We classify such a pseudo-mailbox as
            an "alias" or a "list", depending upon the expansion
            rules:
            (a)  Alias
                 To expand an alias, the recipient mailer simply
                 replaces the pseudo-mailbox address in the envelope
                 with each of the expanded addresses in turn; the rest
                 of the envelope and the message body are left
                 unchanged.  The message is then delivered or
                 forwarded to each expanded address.
            (b)  List
                 A mailing list may be said to operate by
                 "redistribution" rather than by "forwarding".  To

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                 expand a list, the recipient mailer replaces the
                 pseudo-mailbox address in the envelope with each of
                 the expanded addresses in turn. The return address in
                 the envelope is changed so that all error messages
                 generated by the final deliveries will be returned to
                 a list administrator, not to the message originator,
                 who generally has no control over the contents of the
                 list and will typically find error messages annoying.
    5.3.7  Mail Gatewaying
       Gatewaying mail between different mail environments, i.e.,
       different mail formats and protocols, is complex and does not
       easily yield to standardization.  See for example [SMTP:5a],
       [SMTP:5b].  However, some general requirements may be given for
       a gateway between the Internet and another mail environment.
       (A)  Header fields MAY be rewritten when necessary as messages
            are gatewayed across mail environment boundaries.
            DISCUSSION:
                 This may involve interpreting the local-part of the
                 destination address, as suggested in Section 5.2.16.
                 The other mail systems gatewayed to the Internet
                 generally use a subset of RFC-822 headers, but some
                 of them do not have an equivalent to the SMTP
                 envelope.  Therefore, when a message leaves the
                 Internet environment, it may be necessary to fold the
                 SMTP envelope information into the message header.  A
                 possible solution would be to create new header
                 fields to carry the envelope information (e.g., "X-
                 SMTP-MAIL:" and "X-SMTP-RCPT:"); however, this would
                 require changes in mail programs in the foreign
                 environment.
       (B)  When forwarding a message into or out of the Internet
            environment, a gateway MUST prepend a Received: line, but
            it MUST NOT alter in any way a Received: line that is
            already in the header.
            DISCUSSION:
                 This requirement is a subset of the general
                 "Received:" line requirement of Section 5.2.8; it is
                 restated here for emphasis.
                 Received: fields of messages originating from other

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                 environments may not conform exactly to RFC822.
                 However, the most important use of Received: lines is
                 for debugging mail faults, and this debugging can be
                 severely hampered by well-meaning gateways that try
                 to "fix" a Received: line.
                 The gateway is strongly encouraged to indicate the
                 environment and protocol in the "via" clauses of
                 Received field(s) that it supplies.
       (C)  From the Internet side, the gateway SHOULD accept all
            valid address formats in SMTP commands and in RFC-822
            headers, and all valid RFC-822 messages.  Although a
            gateway must accept an RFC-822 explicit source route
            ("@...:" format) in either the RFC-822 header or in the
            envelope, it MAY or may not act on the source route; see
            Sections 5.2.6 and 5.2.19.
            DISCUSSION:
                 It is often tempting to restrict the range of
                 addresses accepted at the mail gateway to simplify
                 the translation into addresses for the remote
                 environment.  This practice is based on the
                 assumption that mail users have control over the
                 addresses their mailers send to the mail gateway.  In
                 practice, however, users have little control over the
                 addresses that are finally sent; their mailers are
                 free to change addresses into any legal RFC-822
                 format.
       (D)  The gateway MUST ensure that all header fields of a
            message that it forwards into the Internet meet the
            requirements for Internet mail.  In particular, all
            addresses in "From:", "To:", "Cc:", etc., fields must be
            transformed (if necessary) to satisfy RFC-822 syntax, and
            they must be effective and useful for sending replies.
       (E)  The translation algorithm used to convert mail from the
            Internet protocols to another environment's protocol
            SHOULD try to ensure that error messages from the foreign
            mail environment are delivered to the return path from the
            SMTP envelope, not to the sender listed in the "From:"
            field of the RFC-822 message.
            DISCUSSION:
                 Internet mail lists usually place the address of the
                 mail list maintainer in the envelope but leave the

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                 original message header intact (with the "From:"
                 field containing the original sender).  This yields
                 the behavior the average recipient expects: a reply
                 to the header gets sent to the original sender, not
                 to a mail list maintainer; however, errors get sent
                 to the maintainer (who can fix the problem) and not
                 the sender (who probably cannot).
       (F)  Similarly, when forwarding a message from another
            environment into the Internet, the gateway SHOULD set the
            envelope return path in accordance with an error message
            return address, if any, supplied by the foreign
            environment.
    5.3.8  Maximum Message Size
       Mailer software MUST be able to send and receive messages of at
       least 64K bytes in length (including header), and a much larger
       maximum size is highly desirable.
       DISCUSSION:
            Although SMTP does not define the maximum size of a
            message, many systems impose implementation limits.
            The current de facto minimum limit in the Internet is 64K
            bytes.  However, electronic mail is used for a variety of
            purposes that create much larger messages.  For example,
            mail is often used instead of FTP for transmitting ASCII
            files, and in particular to transmit entire documents.  As
            a result, messages can be 1 megabyte or even larger.  We
            note that the present document together with its lower-
            layer companion contains 0.5 megabytes.

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 5.4  SMTP REQUIREMENTS SUMMARY
                                             |          | | | |S| |
                                             |          | | | |H| |F
                                             |          | | | |O|M|o
                                             |          | |S| |U|U|o
                                             |          | |H| |L|S|t
                                             |          |M|O| |D|T|n
                                             |          |U|U|M| | |o
                                             |          |S|L|A|N|N|t
                                             |          |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ———————————————–|———-|-|-|-|-|-|–

                                             |          | | | | | |

RECEIVER-SMTP: | | | | | | |

Implement VRFY                               |5.2.3     |x| | | | |
Implement EXPN                               |5.2.3     | |x| | | |
  EXPN, VRFY configurable                    |5.2.3     | | |x| | |
Implement SEND, SOML, SAML                   |5.2.4     | | |x| | |
Verify HELO parameter                        |5.2.5     | | |x| | |
  Refuse message with bad HELO               |5.2.5     | | | | |x|
Accept explicit src-route syntax in env.     |5.2.6     |x| | | | |
Support "postmaster"                         |5.2.7     |x| | | | |
Process RCPT when received (except lists)    |5.2.7     | | |x| | |
    Long delay of RCPT responses             |5.2.7     | | | | |x|
                                             |          | | | | | |
Add Received: line                           |5.2.8     |x| | | | |
    Received: line include domain literal    |5.2.8     | |x| | | |
Change previous Received: line               |5.2.8     | | | | |x|
Pass Return-Path info (final deliv/gwy)      |5.2.8     |x| | | | |
Support empty reverse path                   |5.2.9     |x| | | | |
Send only official reply codes               |5.2.10    | |x| | | |
Send text from RFC-821 when appropriate      |5.2.10    | |x| | | |
Delete "." for transparency                  |5.2.11    |x| | | | |
Accept and recognize self domain literal(s)  |5.2.17    |x| | | | |
                                             |          | | | | | |
Error message about error message            |5.3.1     | | | | |x|
Keep pending listen on SMTP port             |5.3.1.2   | |x| | | |
Provide limit on recv concurrency            |5.3.1.2   | | |x| | |
Wait at least 5 mins for next sender cmd     |5.3.2     | |x| | | |
Avoidable delivery failure after "250 OK"    |5.3.3     | | | | |x|
Send error notification msg after accept     |5.3.3     |x| | | | |
  Send using null return path                |5.3.3     |x| | | | |
  Send to envelope return path               |5.3.3     | |x| | | |
  Send to null address                       |5.3.3     | | | | |x|
  Strip off explicit src route               |5.3.3     | |x| | | |
Minimize acceptance delay (RFC-1047)         |5.3.3     |x| | | | |

———————————————–|———-|-|-|-|-|-|–

Internet Engineering Task Force [Page 69]

RFC1123 MAIL – SMTP & RFC-822 October 1989

                                             |          | | | | | |

SENDER-SMTP: | | | | | | |

Canonicalized domain names in MAIL, RCPT     |5.2.2     |x| | | | |
Implement SEND, SOML, SAML                   |5.2.4     | | |x| | |
Send valid principal host name in HELO       |5.2.5     |x| | | | |
Send explicit source route in RCPT TO:       |5.2.6     | | | |x| |
Use only reply code to determine action      |5.2.10    |x| | | | |
Use only high digit of reply code when poss. |5.2.10    | |x| | | |
Add "." for transparency                     |5.2.11    |x| | | | |
                                             |          | | | | | |
Retry messages after soft failure            |5.3.1.1   |x| | | | |
  Delay before retry                         |5.3.1.1   |x| | | | |
  Configurable retry parameters              |5.3.1.1   |x| | | | |
  Retry once per each queued dest host       |5.3.1.1   | |x| | | |
Multiple RCPT's for same DATA                |5.3.1.1   | |x| | | |
Support multiple concurrent transactions     |5.3.1.1   | | |x| | |
  Provide limit on concurrency               |5.3.1.1   | |x| | | |
                                             |          | | | | | |
Timeouts on all activities                   |5.3.1     |x| | | | |
  Per-command timeouts                       |5.3.2     | |x| | | |
  Timeouts easily reconfigurable             |5.3.2     | |x| | | |
  Recommended times                          |5.3.2     | |x| | | |
Try alternate addr's in order                |5.3.4     |x| | | | |
  Configurable limit on alternate tries      |5.3.4     | | |x| | |
  Try at least two alternates                |5.3.4     | |x| | | |
Load-split across equal MX alternates        |5.3.4     | |x| | | |
Use the Domain Name System                   |5.3.5     |x| | | | |
  Support MX records                         |5.3.5     |x| | | | |
  Use WKS records in MX processing           |5.2.12    | | | |x| |

———————————————–|———-|-|-|-|-|-|–

                                             |          | | | | | |

MAIL FORWARDING: | | | | | | |

Alter existing header field(s)               |5.2.6     | | | |x| |
Implement relay function: 821/section 3.6    |5.2.6     | | |x| | |
  If not, deliver to RHS domain              |5.2.6     | |x| | | |
Interpret 'local-part' of addr               |5.2.16    | | | | |x|
                                             |          | | | | | |

MAILING LISTS AND ALIASES | | | | | | |

Support both                                 |5.3.6     | |x| | | |
Report mail list error to local admin.       |5.3.6     |x| | | | |
                                             |          | | | | | |

MAIL GATEWAYS: | | | | | | |

Embed foreign mail route in local-part       |5.2.16    | | |x| | |
Rewrite header fields when necessary         |5.3.7     | | |x| | |
Prepend Received: line                       |5.3.7     |x| | | | |
Change existing Received: line               |5.3.7     | | | | |x|
Accept full RFC-822 on Internet side         |5.3.7     | |x| | | |
Act on RFC-822 explicit source route         |5.3.7     | | |x| | |

Internet Engineering Task Force [Page 70]

RFC1123 MAIL – SMTP & RFC-822 October 1989

Send only valid RFC-822 on Internet side     |5.3.7     |x| | | | |
Deliver error msgs to envelope addr          |5.3.7     | |x| | | |
Set env return path from err return addr     |5.3.7     | |x| | | |
                                             |          | | | | | |

USER AGENT – RFC-822 | | | | | | |

Allow user to enter <route> address          |5.2.6     | | | |x| |
Support RFC-1049 Content Type field          |5.2.13    | | |x| | |
Use 4-digit years                            |5.2.14    | |x| | | |
Generate numeric timezones                   |5.2.14    | |x| | | |
Accept all timezones                         |5.2.14    |x| | | | |
Use non-num timezones from RFC-822           |5.2.14    |x| | | | |
Omit phrase before route-addr                |5.2.15    | | |x| | |
Accept and parse dot.dec. domain literals    |5.2.17    |x| | | | |
Accept all RFC-822 address formats           |5.2.18    |x| | | | |
Generate invalid RFC-822 address format      |5.2.18    | | | | |x|
Fully-qualified domain names in header       |5.2.18    |x| | | | |
Create explicit src route in header          |5.2.19    | | | |x| |
Accept explicit src route in header          |5.2.19    |x| | | | |
                                             |          | | | | | |

Send/recv at least 64KB messages |5.3.8 |x| | | | |

Internet Engineering Task Force [Page 71]

RFC1123 SUPPORT SERVICES – DOMAINS October 1989

6. SUPPORT SERVICES

 6.1 DOMAIN NAME TRANSLATION
    6.1.1 INTRODUCTION
       Every host MUST implement a resolver for the Domain Name System
       (DNS), and it MUST implement a mechanism using this DNS
       resolver to convert host names to IP addresses and vice-versa
       [DNS:1, DNS:2].
       In addition to the DNS, a host MAY also implement a host name
       translation mechanism that searches a local Internet host
       table.  See Section 6.1.3.8 for more information on this
       option.
       DISCUSSION:
            Internet host name translation was originally performed by
            searching local copies of a table of all hosts.  This
            table became too large to update and distribute in a
            timely manner and too large to fit into many hosts, so the
            DNS was invented.
            The DNS creates a distributed database used primarily for
            the translation between host names and host addresses.
            Implementation of DNS software is required.  The DNS
            consists of two logically distinct parts: name servers and
            resolvers (although implementations often combine these
            two logical parts in the interest of efficiency) [DNS:2].
            Domain name servers store authoritative data about certain
            sections of the database and answer queries about the
            data.  Domain resolvers query domain name servers for data
            on behalf of user processes.  Every host therefore needs a
            DNS resolver; some host machines will also need to run
            domain name servers.  Since no name server has complete
            information, in general it is necessary to obtain
            information from more than one name server to resolve a
            query.
    6.1.2  PROTOCOL WALK-THROUGH
       An implementor must study references [DNS:1] and [DNS:2]
       carefully.  They provide a thorough description of the theory,
       protocol, and implementation of the domain name system, and
       reflect several years of experience.

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       6.1.2.1  Resource Records with Zero TTL: RFC-1035 Section 3.2.1
          All DNS name servers and resolvers MUST properly handle RRs
          with a zero TTL: return the RR to the client but do not
          cache it.
          DISCUSSION:
               Zero TTL values are interpreted to mean that the RR can
               only be used for the transaction in progress, and
               should not be cached; they are useful for extremely
               volatile data.
       6.1.2.2  QCLASS Values: RFC-1035 Section 3.2.5
          A query with "QCLASS=*" SHOULD NOT be used unless the
          requestor is seeking data from more than one class.  In
          particular, if the requestor is only interested in Internet
          data types, QCLASS=IN MUST be used.
       6.1.2.3  Unused Fields: RFC-1035 Section 4.1.1
          Unused fields in a query or response message MUST be zero.
       6.1.2.4  Compression: RFC-1035 Section 4.1.4
          Name servers MUST use compression in responses.
          DISCUSSION:
               Compression is essential to avoid overflowing UDP
               datagrams; see Section 6.1.3.2.
       6.1.2.5  Misusing Configuration Info: RFC-1035 Section 6.1.2
          Recursive name servers and full-service resolvers generally
          have some configuration information containing hints about
          the location of root or local name servers.  An
          implementation MUST NOT include any of these hints in a
          response.
          DISCUSSION:
               Many implementors have found it convenient to store
               these hints as if they were cached data, but some
               neglected to ensure that this "cached data" was not
               included in responses.  This has caused serious
               problems in the Internet when the hints were obsolete
               or incorrect.

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

    6.1.3  SPECIFIC ISSUES
       6.1.3.1  Resolver Implementation
          A name resolver SHOULD be able to multiplex concurrent
          requests if the host supports concurrent processes.
          In implementing a DNS resolver, one of two different models
          MAY optionally be chosen: a full-service resolver, or a stub
          resolver.
          (A)  Full-Service Resolver
               A full-service resolver is a complete implementation of
               the resolver service, and is capable of dealing with
               communication failures, failure of individual name
               servers, location of the proper name server for a given
               name, etc.  It must satisfy the following requirements:
               o    The resolver MUST implement a local caching
                    function to avoid repeated remote access for
                    identical requests, and MUST time out information
                    in the cache.
               o    The resolver SHOULD be configurable with start-up
                    information pointing to multiple root name servers
                    and multiple name servers for the local domain.
                    This insures that the resolver will be able to
                    access the whole name space in normal cases, and
                    will be able to access local domain information
                    should the local network become disconnected from
                    the rest of the Internet.
          (B)  Stub Resolver
               A "stub resolver" relies on the services of a recursive
               name server on the connected network or a "nearby"
               network.  This scheme allows the host to pass on the
               burden of the resolver function to a name server on
               another host.  This model is often essential for less
               capable hosts, such as PCs, and is also recommended
               when the host is one of several workstations on a local
               network, because it allows all of the workstations to
               share the cache of the recursive name server and hence
               reduce the number of domain requests exported by the
               local network.

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

               At a minimum, the stub resolver MUST be capable of
               directing its requests to redundant recursive name
               servers.  Note that recursive name servers are allowed
               to restrict the sources of requests that they will
               honor, so the host administrator must verify that the
               service will be provided.  Stub resolvers MAY implement
               caching if they choose, but if so, MUST timeout cached
               information.
       6.1.3.2  Transport Protocols
          DNS resolvers and recursive servers MUST support UDP, and
          SHOULD support TCP, for sending (non-zone-transfer) queries.
          Specifically, a DNS resolver or server that is sending a
          non-zone-transfer query MUST send a UDP query first.  If the
          Answer section of the response is truncated and if the
          requester supports TCP, it SHOULD try the query again using
          TCP.
          DNS servers MUST be able to service UDP queries and SHOULD
          be able to service TCP queries.  A name server MAY limit the
          resources it devotes to TCP queries, but it SHOULD NOT
          refuse to service a TCP query just because it would have
          succeeded with UDP.
          Truncated responses MUST NOT be saved (cached) and later
          used in such a way that the fact that they are truncated is
          lost.
          DISCUSSION:
               UDP is preferred over TCP for queries because UDP
               queries have much lower overhead, both in packet count
               and in connection state.  The use of UDP is essential
               for heavily-loaded servers, especially the root
               servers.  UDP also offers additional robustness, since
               a resolver can attempt several UDP queries to different
               servers for the cost of a single TCP query.
               It is possible for a DNS response to be truncated,
               although this is a very rare occurrence in the present
               Internet DNS.  Practically speaking, truncation cannot
               be predicted, since it is data-dependent.  The
               dependencies include the number of RRs in the answer,
               the size of each RR, and the savings in space realized
               by the name compression algorithm.  As a rule of thumb,
               truncation in NS and MX lists should not occur for
               answers containing 15 or fewer RRs.

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               Whether it is possible to use a truncated answer
               depends on the application.  A mailer must not use a
               truncated MX response, since this could lead to mail
               loops.
               Responsible practices can make UDP suffice in the vast
               majority of cases.  Name servers must use compression
               in responses.  Resolvers must differentiate truncation
               of the Additional section of a response (which only
               loses extra information) from truncation of the Answer
               section (which for MX records renders the response
               unusable by mailers).  Database administrators should
               list only a reasonable number of primary names in lists
               of name servers, MX alternatives, etc.
               However, it is also clear that some new DNS record
               types defined in the future will contain information
               exceeding the 512 byte limit that applies to UDP, and
               hence will require TCP.  Thus, resolvers and name
               servers should implement TCP services as a backup to
               UDP today, with the knowledge that they will require
               the TCP service in the future.
          By private agreement, name servers and resolvers MAY arrange
          to use TCP for all traffic between themselves.  TCP MUST be
          used for zone transfers.
          A DNS server MUST have sufficient internal concurrency that
          it can continue to process UDP queries while awaiting a
          response or performing a zone transfer on an open TCP
          connection [DNS:2].
          A server MAY support a UDP query that is delivered using an
          IP broadcast or multicast address.  However, the Recursion
          Desired bit MUST NOT be set in a query that is multicast,
          and MUST be ignored by name servers receiving queries via a
          broadcast or multicast address.  A host that sends broadcast
          or multicast DNS queries SHOULD send them only as occasional
          probes, caching the IP address(es) it obtains from the
          response(s) so it can normally send unicast queries.
          DISCUSSION:
               Broadcast or (especially) IP multicast can provide a
               way to locate nearby name servers without knowing their
               IP addresses in advance.  However, general broadcasting
               of recursive queries can result in excessive and
               unnecessary load on both network and servers.

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

       6.1.3.3  Efficient Resource Usage
          The following requirements on servers and resolvers are very
          important to the health of the Internet as a whole,
          particularly when DNS services are invoked repeatedly by
          higher level automatic servers, such as mailers.
          (1)  The resolver MUST implement retransmission controls to
               insure that it does not waste communication bandwidth,
               and MUST impose finite bounds on the resources consumed
               to respond to a single request.  See [DNS:2] pages 43-
               44 for specific recommendations.
          (2)  After a query has been retransmitted several times
               without a response, an implementation MUST give up and
               return a soft error to the application.
          (3)  All DNS name servers and resolvers SHOULD cache
               temporary failures, with a timeout period of the order
               of minutes.
               DISCUSSION:
                    This will prevent applications that immediately
                    retry soft failures (in violation of Section 2.2
                    of this document) from generating excessive DNS
                    traffic.
          (4)  All DNS name servers and resolvers SHOULD cache
               negative responses that indicate the specified name, or
               data of the specified type, does not exist, as
               described in [DNS:2].
          (5)  When a DNS server or resolver retries a UDP query, the
               retry interval SHOULD be constrained by an exponential
               backoff algorithm, and SHOULD also have upper and lower
               bounds.
               IMPLEMENTATION:
                    A measured RTT and variance (if available) should
                    be used to calculate an initial retransmission
                    interval.  If this information is not available, a
                    default of no less than 5 seconds should be used.
                    Implementations may limit the retransmission
                    interval, but this limit must exceed twice the
                    Internet maximum segment lifetime plus service
                    delay at the name server.
          (6)  When a resolver or server receives a Source Quench for

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

               a query it has issued, it SHOULD take steps to reduce
               the rate of querying that server in the near future.  A
               server MAY ignore a Source Quench that it receives as
               the result of sending a response datagram.
               IMPLEMENTATION:
                    One recommended action to reduce the rate is to
                    send the next query attempt to an alternate
                    server, if there is one available.  Another is to
                    backoff the retry interval for the same server.
       6.1.3.4  Multihomed Hosts
          When the host name-to-address function encounters a host
          with multiple addresses, it SHOULD rank or sort the
          addresses using knowledge of the immediately connected
          network number(s) and any other applicable performance or
          history information.
          DISCUSSION:
               The different addresses of a multihomed host generally
               imply different Internet paths, and some paths may be
               preferable to others in performance, reliability, or
               administrative restrictions.  There is no general way
               for the domain system to determine the best path.  A
               recommended approach is to base this decision on local
               configuration information set by the system
               administrator.
          IMPLEMENTATION:
               The following scheme has been used successfully:
               (a)  Incorporate into the host configuration data a
                    Network-Preference List, that is simply a list of
                    networks in preferred order.  This list may be
                    empty if there is no preference.
               (b)  When a host name is mapped into a list of IP
                    addresses, these addresses should be sorted by
                    network number, into the same order as the
                    corresponding networks in the Network-Preference
                    List.  IP addresses whose networks do not appear
                    in the Network-Preference List should be placed at
                    the end of the list.

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

       6.1.3.5  Extensibility
          DNS software MUST support all well-known, class-independent
          formats [DNS:2], and SHOULD be written to minimize the
          trauma associated with the introduction of new well-known
          types and local experimentation with non-standard types.
          DISCUSSION:
               The data types and classes used by the DNS are
               extensible, and thus new types will be added and old
               types deleted or redefined.  Introduction of new data
               types ought to be dependent only upon the rules for
               compression of domain names inside DNS messages, and
               the translation between printable (i.e., master file)
               and internal formats for Resource Records (RRs).
               Compression relies on knowledge of the format of data
               inside a particular RR.  Hence compression must only be
               used for the contents of well-known, class-independent
               RRs, and must never be used for class-specific RRs or
               RR types that are not well-known.  The owner name of an
               RR is always eligible for compression.
               A name server may acquire, via zone transfer, RRs that
               the server doesn't know how to convert to printable
               format.  A resolver can receive similar information as
               the result of queries.  For proper operation, this data
               must be preserved, and hence the implication is that
               DNS software cannot use textual formats for internal
               storage.
               The DNS defines domain name syntax very generally -- a
               string of labels each containing up to 63 8-bit octets,
               separated by dots, and with a maximum total of 255
               octets.  Particular applications of the DNS are
               permitted to further constrain the syntax of the domain
               names they use, although the DNS deployment has led to
               some applications allowing more general names.  In
               particular, Section 2.1 of this document liberalizes
               slightly the syntax of a legal Internet host name that
               was defined in RFC-952 [DNS:4].
       6.1.3.6  Status of RR Types
          Name servers MUST be able to load all RR types except MD and
          MF from configuration files.  The MD and MF types are
          obsolete and MUST NOT be implemented; in particular, name
          servers MUST NOT load these types from configuration files.

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RFC1123 SUPPORT SERVICES – DOMAINS October 1989

          DISCUSSION:
               The RR types MB, MG, MR, NULL, MINFO and RP are
               considered experimental, and applications that use the
               DNS cannot expect these RR types to be supported by
               most domains.  Furthermore these types are subject to
               redefinition.
               The TXT and WKS RR types have not been widely used by
               Internet sites; as a result, an application cannot rely
               on the the existence of a TXT or WKS RR in most
               domains.
       6.1.3.7  Robustness
          DNS software may need to operate in environments where the
          root servers or other servers are unavailable due to network
          connectivity or other problems.  In this situation, DNS name
          servers and resolvers MUST continue to provide service for
          the reachable part of the name space, while giving temporary
          failures for the rest.
          DISCUSSION:
               Although the DNS is meant to be used primarily in the
               connected Internet, it should be possible to use the
               system in networks which are unconnected to the
               Internet.  Hence implementations must not depend on
               access to root servers before providing service for
               local names.
       6.1.3.8  Local Host Table
          DISCUSSION:
               A host may use a local host table as a backup or
               supplement to the DNS.  This raises the question of
               which takes precedence, the DNS or the host table; the
               most flexible approach would make this a configuration
               option.
               Typically, the contents of such a supplementary host
               table will be determined locally by the site.  However,
               a publically-available table of Internet hosts is
               maintained by the DDN Network Information Center (DDN
               NIC), with a format documented in [DNS:4].  This table
               can be retrieved from the DDN NIC using a protocol
               described in [DNS:5].  It must be noted that this table
               contains only a small fraction of all Internet hosts.
               Hosts using this protocol to retrieve the DDN NIC host
               table should use the VERSION command to check if the

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               table has changed before requesting the entire table
               with the ALL command.  The VERSION identifier should be
               treated as an arbitrary string and tested only for
               equality; no numerical sequence may be assumed.
               The DDN NIC host table includes administrative
               information that is not needed for host operation and
               is therefore not currently included in the DNS
               database; examples include network and gateway entries.
               However, much of this additional information will be
               added to the DNS in the future.  Conversely, the DNS
               provides essential services (in particular, MX records)
               that are not available from the DDN NIC host table.
    6.1.4  DNS USER INTERFACE
       6.1.4.1  DNS Administration
          This document is concerned with design and implementation
          issues in host software, not with administrative or
          operational issues.  However, administrative issues are of
          particular importance in the DNS, since errors in particular
          segments of this large distributed database can cause poor
          or erroneous performance for many sites.  These issues are
          discussed in [DNS:6] and [DNS:7].
       6.1.4.2  DNS User Interface
          Hosts MUST provide an interface to the DNS for all
          application programs running on the host.  This interface
          will typically direct requests to a system process to
          perform the resolver function [DNS:1, 6.1:2].
          At a minimum, the basic interface MUST support a request for
          all information of a specific type and class associated with
          a specific name, and it MUST return either all of the
          requested information, a hard error code, or a soft error
          indication.  When there is no error, the basic interface
          returns the complete response information without
          modification, deletion, or ordering, so that the basic
          interface will not need to be changed to accommodate new
          data types.
          DISCUSSION:
               The soft error indication is an essential part of the
               interface, since it may not always be possible to
               access particular information from the DNS; see Section
               6.1.3.3.

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          A host MAY provide other DNS interfaces tailored to
          particular functions, transforming the raw domain data into
          formats more suited to these functions.  In particular, a
          host MUST provide a DNS interface to facilitate translation
          between host addresses and host names.
       6.1.4.3 Interface Abbreviation Facilities
          User interfaces MAY provide a method for users to enter
          abbreviations for commonly-used names.  Although the
          definition of such methods is outside of the scope of the
          DNS specification, certain rules are necessary to insure
          that these methods allow access to the entire DNS name space
          and to prevent excessive use of Internet resources.
          If an abbreviation method is provided, then:
          (a)  There MUST be some convention for denoting that a name
               is already complete, so that the abbreviation method(s)
               are suppressed.  A trailing dot is the usual method.
          (b)  Abbreviation expansion MUST be done exactly once, and
               MUST be done in the context in which the name was
               entered.
          DISCUSSION:
               For example, if an abbreviation is used in a mail
               program for a destination, the abbreviation should be
               expanded into a full domain name and stored in the
               queued message with an indication that it is already
               complete.  Otherwise, the abbreviation might be
               expanded with a mail system search list, not the
               user's, or a name could grow due to repeated
               canonicalizations attempts interacting with wildcards.
          The two most common abbreviation methods are:
          (1)  Interface-level aliases
               Interface-level aliases are conceptually implemented as
               a list of alias/domain name pairs. The list can be
               per-user or per-host, and separate lists can be
               associated with different functions, e.g. one list for
               host name-to-address translation, and a different list
               for mail domains.  When the user enters a name, the
               interface attempts to match the name to the alias
               component of a list entry, and if a matching entry can

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               be found, the name is replaced by the domain name found
               in the pair.
               Note that interface-level aliases and CNAMEs are
               completely separate mechanisms; interface-level aliases
               are a local matter while CNAMEs are an Internet-wide
               aliasing mechanism which is a required part of any DNS
               implementation.
          (2)  Search Lists
               A search list is conceptually implemented as an ordered
               list of domain names.  When the user enters a name, the
               domain names in the search list are used as suffixes to
               the user-supplied name, one by one, until a domain name
               with the desired associated data is found, or the
               search list is exhausted.  Search lists often contain
               the name of the local host's parent domain or other
               ancestor domains.  Search lists are often per-user or
               per-process.
               It SHOULD be possible for an administrator to disable a
               DNS search-list facility.  Administrative denial may be
               warranted in some cases, to prevent abuse of the DNS.
               There is danger that a search-list mechanism will
               generate excessive queries to the root servers while
               testing whether user input is a complete domain name,
               lacking a final period to mark it as complete.  A
               search-list mechanism MUST have one of, and SHOULD have
               both of, the following two provisions to prevent this:
               (a)  The local resolver/name server can implement
                    caching  of negative responses (see Section
                    6.1.3.3).
               (b)  The search list expander can require two or more
                    interior dots in a generated domain name before it
                    tries using the name in a query to non-local
                    domain servers, such as the root.
               DISCUSSION:
                    The intent of this requirement is to avoid
                    excessive delay for the user as the search list is
                    tested, and more importantly to prevent excessive
                    traffic to the root and other high-level servers.
                    For example, if the user supplied a name "X" and
                    the search list contained the root as a component,

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                    a query would have to consult a root server before
                    the next search list alternative could be tried.
                    The resulting load seen by the root servers and
                    gateways near the root would be multiplied by the
                    number of hosts in the Internet.
                    The negative caching alternative limits the effect
                    to the first time a name is used.  The interior
                    dot rule is simpler to implement but can prevent
                    easy use of some top-level names.
    6.1.5  DOMAIN NAME SYSTEM REQUIREMENTS SUMMARY
                                             |           | | | |S| |
                                             |           | | | |H| |F
                                             |           | | | |O|M|o
                                             |           | |S| |U|U|o
                                             |           | |H| |L|S|t
                                             |           |M|O| |D|T|n
                                             |           |U|U|M| | |o
                                             |           |S|L|A|N|N|t
                                             |           |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ———————————————–|———–|-|-|-|-|-|– GENERAL ISSUES | | | | | | |

                                             |           | | | | | |

Implement DNS name-to-address conversion |6.1.1 |x| | | | | Implement DNS address-to-name conversion |6.1.1 |x| | | | | Support conversions using host table |6.1.1 | | |x| | | Properly handle RR with zero TTL |6.1.2.1 |x| | | | | Use QCLASS=* unnecessarily |6.1.2.2 | |x| | | |

Use QCLASS=IN for Internet class             |6.1.2.2    |x| | | | |

Unused fields zero |6.1.2.3 |x| | | | | Use compression in responses |6.1.2.4 |x| | | | |

                                             |           | | | | | |

Include config info in responses |6.1.2.5 | | | | |x| Support all well-known, class-indep. types |6.1.3.5 |x| | | | | Easily expand type list |6.1.3.5 | |x| | | | Load all RR types (except MD and MF) |6.1.3.6 |x| | | | | Load MD or MF type |6.1.3.6 | | | | |x| Operate when root servers, etc. unavailable |6.1.3.7 |x| | | | | ———————————————–|———–|-|-|-|-|-|– RESOLVER ISSUES: | | | | | | |

                                             |           | | | | | |

Resolver support multiple concurrent requests |6.1.3.1 | |x| | | | Full-service resolver: |6.1.3.1 | | |x| | |

Local caching                                |6.1.3.1    |x| | | | |

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Information in local cache times out         |6.1.3.1    |x| | | | |
Configurable with starting info              |6.1.3.1    | |x| | | |

Stub resolver: |6.1.3.1 | | |x| | |

Use redundant recursive name servers         |6.1.3.1    |x| | | | |
Local caching                                |6.1.3.1    | | |x| | |
Information in local cache times out         |6.1.3.1    |x| | | | |

Support for remote multi-homed hosts: | | | | | | |

Sort multiple addresses by preference list   |6.1.3.4    | |x| | | |
                                             |           | | | | | |

———————————————–|———–|-|-|-|-|-|– TRANSPORT PROTOCOLS: | | | | | | |

                                             |           | | | | | |

Support UDP queries |6.1.3.2 |x| | | | | Support TCP queries |6.1.3.2 | |x| | | |

Send query using UDP first                   |6.1.3.2    |x| | | | |1
Try TCP if UDP answers are truncated         |6.1.3.2    | |x| | | |

Name server limit TCP query resources |6.1.3.2 | | |x| | |

Punish unnecessary TCP query                 |6.1.3.2    | | | |x| |

Use truncated data as if it were not |6.1.3.2 | | | | |x| Private agreement to use only TCP |6.1.3.2 | | |x| | | Use TCP for zone transfers |6.1.3.2 |x| | | | | TCP usage not block UDP queries |6.1.3.2 |x| | | | | Support broadcast or multicast queries |6.1.3.2 | | |x| | |

RD bit set in query                          |6.1.3.2    | | | | |x|
RD bit ignored by server is b'cast/m'cast    |6.1.3.2    |x| | | | |
Send only as occasional probe for addr's     |6.1.3.2    | |x| | | |

———————————————–|———–|-|-|-|-|-|– RESOURCE USAGE: | | | | | | |

                                             |           | | | | | |

Transmission controls, per [DNS:2] |6.1.3.3 |x| | | | |

Finite bounds per request                    |6.1.3.3    |x| | | | |

Failure after retries ⇒ soft error |6.1.3.3 |x| | | | | Cache temporary failures |6.1.3.3 | |x| | | | Cache negative responses |6.1.3.3 | |x| | | | Retries use exponential backoff |6.1.3.3 | |x| | | |

Upper, lower bounds                          |6.1.3.3    | |x| | | |

Client handle Source Quench |6.1.3.3 | |x| | | | Server ignore Source Quench |6.1.3.3 | | |x| | | ———————————————–|———–|-|-|-|-|-|– USER INTERFACE: | | | | | | |

                                             |           | | | | | |

All programs have access to DNS interface |6.1.4.2 |x| | | | | Able to request all info for given name |6.1.4.2 |x| | | | | Returns complete info or error |6.1.4.2 |x| | | | | Special interfaces |6.1.4.2 | | |x| | |

Name<->Address translation                   |6.1.4.2    |x| | | | |
                                             |           | | | | | |

Abbreviation Facilities: |6.1.4.3 | | |x| | |

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Convention for complete names                |6.1.4.3    |x| | | | |
Conversion exactly once                      |6.1.4.3    |x| | | | |
Conversion in proper context                 |6.1.4.3    |x| | | | |
Search list:                                 |6.1.4.3    | | |x| | |
  Administrator can disable                  |6.1.4.3    | |x| | | |
  Prevention of excessive root queries       |6.1.4.3    |x| | | | |
    Both methods                             |6.1.4.3    | |x| | | |

———————————————–|———–|-|-|-|-|-|– ———————————————–|———–|-|-|-|-|-|–

1. Unless there is private agreement between particular resolver and

   particular server.

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 6.2  HOST INITIALIZATION
    6.2.1  INTRODUCTION
       This section discusses the initialization of host software
       across a connected network, or more generally across an
       Internet path.  This is necessary for a diskless host, and may
       optionally be used for a host with disk drives.  For a diskless
       host, the initialization process is called "network booting"
       and is controlled by a bootstrap program located in a boot ROM.
       To initialize a diskless host across the network, there are two
       distinct phases:
       (1)  Configure the IP layer.
            Diskless machines often have no permanent storage in which
            to store network configuration information, so that
            sufficient configuration information must be obtained
            dynamically to support the loading phase that follows.
            This information must include at least the IP addresses of
            the host and of the boot server.  To support booting
            across a gateway, the address mask and a list of default
            gateways are also required.
       (2)  Load the host system code.
            During the loading phase, an appropriate file transfer
            protocol is used to copy the system code across the
            network from the boot server.
       A host with a disk may perform the first step, dynamic
       configuration.  This is important for microcomputers, whose
       floppy disks allow network configuration information to be
       mistakenly duplicated on more than one host.  Also,
       installation of new hosts is much simpler if they automatically
       obtain their configuration information from a central server,
       saving administrator time and decreasing the probability of
       mistakes.
    6.2.2  REQUIREMENTS
       6.2.2.1  Dynamic Configuration
          A number of protocol provisions have been made for dynamic
          configuration.
          o    ICMP Information Request/Reply messages

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               This obsolete message pair was designed to allow a host
               to find the number of the network it is on.
               Unfortunately, it was useful only if the host already
               knew the host number part of its IP address,
               information that hosts requiring dynamic configuration
               seldom had.
          o    Reverse Address Resolution Protocol (RARP) [BOOT:4]
               RARP is a link-layer protocol for a broadcast medium
               that allows a host to find its IP address given its
               link layer address.  Unfortunately, RARP does not work
               across IP gateways and therefore requires a RARP server
               on every network.  In addition, RARP does not provide
               any other configuration information.
          o    ICMP Address Mask Request/Reply messages
               These ICMP messages allow a host to learn the address
               mask for a particular network interface.
          o    BOOTP Protocol [BOOT:2]
               This protocol allows a host to determine the IP
               addresses of the local host and the boot server, the
               name of an appropriate boot file, and optionally the
               address mask and list of default gateways.  To locate a
               BOOTP server, the host broadcasts a BOOTP request using
               UDP.  Ad hoc gateway extensions have been used to
               transmit the BOOTP broadcast through gateways, and in
               the future the IP Multicasting facility will provide a
               standard mechanism for this purpose.
          The suggested approach to dynamic configuration is to use
          the BOOTP protocol with the extensions defined in "BOOTP
          Vendor Information Extensions" RFC-1084 [BOOT:3].  RFC-1084
          defines some important general (not vendor-specific)
          extensions.  In particular, these extensions allow the
          address mask to be supplied in BOOTP; we RECOMMEND that the
          address mask be supplied in this manner.
          DISCUSSION:
               Historically, subnetting was defined long after IP, and
               so a separate mechanism (ICMP Address Mask messages)
               was designed to supply the address mask to a host.
               However, the IP address mask and the corresponding IP
               address conceptually form a pair, and for operational

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               simplicity they ought to be defined at the same time
               and by the same mechanism, whether a configuration file
               or a dynamic mechanism like BOOTP.
               Note that BOOTP is not sufficiently general to specify
               the configurations of all interfaces of a multihomed
               host.  A multihomed host must either use BOOTP
               separately for each interface, or configure one
               interface using BOOTP to perform the loading, and
               perform the complete initialization from a file later.
               Application layer configuration information is expected
               to be obtained from files after loading of the system
               code.
       6.2.2.2  Loading Phase
          A suggested approach for the loading phase is to use TFTP
          [BOOT:1] between the IP addresses established by BOOTP.
          TFTP to a broadcast address SHOULD NOT be used, for reasons
          explained in Section 4.2.3.4.

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 6.3  REMOTE MANAGEMENT
    6.3.1  INTRODUCTION
       The Internet community has recently put considerable effort
       into the development of network management protocols.  The
       result has been a two-pronged approach [MGT:1, MGT:6]:  the
       Simple Network Management Protocol (SNMP) [MGT:4] and the
       Common Management Information Protocol over TCP (CMOT) [MGT:5].
       In order to be managed using SNMP or CMOT, a host will need to
       implement an appropriate management agent.  An Internet host
       SHOULD include an agent for either SNMP or CMOT.
       Both SNMP and CMOT operate on a Management Information Base
       (MIB) that defines a collection of management values.  By
       reading and setting these values, a remote application may
       query and change the state of the managed system.
       A standard MIB [MGT:3] has been defined for use by both
       management protocols, using data types defined by the Structure
       of Management Information (SMI) defined in [MGT:2].  Additional
       MIB variables can be introduced under the "enterprises" and
       "experimental" subtrees of the MIB naming space [MGT:2].
       Every protocol module in the host SHOULD implement the relevant
       MIB variables.  A host SHOULD implement the MIB variables as
       defined in the most recent standard MIB, and MAY implement
       other MIB variables when appropriate and useful.
    6.3.2  PROTOCOL WALK-THROUGH
       The MIB is intended to cover both hosts and gateways, although
       there may be detailed differences in MIB application to the two
       cases.  This section contains the appropriate interpretation of
       the MIB for hosts.  It is likely that later versions of the MIB
       will include more entries for host management.
       A managed host must implement the following groups of MIB
       object definitions: System, Interfaces, Address Translation,
       IP, ICMP, TCP, and UDP.
       The following specific interpretations apply to hosts:
       o    ipInHdrErrors
            Note that the error "time-to-live exceeded" can occur in a
            host only when it is forwarding a source-routed datagram.

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       o    ipOutNoRoutes
            This object counts datagrams discarded because no route
            can be found.  This may happen in a host if all the
            default gateways in the host's configuration are down.
       o    ipFragOKs, ipFragFails, ipFragCreates
            A host that does not implement intentional fragmentation
            (see "Fragmentation" section of [INTRO:1]) MUST return the
            value zero for these three objects.
       o    icmpOutRedirects
            For a host, this object MUST always be zero, since hosts
            do not send Redirects.
       o    icmpOutAddrMaskReps
            For a host, this object MUST always be zero, unless the
            host is an authoritative source of address mask
            information.
       o    ipAddrTable
            For a host, the "IP Address Table" object is effectively a
            table of logical interfaces.
       o    ipRoutingTable
            For a host, the "IP Routing Table" object is effectively a
            combination of the host's Routing Cache and the static
            route table described in "Routing Outbound Datagrams"
            section of [INTRO:1].
            Within each ipRouteEntry, ipRouteMetric1...4 normally will
            have no meaning for a host and SHOULD always be -1, while
            ipRouteType will normally have the value "remote".
            If destinations on the connected network do not appear in
            the Route Cache (see "Routing Outbound Datagrams section
            of [INTRO:1]), there will be no entries with ipRouteType
            of "direct".
       DISCUSSION:
            The current MIB does not include Type-of-Service in an
            ipRouteEntry, but a future revision is expected to make

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            this addition.
            We also expect the MIB to be expanded to allow the remote
            management of applications (e.g., the ability to partially
            reconfigure mail systems).  Network service applications
            such as mail systems should therefore be written with the
            "hooks" for remote management.
    6.3.3  MANAGEMENT REQUIREMENTS SUMMARY
                                             |           | | | |S| |
                                             |           | | | |H| |F
                                             |           | | | |O|M|o
                                             |           | |S| |U|U|o
                                             |           | |H| |L|S|t
                                             |           |M|O| |D|T|n
                                             |           |U|U|M| | |o
                                             |           |S|L|A|N|N|t
                                             |           |T|D|Y|O|O|t

FEATURE |SECTION | | | |T|T|e ———————————————–|———–|-|-|-|-|-|– Support SNMP or CMOT agent |6.3.1 | |x| | | | Implement specified objects in standard MIB |6.3.1 | |x| | | |

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7. REFERENCES

 This section lists the primary references with which every
 implementer must be thoroughly familiar.  It also lists some
 secondary references that are suggested additional reading.
 INTRODUCTORY REFERENCES:
 [INTRO:1] "Requirements for Internet Hosts -- Communication Layers,"
      IETF Host Requirements Working Group, R. Braden, Ed., RFC-1122,
      October 1989.
 [INTRO:2]  "DDN Protocol Handbook," NIC-50004, NIC-50005, NIC-50006,
      (three volumes), SRI International, December 1985.
 [INTRO:3]  "Official Internet Protocols," J. Reynolds and J. Postel,
      RFC-1011, May 1987.
      This document is republished periodically with new RFC numbers;
      the latest version must be used.
 [INTRO:4]  "Protocol Document Order Information," O. Jacobsen and J.
      Postel, RFC-980, March 1986.
 [INTRO:5]  "Assigned Numbers," J. Reynolds and J. Postel, RFC-1010,
      May 1987.
      This document is republished periodically with new RFC numbers;
      the latest version must be used.
 TELNET REFERENCES:
 [TELNET:1]  "Telnet Protocol Specification," J. Postel and J.
      Reynolds, RFC-854, May 1983.
 [TELNET:2]  "Telnet Option Specification," J. Postel and J. Reynolds,
      RFC-855, May 1983.
 [TELNET:3]  "Telnet Binary Transmission," J. Postel and J. Reynolds,
      RFC-856, May 1983.
 [TELNET:4]  "Telnet Echo Option," J. Postel and J. Reynolds, RFC-857,
      May 1983.
 [TELNET:5]  "Telnet Suppress Go Ahead Option," J. Postel and J.

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      Reynolds, RFC-858, May 1983.
 [TELNET:6]  "Telnet Status Option," J. Postel and J. Reynolds, RFC-
      859, May 1983.
 [TELNET:7]  "Telnet Timing Mark Option," J. Postel and J. Reynolds,
      RFC-860, May 1983.
 [TELNET:8]  "Telnet Extended Options List," J. Postel and J.
      Reynolds, RFC-861, May 1983.
 [TELNET:9]  "Telnet End-Of-Record Option," J. Postel, RFC-855,
      December 1983.
 [TELNET:10] "Telnet Terminal-Type Option," J. VanBokkelen, RFC-1091,
      February 1989.
      This document supercedes RFC-930.
 [TELNET:11] "Telnet Window Size Option," D. Waitzman, RFC-1073,
      October 1988.
 [TELNET:12] "Telnet Linemode Option," D. Borman, RFC-1116, August
      1989.
 [TELNET:13] "Telnet Terminal Speed Option," C. Hedrick, RFC-1079,
      December 1988.
 [TELNET:14] "Telnet Remote Flow Control Option," C. Hedrick, RFC-
      1080, November 1988.
 SECONDARY TELNET REFERENCES:
 [TELNET:15] "Telnet Protocol," MIL-STD-1782, U.S. Department of
      Defense, May 1984.
      This document is intended to describe the same protocol as RFC-
      854.  In case of conflict, RFC-854 takes precedence, and the
      present document takes precedence over both.
 [TELNET:16] "SUPDUP Protocol," M. Crispin, RFC-734, October 1977.
 [TELNET:17] "Telnet SUPDUP Option," M. Crispin, RFC-736, October
      1977.
 [TELNET:18] "Data Entry Terminal Option," J. Day, RFC-732, June 1977.

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 [TELNET:19] "TELNET Data Entry Terminal option -- DODIIS
      Implementation," A. Yasuda and T. Thompson, RFC-1043, February
      1988.
 FTP REFERENCES:
 [FTP:1]  "File Transfer Protocol," J. Postel and J. Reynolds, RFC-
      959, October 1985.
 [FTP:2]  "Document File Format Standards," J. Postel, RFC-678,
      December 1974.
 [FTP:3]  "File Transfer Protocol," MIL-STD-1780, U.S. Department of
      Defense, May 1984.
      This document is based on an earlier version of the FTP
      specification (RFC-765) and is obsolete.
 TFTP REFERENCES:
 [TFTP:1]  "The TFTP Protocol Revision 2," K. Sollins, RFC-783, June
      1981.
 MAIL REFERENCES:
 [SMTP:1]  "Simple Mail Transfer Protocol," J. Postel, RFC-821, August
      1982.
 [SMTP:2]  "Standard For The Format of ARPA Internet Text Messages,"
      D. Crocker, RFC-822, August 1982.
      This document obsoleted an earlier specification, RFC-733.
 [SMTP:3]  "Mail Routing and the Domain System," C. Partridge, RFC-
      974, January 1986.
      This RFC describes the use of MX records, a mandatory extension
      to the mail delivery process.
 [SMTP:4]  "Duplicate Messages and SMTP," C. Partridge, RFC-1047,
      February 1988.

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 [SMTP:5a]  "Mapping between X.400 and RFC 822," S. Kille, RFC-987,
      June 1986.
 [SMTP:5b]  "Addendum to RFC-987," S. Kille, RFC-???, September 1987.
      The two preceding RFC's define a proposed standard for
      gatewaying mail between the Internet and the X.400 environments.
 [SMTP:6]  "Simple Mail Transfer Protocol,"  MIL-STD-1781, U.S.
      Department of Defense, May 1984.
      This specification is intended to describe the same protocol as
      does RFC-821.  However, MIL-STD-1781 is incomplete; in
      particular, it does not include MX records [SMTP:3].
 [SMTP:7]  "A Content-Type Field for Internet Messages," M. Sirbu,
      RFC-1049, March 1988.
 DOMAIN NAME SYSTEM REFERENCES:
 [DNS:1]  "Domain Names - Concepts and Facilities," P. Mockapetris,
      RFC-1034, November 1987.
      This document and the following one obsolete RFC-882, RFC-883,
      and RFC-973.
 [DNS:2]  "Domain Names - Implementation and Specification," RFC-1035,
      P. Mockapetris, November 1987.
 [DNS:3]  "Mail Routing and the Domain System," C. Partridge, RFC-974,
      January 1986.
 [DNS:4]  "DoD Internet Host Table Specification," K. Harrenstein,
      RFC-952, M. Stahl, E. Feinler, October 1985.
      SECONDARY DNS REFERENCES:
 [DNS:5]  "Hostname Server," K. Harrenstein, M. Stahl, E. Feinler,
      RFC-953, October 1985.
 [DNS:6]  "Domain Administrators Guide," M. Stahl, RFC-1032, November
      1987.

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 [DNS:7]  "Domain Administrators Operations Guide," M. Lottor, RFC-
      1033, November 1987.
 [DNS:8]  "The Domain Name System Handbook," Vol. 4 of Internet
      Protocol Handbook, NIC 50007, SRI Network Information Center,
      August 1989.
 SYSTEM INITIALIZATION REFERENCES:
 [BOOT:1] "Bootstrap Loading Using TFTP," R. Finlayson, RFC-906, June
      1984.
 [BOOT:2] "Bootstrap Protocol (BOOTP)," W. Croft and J. Gilmore, RFC-
      951, September 1985.
 [BOOT:3] "BOOTP Vendor Information Extensions," J. Reynolds, RFC-
      1084, December 1988.
      Note: this RFC revised and obsoleted RFC-1048.
 [BOOT:4] "A Reverse Address Resolution Protocol," R. Finlayson, T.
      Mann, J. Mogul, and M. Theimer, RFC-903, June 1984.
 MANAGEMENT REFERENCES:
 [MGT:1]  "IAB Recommendations for the Development of Internet Network
      Management Standards," V. Cerf, RFC-1052, April 1988.
 [MGT:2]  "Structure and Identification of Management Information for
      TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1065,
      August 1988.
 [MGT:3]  "Management Information Base for Network Management of
      TCP/IP-based internets," M. Rose and K. McCloghrie, RFC-1066,
      August 1988.
 [MGT:4]  "A Simple Network Management Protocol," J. Case, M. Fedor,
      M. Schoffstall, and C. Davin, RFC-1098, April 1989.
 [MGT:5]  "The Common Management Information Services and Protocol
      over TCP/IP," U. Warrier and L. Besaw, RFC-1095, April 1989.
 [MGT:6]  "Report of the Second Ad Hoc Network Management Review
      Group," V. Cerf, RFC-1109, August 1989.

Internet Engineering Task Force [Page 97]

RFC1123 SUPPORT SERVICES – MANAGEMENT October 1989

Security Considerations

 There are many security issues in the application and support
 programs of host software, but a full discussion is beyond the scope
 of this RFC.  Security-related issues are mentioned in sections
 concerning TFTP (Sections 4.2.1, 4.2.3.4, 4.2.3.5), the SMTP VRFY and
 EXPN commands (Section 5.2.3), the SMTP HELO command (5.2.5), and the
 SMTP DATA command (Section 5.2.8).

Author's Address

 Robert Braden
 USC/Information Sciences Institute
 4676 Admiralty Way
 Marina del Rey, CA 90292-6695
 Phone: (213) 822 1511
 EMail: Braden@ISI.EDU

Internet Engineering Task Force [Page 98]

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