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rfc:rfc882

Network Working Group P. Mockapetris Request for Comments: 882 ISI

                                                         November 1983
               DOMAIN NAMES - CONCEPTS and FACILITIES
      +-----------------------------------------------------+
      |                                                     |
      | This RFC introduces domain style names, their use   |
      | for ARPA Internet mail and host address support,    |
      | and the protocols and servers used to implement     |
      | domain name facilities.                             |
      |                                                     |
      | This memo describes the conceptual framework of the |
      | domain system and some uses, but it omits many      |
      | uses, fields, and implementation details.  A        |
      | complete specification of formats, timeouts, etc.   |
      | is presented in RFC 883, "Domain Names -            |
      | Implementation and Specification".  That RFC        |
      | assumes that the reader is familiar with the        |
      | concepts discussed in this memo.                    |
      |                                                     |
      +-----------------------------------------------------+

INTRODUCTION

 The need for domain names
    As applications grow to span multiple hosts, then networks, and
    finally internets, these applications must also span multiple
    administrative boundaries and related methods of operation
    (protocols, data formats, etc).  The number of resources (for
    example mailboxes), the number of locations for resources, and the
    diversity of such an environment cause formidable problems when we
    wish to create consistent methods for referencing particular
    resources that are similar but scattered throughout the
    environment.
    The ARPA Internet illustrates the size-related problems; it is a
    large system and is likely to grow much larger.  The need to have
    a mapping between host names (e.g., USC-ISIF) and ARPA Internet
    addresses (e.g., 10.2.0.52) is beginning to stress the existing
    mechanisms.  Currently hosts in the ARPA Internet are registered
    with the Network Information Center (NIC) and listed in a global
    table (available as the file <NETINFO>HOSTS.TXT on the SRI-NIC
    host) [1].  The size of this table, and especially the frequency
    of updates to the table are near the limit of manageability.  What
    is needed is a distributed database that performs the same
    function, and hence avoids the problems caused by a centralized
    database.
    The problem for computer mail is more severe.  While mail system
    implementers long ago recognized the impossibility of centralizing

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                                Domain Names - Concepts and Facilities
    mailbox names, they have also created an increasingly large and
    irregular set of methods for identifying the location of a
    mailbox.  Some of these methods involve the use of routes and
    forwarding hosts as part of the mail destination address, and
    consequently force the mail user to know multiple address formats,
    the capabilities of various forwarders, and ad hoc tricks for
    passing address specifications through intermediaries.
    These problems have common characteristics that suggest the nature
    of any solution:
       The basic need is for a consistent name space which will be
       used for referring to resources.  In order to avoid the
       problems caused by ad hoc encodings, names should not contain
       addresses, routes, or similar information as part of the name.
       The sheer size of the database and frequency of updates suggest
       that it must be maintained in a distributed manner, with local
       caching to improve performance.  Approaches that attempt to
       collect a consistent copy of the entire database will become
       more and more expensive and difficult, and hence should be
       avoided.  The same principle holds for the structure of the
       name space, and in particular mechanisms for creating and
       deleting names; these should also be distributed.
       The costs of implementing such a facility dictate that it be
       generally useful, and not restricted to a single application.
       We should be able to use names to retrieve host addresses,
       mailbox data, and other as yet undetermined information.
       Because we want the name space to be useful in dissimilar
       networks, it is unlikely that all users of domain names will be
       able to agree on the set of resources or resource information
       that names will be used to retrieve.  Hence names refer to a
       set of resources, and queries contain resource identifiers.
       The only standard types of information that we expect to see
       throughout the name space is structuring information for the
       name space itself, and resources that are described using
       domain names and no nonstandard data.
       We also want the name server transactions to be independent of
       the communications system that carries them. Some systems may
       wish to use datagrams for simple queries and responses, and
       only establish virtual circuits for transactions that need the
       reliability (e.g. database updates, long transactions); other
       systems will use virtual circuits exclusively.

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                                Domain Names - Concepts and Facilities
 Elements of the solution
    The proposed solution has three major components:
       The DOMAIN NAME SPACE, which is a specification for a tree
       structured name space.  Conceptually, each node and leaf of the
       domain name space tree names a set of information, and query
       operations are attempts to extract specific types of
       information from a particular set.  A query names the domain
       name of interest and describes the type of resource information
       that is desired.  For example, the ARPA Internet uses some of
       its domain names to identify hosts; queries for address
       resources return ARPA Internet host addresses.  However, to
       preserve the generality of the domain mechanism, domain names
       are not required to have a one-to-one correspondence with host
       names, host addresses, or any other type of information.
       NAME SERVERS are server programs which hold information about
       the domain tree's structure and set information.  A name server
       may cache structure or set information about any part of the
       domain tree, but in general a particular name server has
       complete information about a subset of the domain space, and
       pointers to other name servers that can be used to lead to
       information from any part of the domain tree.  Name servers
       know the parts of the domain tree for which they have complete
       information; these parts are called ZONEs; a name server is an
       AUTHORITY for these parts of the name space.
       RESOLVERS are programs that extract information from name
       servers in response to user requests.  Resolvers must be able
       to access at least one name server and use that name server's
       information to answer a query directly, or pursue the query
       using referrals to other name servers.  A resolver will
       typically be a system routine that is directly accessible to
       user programs; hence no protocol is necessary between the
       resolver and the user program.
    These three components roughly correspond to the three layers or
    views of the domain system:
       From the user's point of view, the domain system is accessed
       through simple procedure or OS calls to resolvers.  The domain
       space consists of a single tree and the user can request
       information from any section of the tree.
       From the resolver's point of view, the domain system is
       composed of an unknown number of name servers.  Each name
       server has one or more pieces of the whole domain tree's data,

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                                Domain Names - Concepts and Facilities
       but the resolver views each of these databases as essentially
       static.
       From a name server's point of view, the domain system consists
       of separate sets of local information called zones.  The name
       server has local copies of some of the zones.  The name server
       must periodically refresh its zones from master copies in local
       files or foreign name servers.  The name server must
       concurrently process queries that arrive from resolvers using
       the local zones.
    In the interests of performance, these layers blur a bit.  For
    example, resolvers on the same machine as a name server may share
    a database and may also introduce foreign information for use in
    later queries.  This cached information is treated differently
    from the authoritative data in zones.
 Database model
    The organization of the domain system derives from some
    assumptions about the needs and usage patterns of its user
    community and is designed to avoid many of the the complicated
    problems found in general purpose database systems.
    The assumptions are:
       The size of the total database will initially be proportional
       to the number of hosts using the system, but will eventually
       grow to be proportional to the number of users on those hosts
       as mailboxes and other information are added to the domain
       system.
       Most of the data in the system will change very slowly (e.g.,
       mailbox bindings, host addresses), but that the system should
       be able to deal with subsets that change more rapidly (on the
       order of minutes).
       The administrative boundaries used to distribute responsibility
       for the database will usually correspond to organizations that
       have one or more hosts.  Each organization that has
       responsibility for a particular set of domains will provide
       redundant name servers, either on the organization's own hosts
       or other hosts that the organization arranges to use.
       Clients of the domain system should be able to identify trusted
       name servers they prefer to use before accepting referrals to
       name servers outside of this "trusted" set.
       Access to information is more critical than instantaneous

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                                Domain Names - Concepts and Facilities
       updates or guarantees of consistency.  Hence the update process
       allows updates to percolate out though the users of the domain
       system rather than guaranteeing that all copies are
       simultaneously updated.  When updates are unavailable due to
       network or host failure, the usual course is to believe old
       information while continuing efforts to update it.  The general
       model is that copies are distributed with timeouts for
       refreshing.  The distributor sets the timeout value and the
       recipient of the distribution is responsible for performing the
       refresh.  In special situations, very short intervals can be
       specified, or the owner can prohibit copies.
       Some users will wish to access the database via datagrams;
       others will prefer virtual circuits.  The domain system is
       designed so that simple queries and responses can use either
       style, although refreshing operations need the reliability of
       virtual circuits.  The same overall message format is used for
       all communication.  The domain system does not assume any
       special properties of the communications system, and hence
       could be used with any datagram or virtual circuit protocol.
       In any system that has a distributed database, a particular
       name server may be presented with a query that can only be
       answered by some other server.  The two general approaches to
       dealing with this problem are "recursive", in which the first
       server pursues the query for the client at another server, and
       "iterative", in which the server refers the client to another
       server and lets the client pursue the query.  Both approaches
       have advantages and disadvantages, but the iterative approach
       is preferred for the datagram style of access.  The domain
       system requires implementation of the iterative approach, but
       allows the recursive approach as an option.  The optional
       recursive style is discussed in [14], and omitted from further
       discussion in this memo.
    The domain system assumes that all data originates in master files
    scattered through the hosts that use the domain system.  These
    master files are updated by local system administrators.  Master
    files are text files that are read by a local name server, and
    hence become available to users of the domain system.  A standard
    format for these files is given in [14].
    The standard format allows these files to be exchanged between
    hosts (via FTP, mail, or some other mechanism); this facility is
    useful when an organization wants a domain, but doesn't want to
    support a name server.  The organization can maintain the master
    files locally using a text editor, transfer them to a foreign host
    which runs a name server, and then arrange with the system
    administrator of the name server to get the files loaded.

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                                Domain Names - Concepts and Facilities
    Each host's name servers and resolvers are configured by a local
    system administrator.  For a name server, this configuration data
    includes the identity of local master files and instructions on
    which non-local master files are to be loaded from foreign
    servers.  The name server uses the master files or copies to load
    its zones.  For resolvers, the configuration data identifies the
    name servers which should be the primary sources of information.
    The domain system defines procedures for accessing the data and
    for referrals to other name servers.  The domain system also
    defines procedures for caching retrieved data and for periodic
    refreshing of data defined by the system administrator.
    The system administrators provide:
       The definition of zone boundaries
       Master files of data
       Updates to master files
       Statements of the refresh policies desired
    The domain system provides:
       Standard formats for resource data
       Standard methods for querying the database
       Standard methods for name servers to refresh local data from
       foreign name servers

DOMAIN NAME SPACE

 Name space specifications and terminology
    The domain name space is a tree structure.  Each node and leaf on
    the tree corresponds to a resource set (which may be empty).  Each
    node and leaf has an associated label.  Labels are NOT guaranteed
    to be unique, with the exception of the root node, which has a
    null label.  The domain name of a node or leaf is the path from
    the root of the tree to the node or leaf.  By convention, the
    labels that compose a domain name are read left to right, from the
    most specific (lowest) to the least specific (highest).
    Internally, programs that manipulate domain names represent them
    as sequences of labels, where each label is a length octet
    followed by an octet string.  Because all domain names end at the
    root, which has a null string for a label, these internal

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                                Domain Names - Concepts and Facilities
    representations can use a length byte of zero to terminate a
    domain name.  When domain names are printed, labels in a path are
    separated by dots (".").  The root label and its associated dot
    are omitted from printed domain names, but the root can be named
    by a null domain name (" " in this memo).
    To simplify implementations, the total number of octets that
    represent label octets and label lengths is limited to 255.  Thus
    a printed domain name can be up to 254 characters.
    A special label is defined that matches any other label.  This
    label is the asterisk or "*".  An asterisk matches a single label.
    Thus *.ARPA matches FOO.ARPA, but does not match FOO.BAR.ARPA.
    The asterisk is mainly used to create default resource records at
    the boundary between protocol families, and requires prudence in
    its use.
    A domain is identified by a domain name, and consists of that part
    of the domain name space that is at or below the domain name which
    specifies the domain.  A domain is a subdomain of another domain
    if it is contained within that domain.  This relationship can be
    tested by seeing if the subdomain's name has the containing
    domain's name as the right part of its name.  For example, A.B.C.D
    is a subdomain of B.C.D, C.D, D, and " ".
    This tree structure is intended to parallel the administrative
    organization and delegation of authority.  Potentially, each node
    or leaf on the tree can create new subdomains ad infinitum.  In
    practice, this delegation can be limited by the administrator of
    the name servers that manage the domain space and resource data.
    The following figure shows an example of a domain name space.
                                 |                            
              +------------------+------------------+         
              |                  |                  |         
            COLORS            FLAVORS             TRUTH       
              |                  |                            
        +-----+-----+            |                            
        |     |     |         NATURAL                         
       RED  BLUE  GREEN          |                            
                                 |                            
                 +---------------+---------------+            
                 |               |               |            
             CHOCOLATE        VANILLA        STRAWBERRY       
    In this example, the root domain has three immediate subdomains:
    COLORS, FLAVORS, and TRUTH.  The FLAVORS domain has one immediate
    subdomain named NATURAL.FLAVORS.  All of the leaves are also

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                                Domain Names - Concepts and Facilities
    domains.  This domain tree has the names " "(the root), COLORS,
    RED.COLORS, BLUE.COLORS, GREEN.COLORS, FLAVORS, NATURAL.FLAVORS,
    CHOCOLATE.NATURAL.FLAVORS, VANILLA.NATURAL.FLAVORS,
    STRAWBERRY.NATURAL.FLAVORS, and TRUTH.  If we wished to add a new
    domain of ARTIFICIAL under FLAVORS, FLAVORS would typically be the
    administrative entity that would decide; if we wished to create
    CHIP and MOCHA names under CHOCOLATE, CHOCOLATE.NATURAL.FLAVORS
    would typically be the appropriate administrative entity.
 Resource set information
    A domain name identifies a set of resource information.  The set
    of resource information associated with a particular name is
    composed of separate resource records (RRs).
    Each resource record has the following major components:
       The domain name which identifies resource set that holds this
       record, and hence the "owner" of the information.  For example,
       a RR that specifies a host address has a domain name the
       specifies the host having that address.  Thus F.ISI.ARPA might
       be the owner of a RR which specified an address field of
       10.2.0.52.  Since name servers typically store their resource
       information in tree structures paralleling the organization of
       the domain space, this information can usually be stored
       implicitly in the database; however it is always included in
       each resource record carried in a message.
       Other information used to manage the RR, such as length fields,
       timeouts, etc.  This information is omitted in much of this
       memo, but is discussed in [14].
       A resource type field that specifies the type of the resource
       in this resource record.  Types refer to abstract resources
       such as host addresses or mail delivery agents.  The type field
       is two octets long and uses an encoding that is standard
       throughout the domain name system.
       A class field identifies the format of the resource data, such
       as the ARPA Internet format (IN) or the Computer Science
       Network format (CSNET), for certain RR types (such as address
       data).  Note that while the class may separate different
       protocol families, networks, etc. it does not do so in all
       cases.  For example, the IN class uses 32 bit IP addresses
       exclusively, but the CSNET class uses 32 bit IP addresses, X.25
       addresses, and phone numbers.  Thus the class field should be
       used as a guide for interpreting the resource data.  The class
       field is two octets long and uses an encoding that is standard
       throughout the domain name system.

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                                Domain Names - Concepts and Facilities
       Resource data that describes the resource.  The format of this
       data can be determined given the type and class fields, but
       always starts with a two octet length field that allows a name
       server or resolver to determine the boundaries of the resource
       data in any transaction, even if it cannot "understand" the
       resource data itself.  Thus name servers and resolvers can hold
       and pass on records which they cannot interpret.  The format of
       the internal data is restricted only by the maximum length of
       65535 octets; for example the host address record might specify
       a fixed 32 bit number for one class, and a variable length list
       of addresses in another class.
    While the class field in effect partitions the resource data in
    the domain name system into separate parallel sections according
    to class, services can span class boundaries if they use
    compatible resource data formats.  For example, the domain name
    system uses compatible formats for structure information, and the
    mail data decouples mail agent identification from details of how
    to contact the agent (e.g. host addresses).
    This memo uses the following types in its examples:
       A     - the host address associated with the domain name
       MF    - identifies a mail forwarder for the domain
       MD    - identifies a mail destination for the domain
       NS    - the authoritative name server for the domain
       SOA   - identifies the start of a zone of authority
       CNAME - identifies the canonical name of an alias
    This memo uses the following classes in its examples:
       IN - the ARPA Internet system
       CS - the CSNET system
    The first type of resource record holds a host name to host
    address binding.  Its fields are:
+--------+--------+--------+--------------//----------------------+
|<owner> |   A    | <class>| <class specific address>information  |
+--------+--------+--------+--------------//----------------------+

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                                Domain Names - Concepts and Facilities
    The content of the class specific information varies according to
    the value in the CLASS field; for the ARPA Internet, it is the 32
    bit ARPA Internet address of the host, for the CSNET it might be
    the phone number of the host.  For example, F.ISI.ARPA might have
    two A records of the form:
     +----------+--------+--------+----------------------------+
     |F.ISI.ARPA|   A    |   IN   |          10.2.0.52         |
     +----------+--------+--------+----------------------------+
                                and
     +----------+--------+--------+----------------------------+
     |F.ISI.ARPA|   A    |   CS   |         213-822-2112       |
     +----------+--------+--------+----------------------------+
    Note that the data formats for the A type are class dependent, and
    the Internet address and phone number formats shown above are for
    purposes of illustration only.  The actual data formats are
    specified in [14].  For example, CS class data for type A records
    might actually be a list of Internet addresses, phone numbers and
    TELENET addresses.
    The mail forwarder (MF) and mail delivery (MD) records have the
    following format:
      +--------+--------+--------+----------------------------+
      |<owner> | MD/MF  | <class>|       <domain name>        |
      +--------+--------+--------+----------------------------+
    The <domain name> field is a domain name of the host that will
    handle mail; note that this domain name may be completely
    different from the domain name which names the resource record.
    For example, F.ISI.ARPA might have two records of the form:
     +----------+--------+--------+----------------------------+
     |F.ISI.ARPA|  MD    |   IN   |         F.ISI.ARPA         |
     +----------+--------+--------+----------------------------+
                                and
     +----------+--------+--------+----------------------------+
     |F.ISI.ARPA|  MF    |   IN   |         B.ISI.ARPA         |
     +----------+--------+--------+----------------------------+
    These records mean that mail for F.ISI.ARPA can either be
    delivered to the host F.ISI.ARPA or forwarded to B.ISI.ARPA, which
    will accept responsibility for its eventual delivery.  In
    principle, an additional name lookup is required to map the domain
    name of the host to the appropriate address, in practice this
    information is usually returned in the response to the mail query.
    The SOA and NS types of resource records are used to define limits

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                                Domain Names - Concepts and Facilities
    of authority.  The domain name given by the owner field of a SOA
    record is the start of a zone; the domain name given by the owner
    field of a NS record identifies a point in the name space where
    authority has been delegated, and hence marks the zone boundary.
    Except in the case where a name server delegates authority to
    itself, the SOA identifies the top limit of authority, and NS
    records define the first name outside of a zone.  These resource
    records have a standard format for all of the name space:
    +----------+--------+--------+-----------------------------+
    | <owner>  |   SOA  | <class>|       <domain name, etc>    |
    +----------+--------+--------+-----------------------------+
                                  
    +----------+--------+--------+-----------------------------+
    | <owner>  |   NS   | <class>|       <domain name>         |
    +----------+--------+--------+-----------------------------+
    The SOA record marks the start of a zone when it is present in a
    database; the NS record both marks the end of a zone started by an
    SOA (if a higher SOA is present) and also points to a name server
    that has a copy of the zone specified by the <owner. field of the
    NS record.
    The <domain name, etc> in the SOA record specifies the original
    source of the information in the zone and other information used
    by name servers to organize their activities.  SOA records are
    never cached (otherwise they would create false zones); they can
    only be created in special name server maintenance operations.
    The NS record says that a name server which is authoritative for
    records of the given CLASS can be found at <domain name>.
 Queries
    Queries to a name server must include a domain name which
    identifies the target resource set (QNAME), and the type and class
    of desired resource records.  The type and class fields in a query
    can include any of the corresponding type and class fields that
    are defined for resource records; in addition, the query type
    (QTYPE) and query class (QCLASS) fields may contain special values
    that match more than one of the corresponding fields in RRs.
    For example, the QTYPE field may contain:
       MAILA - matches all mail agent RRs (e.g. MD and MF).
  • - matches any RR type.

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    The QCLASS field may contain:
  • - matches any RR class.
    Using the query domain name, QTYPE, and QCLASS, the name server
    looks for matching RRs.  In addition to relevant records, the name
    server may return RRs that point toward a name server that has the
    desired information or RRs that are expected to be useful in
    interpreting the relevant RRs.  For example a name server that
    doesn't have the requested information may know a name server that
    does; a name server that returns a domain name in a relevant RR
    may also return the RR that binds that domain name to an address.
    Note that the QCLASS=* construct requires special interpretation
    regarding authority.  Since a name server may not know all of the
    classes available in the domain system, it can never know if it is
    authoritative for all classes.  Hence responses to QCLASS=*
    queries can never be authoritative.
 Example space
    For purposes of exposition, the following name space is used for
    the remainder of this memo:
                                  |                            
               +------------------+------------------+         
               |                  |                  |         
              DDN               ARPA               CSNET       
               |                  |                  |         
         +-----+-----+            |            +-----+-----+   
         |     |     |            |            |           |   
        JCS  ARMY  NAVY           |           UDEL        UCI  
                                  |                            
         +--------+---------------+---------------+--------+   
         |        |               |               |        |   
        DTI      MIT             ISI             UDEL     NBS  
                  |               |                            
              +---+---+       +---+---+                        
              |       |       |   |   |                        
             DMS     AI       A   B   F                        

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NAME SERVERS

 Introduction
    Name servers store a distributed database consisting of the
    structure of the domain name space, the resource sets associated
    with domain names, and other information used to coordinate
    actions between name servers.
    In general, a name server will be an authority for all or part of
    a particular domain.  The region covered by this authority is
    called a zone.  Name servers may be responsible for no
    authoritative data, and hence have no zones, or may have several
    zones.  When a name server has multiple zones, the zones may have
    no common borders or zones may be contiguous.
    While administrators should not construct overlapping zones, and
    name servers must defend against overlapping zones, overlapping is
    regarded as a non-fatal flaw in the database.  Hence the measures
    taken to protect against it are omitted for the remainder of this
    memo.  A detailed discussion can be found in [14].
    When presented with a query for a domain name over which it has
    authority, a name server returns the desired resource information
    or an indication that the query refers to a domain name or
    resource that does not exist.  If a name server is presented with
    a query for a domain name that is not within its authority, it may
    have the desired information, but it will also return a response
    that points toward an authoritative name server.  If a name server
    is not an authority for a query, it can never return a negative
    response.
    There is no requirement that a name server for a domain reside in
    a host which has a name in the same domain, although this will
    usually be the case.  There is also no restriction on the number
    of name servers that can have authority over a particular domain;
    most domains will have redundant authoritative name servers.  The
    assumption is that different authoritative copies are identical,
    even though inconsistencies are possible as updates are made.
    Name server functions are designed to allow for very simple
    implementations of name servers.  The simplest name server has a
    static set of information and uses datagrams to receive queries
    and return responses.
    More sophisticated name server implementations can improve the
    performance of their clients by caching information from other
    domains.  Although this information can be acquired in a number of
    ways, the normal method is to store the information acquired by a

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    resolver when the resolver consults other name servers.  In a
    sophisticated host, the resolver and name server will coordinate
    their actions and use a shared database.  This cooperation
    requires the incorporation of a time-to-live (TTL) field in all
    cached resource records.  Caching is discussed in the resolver
    section of this memo; this section is devoted to the actions of a
    name servers that don't cache.
    In order to free simple name servers of the requirement of
    managing these timeouts, simple name servers should only contain
    resource records that are expected to remain constant over very
    long periods or resource records for which the name server is an
    authority.  In the following discussion, the TTL field is assumed
    to be stored in the resource record but is omitted in descriptions
    of databases and responses in the interest of clarity.
 Authority and administrative control of domains
    Although we want to have the potential of delegating the
    privileges of name space management at every node, we don't want
    such delegation to be required.
    Hence we introduce the concept of authority.  Authority is vested
    in name servers.  A name server has authority over all of its
    domain until it delegates authority for a subdomain to some other
    name server.
    Any administrative entity that wishes to establish its own domain
    must provide a name server, and have that server accepted by the
    parent name server (i.e. the name server that has authority over
    the place in the domain name space that will hold the new domain).
    While the principles of authority allow acceptance to be at the
    discretion of parent name servers, the following criteria are used
    by the root, and are recommended to all name servers because they
    are responsible for their children's actions:
       1.  It must register with the parent administrator of domains.
       2.  It must identify a responsible person.
       3.  In must provide redundant name servers.
    The domain name must be registered with the administrator to avoid
    name conflicts and to make the domain related information
    available to other domains.  The central administrator may have
    further requirements, and a domain is not registered until the
    central administrator agrees that all requirements are met.
    There must be a responsible person associated with each domain to

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                                Domain Names - Concepts and Facilities
    be a contact point for questions about the domain, to verify and
    update the domain related information, and to resolve any problems
    (e.g., protocol violations) with hosts in the domain.
    The domain must provide redundant (i.e., two or more) name servers
    to provide the name to address resolution service.  These name
    servers must be accessible from outside the domain (as well as
    inside) and must resolve names for at least all the hosts in the
    domain.
    Once the central administrator is satisfied, he will communicate
    the existence to the appropriate administrators of other domains
    so that they can incorporate NS records for the new name server
    into their databases.
 Name server logic
    The processing steps that a name server performs in responding to
    a query are conceptually simple, although implementations may have
    internal databases that are quite complex.
    For purposes of explanation, we assume that the query consists of
    a type QTYPE, a class QCLASS, and a domain name QNAME; we assume
    that the name server stores its RRs in sets where each set has all
    of the RRs for a particular domain.  Note that this database
    structure and the following algorithms are meant to illustrate one
    possible implementation, rather than a specification of how all
    servers must be implemented.
    The following notation is used:
    ord(DOMAIN-NAME)     returns the number of labels in DOMAIN-NAME.
    findset(DOMAIN-NAME) returns a pointer to the set of stored RRs
                         for DOMAIN-NAME, or NULL if there is no such
                         information.
    set(POINTER)         refers to a set located previously by
                         findset, where POINTER is the value returned
                         by findset.
    relevant(QTYPE,TYPE) returns true if a RR of the specified TYPE is
                         relevant to the specified QTYPE.  For
                         example, relevant(MAILA,MF) is true and
                         relevant(MAILA,NS) is false.
    right(NAME,NUMBER)   returns a domain name that is the rightmost
                         NUMBER labels in the string NAME.

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                                Domain Names - Concepts and Facilities
    copy(RR)             copies the resource record specified by RR
                         into the response.
    The name server code could be represented as the following
    sequence of steps:
   {    find out whether the database makes this server          
        authoritative for the domain name specified by QNAME   } 
   for i:=0 to ord(QNAME) { sequence through all nodes in QNAME }
   do   begin                                                    
        ptr:=findset(right(QNAME,i));                            
        if ptr<>NULL                                             
        then { there is domain data for this domain name }       
             begin                                               
             for all RRs in set(ptr)                             
             do   if type(RR)=NS and class(RR)=QCLASS            
                  then begin                                     
                       auth=false;                               
                       NSptr:=ptr                                
                       end;                                      
             for all RRs in set(ptr)                             
             do   if type(RR)=SOA and class(RR)=QCLASS           
                  then auth:=true                                
                  end                                            
             end;                                                
         end;                                                    
    {    copy out authority search results }                     
    if auth                                                      
    then { if authority check for domain found }                 
         if ptr=null                                             
         then return(Name error)                                 
         else                                                    
    else { if not authority, copy NS RRs }                       
         for all RRs in set(nsptr)                               
         do   if (type(RR)=NS and class(RR)=QCLASS)              
                               or                                
                            (QCLASS=*)                           
              then copy(RR);                                     
    {    Copy all RRs that answer the question }                 
    for all RRs in set(ptr)                                      
    do   if class(RR)=QCLASS and relevant(QTYPE,type(RR))        
         then copy(RR);                                          
    The first section of the code (delimited by the for loop over all

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                                Domain Names - Concepts and Facilities
    of the subnodes of QNAME) discovers whether the name server is
    authoritative for the domain specified by QNAME.  It sequences
    through all containing domains of QNAME, starting at the root.  If
    it encounters a SOA it knows that the name server is authoritative
    unless it finds a lower NS RR which delegates authority.  If the
    name server is authoritative, it sets auth=true; if the name
    server is not authoritative, it sets NSptr to point to the set
    which contains the NS RR closest to the domain specified by QNAME.
    The second section of the code reflects the result of the
    authority search into the response.  If the name server is
    authoritative, the code checks to see that the domain specified by
    QNAME exists; if not, a name error is returned.  If the name
    server is not authoritative, the code copies the RRs for a closer
    name server into the response.
    The last section of the code copies all relevant RRs into the
    response.
    Note that this code is not meant as an actual implementation and
    is incomplete in several aspects.  For example, it doesn't deal
    with providing additional information, wildcards, QCLASS=*, or
    with overlapping zones.  The first two of these issues are dealt
    with in the following discussions, the remaining issues are
    discussed in [14].
 Additional information
    When a resolver returns information to a user program, the
    returned information will often lead to a second query.  For
    example, if a mailer asks a resolver for the appropriate mail
    agent for a particular domain name, the name server queried by the
    resolver returns a domain name that identifies the agent.  In
    general, we would expect that the mailer would then request the
    domain name to address binding for the mail agent, and a new name
    server query would result.
    To avoid this duplication of effort, name servers return
    additional information with a response which satisfies the
    anticipated query.  This information is kept in a separate section
    of the response.  Name servers are required to complete the
    appropriate additional information if such information is
    available, but the requestor should not depend on the presence of
    the information since the name server may not have it.  If the
    resolver caches the additional information, it can respond to the
    second query without an additional network transaction.
    The appropriate information is defined in [14], but generally

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                                Domain Names - Concepts and Facilities
    consists of host to address bindings for domain names in returned
    RRs.
 Aliases and canonical names
    In existing systems, hosts and other resources often have several
    names that identify the same resource.  For example, under current
    ARPA Internet naming support, USC-ISIF and ISIF both identify the
    same host.  Similarly, in the case of mailboxes, many
    organizations provide many names that actually go to the same
    mailbox; for example Mockapetris@ISIF, Mockapetris@ISIB, etc., all
    go to the same mailbox (although the mechanism behind this is
    somewhat complicated).
    Most of these systems have a notion that one of the equivalent set
    of names is the canonical name and all others are aliases.
    The domain system provides a similar feature using the canonical
    name (CNAME) RR.  When a name server fails to find a desired RR in
    a set associated with some domain name, it checks to see if the
    resource set contains a CNAME record with a matching class.  If
    so, the name server includes the CNAME record in the response, and
    continues the query at the domain name specified in the data field
    of the CNAME record.
    Suppose a name server was processing a query with QNAME=ISIF.ARPA,
    QTYPE=A, and QCLASS=IN, and had the following resource records:
          ISIF.ARPA     CNAME   IN     F.ISI.ARPA         
          F.ISI.ARPA    A       IN     10.2.0.52          
    Both of these RRs would be returned in the response.
    In the above example, because ISIF.ARPA has no RRs other than the
    CNAME RR, the resources associated with ISIF.ARPA will appear to
    be exactly those associated with F.ISI.ARPA for the IN CLASS.
    Since the CNAME is effective only when the search fails, a CNAME
    can also be used to construct defaults.  For example, suppose the
    name server had the following set of RRs:
          F.ISI.ARPA    A       IN     10.2.0.52          
          F.ISI.ARPA    MD      IN     F.ISI.ARPA         
          XXXX.ARPA     CNAME   IN     F.ISI.ARPA         
          XXXX.ARPA     MF      IN     A.ISI.ARPA         
    Using this database, type A queries for XXXX.ARPA would return the
    XXXX.ARPA CNAME RR and the F.ISI.ARPA A RR, but MAILA or MF
    queries to XXXX.ARPA would return the XXXX.ARPA MF RR without any
    information from F.ISI.ARPA.  This structure might be used to send

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                                Domain Names - Concepts and Facilities
    mail addressed to XXXX.ARPA to A.ISI.ARPA and to direct TELNET for
    XXXX.ARPA to F.ISI.ARPA.
 Wildcards
    In certain cases, an administrator may wish to associate default
    resource information for all or part of a domain.  For example,
    the CSNET domain administrator may wish to establish IN class mail
    forwarding for all hosts in the CSNET domain without IN
    capability.  In such a case, the domain system provides a special
    label "*" that matches any other label.  Note that "*" matches
    only a single label, and not zero or more than one label.  Note
    also that the "*" is distinct from the "*" values for QCLASS and
    QTYPE.
    The semantics of "*" depend upon whether it appears in a query
    domain name (QNAME) or in a RR in a database.
       When an "*" is used in a QNAME, it can only match a "*" in a
       resource record.
       When "*" appears in a RR in a database, it can never override
       an existing exact match.  For example, if a name server
       received a query for the domain UDEL.CSNET, and had appropriate
       RRs for both UDEL.CSNET and *.CSNET, the UDEL.CSNET RRs would
       be used and the *.CSNET RRs would be ignored.  If a query to
       the same database specified FOO.CSNET, the *.CSNET RR would be
       used, but the corresponding labels from the QNAME would replace
       the "*".  Thus the FOO.CSNET query would match the *.CSNET RR
       and return a RR for FOO.CSNET rather than *.CSNET.
       RRs containing "*" labels are copied exactly when zones are
       transfered via name server maintenance operations.
    These semantics are easily implemented by having the name server
    first search for an exact match for a query, and then replacing
    the leftmost label with a "*" and trying again, repeating the
    process until all labels became "*" or the search succeeded.
    TYPE=* in RRs is prohibited.  If it were to be allowed, the
    requestor would have no way of interpreting the data in the RR
    because this data is type dependent.
    CLASS=* is also prohibited.  Similar effects can be achieved using
    QCLASS=*, and allowing both QCLASS=* and CLASS=* leads to
    complexities without apparent benefit.

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                                Domain Names - Concepts and Facilities
 A scenario
    In our sample domain space, suppose we wanted separate
    administrative control for the root, DDN, ARPA, CSNET, MIT and ISI
    domains.  We might allocate name servers as follows:
                                 |(B.ISI.ARPA)                  
                                 |(UDEL.CSNET)                  
              +------------------+------------------+           
              |                  |                  |           
             DDN               ARPA               CSNET         
              |(JCS.DDN)         |(F.ISI.ARPA)      |(UDEL.ARPA)
        +-----+-----+            |(A.ISI.ARPA)+-----+-----+     
        |     |     |            |            |           |     
       JCS  ARMY  NAVY           |           UDEL        UCI    
                                 |                              
        +--------+---------------+---------------+--------+     
        |        |               |               |        |     
       DTI      MIT             ISI             UDEL     NBS    
                 |(AI.MIT.ARPA)  |(F.ISI.ARPA)                  
             +---+---+       +---+---+                          
             |       |       |   |   |                          
            DMS     AI       A   B   F                          
    In this example the authoritative name server is shown in
    parentheses at the point in the domain tree at which is assumes
    control.
    Thus the root name servers are on B.ISI.ARPA and UDEL.CSNET, the
    DDN name server is on JCS.DDN, the CSNET domain server is on
    UDEL.ARPA, etc.
    In an actual system, all domains should have redundant name
    servers, but in this example only the ARPA domain has redundant
    servers A.ISI.ARPA and F.ISI.ARPA.  (The B.ISI.ARPA and UDEL.CSNET
    name servers happen to be not redundant because they handle
    different classes.)  The F.ISI.ARPA name server has authority over
    the ARPA domain, but delegates authority over the MIT.ARPA domain
    to the name server on AI.MIT.ARPA.  The A.ISI.ARPA name server
    also has authority over the ARPA domain, but delegates both the
    ISI.ARPA and MIT.ARPA domains to other name servers.

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                                Domain Names - Concepts and Facilities
 B.ISI.ARPA Name server for " "
    B.ISI.ARPA has the root name server for the IN class.  Its
    database might contain:
          Domain        Resource Record                   
          " "           SOA     IN     A.ISI.ARPA         
          DDN           NS      IN     JCS.DDN            
          ARPA          NS      IN     F.ISI.ARPA         
          CSNET         NS      IN     UDEL.ARPA          
          " "           NS      IN     B.ISI.ARPA         
          " "           NS      CS     UDEL.CSNET         
                                  
          JCS.DDN       A       IN     9.0.0.1            
          F.ISI.ARPA    A       IN     10.2.0.52          
          UDEL.CSNET    A       CS     302-555-0000       
          UDEL.ARPA     A       IN     10.0.0.96          
    The SOA record for the root is necessary so that the name server
    knows that it is authoritative for the root domain for class IN.
    The contents of the SOA resource record point back to A.ISI.ARPA
    and denote that the master data for the zone of authority is
    originally from this host.  The first three NS records denote
    delegation of authority.  The NS root entry for the B.ISI.ARPA
    name server is necessary so that this name server knows about
    itself, and can respond correctly to a query for NS information
    about the root (for which it is an authority).  The root entry for
    class CS denotes that UDEL.CSNET is the authoritative name server
    for the CS class root.  UDEL.CSNET and UDEL.ARPA may or may not
    refer to the same name server; from this information it is
    impossible to tell.
    If this name server was sent a query specifying QTYPE=MAILA,
    QCLASS=IN, QNAME=F.ISI.ARPA, it would begin processing (using the
    previous algorithm) by determining that it was not an authority
    for F.ISI.ARPA.  The test would note that it had authority at " ",
    but would also note that the authority was delegated at ARPA and
    never reestablished via another SOA.  Thus the response would
    return the NS record for the domain ARPA.
    Any queries presented to this server with QCLASS=CS would result
    in the UDEL.CSNET NS record being returned in the response.

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                                Domain Names - Concepts and Facilities
 F.ISI.ARPA Name server for ARPA and ISI.ARPA
    In the same domain space, the F.ISI.ARPA database for the domains
    ARPA and ISI.ARPA might be:
          Domain        Resource Record                   
          " "           NS      IN     B.ISI.ARPA         
          " "           NS      CS     CSNET.UDEL         
          ARPA          SOA     IN     B.ISI.ARPA         
          ARPA          NS      IN     A.ISI.ARPA         
          ARPA          NS      IN     F.ISI.ARPA         
          MIT.ARPA      NS      IN     AI.MIT.ARPA        
          ISI.ARPA      SOA     IN     F.ISI.ARPA         
          ISI.ARPA      NS      IN     F.ISI.ARPA         
          A.ISI.ARPA    MD      IN     A.ISI.ARPA         
          ISI.ARPA      MD      IN     F.ISI.ARPA         
          A.ISI.ARPA    MF      IN     F.ISI.ARPA         
          B.ISI.ARPA    MD      IN     B.ISI.ARPA         
          B.ISI.ARPA    MF      IN     F.ISI.ARPA         
          F.ISI.ARPA    MD      IN     F.ISI.ARPA         
          F.ISI.ARPA    MF      IN     A.ISI.ARPA         
          DTI.ARPA      MD      IN     DTI.ARPA           
          NBS.ARPA      MD      IN     NBS.ARPA           
          UDEL.ARPA     MD      IN     UDEL.ARPA          
          A.ISI.ARPA    A       IN     10.1.0.32          
          F.ISI.ARPA    A       IN     10.2.0.52          
          B.ISI.ARPA    A       IN     10.3.0.52          
          DTI.ARPA      A       IN     10.0.0.12          
          AI.MIT.ARPA   A       IN     10.2.0.6           
          DMS.MIT.ARPA  A       IN     10.1.0.6           
          NBS.ARPA      A       IN     10.0.0.19          
          UDEL.ARPA     A       IN     10.0.0.96          
    For the IN class, the SOA RR for ARPA denotes that this name
    server is authoritative for the domain ARPA, and that the master
    file for this authority is stored on B.ISI.ARPA.  This zone
    extends to ISI.ARPA, where the database delegates authority back
    to this name server in another zone, and doesn't include the
    domain MIT.ARPA, which is served by a name server on AI.MIT.ARPA.
    This name server is not authoritative for any data in the CS
    class.  It has a pointer to the root server for CS data which
    could be use to resolve CS class queries.
    Suppose this name server received a query of the form
    QNAME=A.ISI.ARPA, QTYPE=A, and QCLASS=IN.  The authority search

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                                Domain Names - Concepts and Facilities
    would notice the NS record for " ", its SOA at ARPA, a delegation
    at ISI.ARPA, and the reassumption of authority at ISI.ARPA.  Hence
    it would know that it was an authority for this query.  It would
    then find the A record for A.ISI.ARPA, and return a datagram
    containing this record.
    Another query might be QNAME=B.ISI.ARPA, QTYPE=MAILA, QCLASS=*.
    In this case the name server would know that it cannot be
    authoritative because of the "*" value of QCLASS, and would look
    for records for domain B.ISI.ARPA that match.  Assuming that the
    name server performs the additional record inclusion mentioned in
    the name server algorithm, the returned datagram would include:
          ISI.ARPA      NS      IN     F.ISI.ARPA         
          " "           NS      CS     UDEL.CSNET         
          B.ISI.ARPA    MD      IN     B.ISI.ARPA         
          B.ISI.ARPA    MF      IN     F.ISI.ARPA         
          B.ISI.ARPA    A       IN     10.3.0.52          
          F.ISI.ARPA    A       IN     10.2.0.52          
    If the query were QNAME=DMS.MIT.ARPA, QTYPE=MAILA, QCLASS=IN, the
    name server would discover that AI.MIT.ARPA was the authoritative
    name server and return the following:
          MIT.ARPA      NS      IN     AI.MIT.ARPA        
          AI.MIT.ARPA   A       IN     10.2.0.6           
    In this case, the requestor is directed to seek information from
    the MIT.ARPA domain name server residing on AI.MIT.ARPA.

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                                Domain Names - Concepts and Facilities
 UDEL.ARPA and UDEL.CSNET name server
    In the previous discussion of the sample domain, we stated that
    UDEL.CSNET and UDEL.ARPA might be the same name server.  In this
    example, we assume that this is the case.  As such, the name
    server is an authority for the root for class CS, and an authority
    for the CSNET domain for class IN.
    This name server deals with mail forwarding between the ARPA
    Internet and CSNET systems.  Its RRs illustrate one approach to
    solving this problem.  The name server has the following resource
    records:
          " "           SOA     CS     UDEL.CSNET         
          " "           NS      CS     UDEL.CSNET         
          " "           NS      IN     B.ISI.ARPA         
          CSNET         SOA     IN     UDEL.ARPA          
          CSNET         NS      IN     UDEL.ARPA          
          ARPA          NS      IN     A.ISI.ARPA         
  • .CSNET MF IN UDEL.ARPA

UDEL.CSNET MD CS UDEL.CSNET

          UCI.CSNET     MD      CS     UCI.CSNET          
          UDEL.ARPA     MD      IN     UDEL.ARPA          
          B.ISI.ARPA    A       IN     10.3.0.52          
          UDEL.ARPA     A       IN     10.0.0.96          
          UDEL.CSNET    A       CS     302-555-0000       
          UCI.CSNET     A       CS     714-555-0000       
    Suppose this name server received a query of the form
    QNAME=UCI.CSNET, QTYPE=MAILA, and QCLASS=IN.  The name server
    would discover it was authoritative for the CSNET domain under
    class IN, but would find no explicit mail data for UCI.CSNET.
    However, using the *.CSNET record, it would construct a reply:
          UCI.CSNET     MF      IN     UDEL.ARPA          
          UDEL.ARPA     A       IN     10.0.0.96          
    If this name server received a query of the form QNAME=UCI.CSNET,
    QTYPE=MAILA, and QCLASS=CS, the name server would return:
          UCI.CSNET     MD      CS     UCI.CSNET          
          UCI.CSNET     A       CS     714-555-0000       
    Note that although this scheme allows for forwarding of all mail
    addressed as <anything>.CSNET, it doesn't help with names that
    have more than two components, e.g. A.B.CSNET.  Although this
    problem could be "fixed" by a series of MF entries for *.*.CSNET,

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                                Domain Names - Concepts and Facilities
  • .*.*.CSNET, etc, a more tasteful solution would be to introduce a

cleverer pattern matching algorithm in the CSNET name server.

 Summary of requirements for name servers
    The requirements for a name server are as follows:
       1. It must be recognized by its parent.
       2. It must have complete resource information for all domain
          names for which it is the authority.
       3. It must periodically refresh authoritative information from
          a master file or name server which holds the master.
       4. If it caches information it must also handle TTL management
          for that information.
       5. It must answer simple queries.
 Inverse queries
    Name servers may also support inverse queries that map a
    particular resource to a domain name or domain names that have
    that resource.  For example, while a query might map a domain name
    to a host address, the corresponding inverse query might map the
    address back to the domain name.
    Implementation of this service is optional in a name server, but
    all name servers must at least be able to understand an inverse
    query message and return an error response.
    The domain system cannot guarantee the completeness or uniqueness
    of inverse queries because the domain system is organized by
    domain name rather than by host address or any other resource
    type.  In general, a resolver or other program that wishes to
    guarantee that an inverse query will work must use a name server
    that is known to have the appropriate data, or ask all name
    servers in a domain of interest.
    For example, if a resolver wishes to perform an inverse query for
    an arbitrary host on the ARPA Internet, it must consult a set of
    name servers sufficient to know that all IN data was considered.
    In practice, a single inverse query to a name server that has a
    fairly comprehensive database should satisfy the vast majority of
    inverse queries.
    A detailed discussion of inverse queries is contained in [14].

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                                Domain Names - Concepts and Facilities
 Completion services
    Some existing systems provide the ability to complete partial
    specifications of arguments.  The general principle is that the
    user types the first few characters of the argument and then hits
    an escape character to prompt the system to complete the rest.
    Some completion systems require that the user type enough of the
    argument to be unique; others do not.
    Other systems allow the user to specify one argument and ask the
    system to fill in other arguments.  For example, many mail systems
    allow the user to specify a username without a host for local mail
    delivery.
    The domain system defines name server completion transactions that
    perform the analogous service for the domain system.
    Implementation of this service is optional in a name server, but
    all name servers must at least be able to understand a completion
    request and return an error response.
    When a resolver wishes to request a completion, it sends a name
    server a message that sets QNAME to the partial string, QTYPE to
    the type of resource desired, and QCLASS to the desired class.
    The completion request also includes a RR for the target domain.
    The target domain RR identifies the preferred location of the
    resource.  In completion requests, QNAME must still have a null
    label to terminate the name, but its presence is ignored.  Note
    that a completion request is not a query, but shares some of the
    same field formats.
    For example, a completion request might contain QTYPE=A, QNAME=B,
    QCLASS=IN and a RR for ISI.ARPA.  This request asks for completion
    for a resource whose name begins with "B" and is "close" to
    ISI.ARPA.  This might be a typical shorthand used in the ISI
    community which uses "B" as a way of referring to B.ISI.ARPA.
    The first step in processing a completion request is to look for a
    "whole label" match.  When the name server receives the request
    mentioned above, it looks at all records that are of type A, class
    IN, and whose domain name starts (on the left) with the labels of
    QNAME, in this case, "B".  If multiple records match, the name
    server selects those whose domain names match (from the right) the
    most labels of the preferred domain name.  If there are still
    multiple candidates, the name server selects the records that have
    the shortest (in terms of octets in the name) domain name.  If
    several records remain, then the name server returns them all.
    If no records are found in the previous algorithm, the name server
    assumes that the rightmost label in QNAME is not complete, and

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                                Domain Names - Concepts and Facilities
    looks for records that match but require addition of characters to
    the rightmost label of QNAME.  For example, the previous search
    would not match BB.ARPA to B, but this search would.  If multiple
    hits are found, the same discarding strategy is followed.
    A detailed discussion of completion can be found in [14].

RESOLVERS

 Introduction
    Resolvers are programs that interface user programs to domain name
    servers.  In the simplest case, a resolver receives a request from
    a user program (e.g. mail programs, TELNET, FTP) in the form of a
    subroutine call, system call etc., and returns the desired
    information in a form compatible with the local host's data
    formats.
    Because a resolver may need to consult several name servers, the
    amount of time that a resolver will take to complete can vary.
    This variance is part of the justification for the split between
    name servers and resolvers; name servers may use datagrams and
    have a response time that is essentially equal to network delay
    plus a short service time, while resolvers may take an essentially
    indeterminate amount of time.
    We expect to see two types of resolvers: simple resolvers that can
    chain through multiple name servers when required, and more
    complicated resolvers that cache resource records for use in
    future queries.
 Simple resolvers
    A simple resolver needs the following capabilities:
    1. It must know how to access a name server, and should know the
       authoritative name server for the host that it services.
    2. It must know the protocol capabilities for its clients so that
       it can set the class fields of the queries it sends to return
       information that is useful to its clients.  If the resolver
       serves a client that has multiple protocol capabilities, it
       should be able to support the preferences of the client.
       The resolver for a multiple protocol client can either collect
       information for all classes using the * class value, or iterate
       on the classes supported by the client.  Note that in either
       case, the resolver must understand the preferences of the host.
       For example, the host that supports both CSNET and ARPA

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                                Domain Names - Concepts and Facilities
       Internet protocols might prefer mail delivery (MD) to mail
       forwarding (MF), regardless of protocol, or might prefer one
       protocol regardless of whether MD or MF is required.  Care is
       required to prevent loops.
    3. The resolver must be capable of chaining through multiple name
       servers to get to an authoritative name server for any query.
       The resolver should guard against loops in referrals; a simple
       policy is to discard referrals that don't match more of the
       query name than the referring name server, and also to avoid
       querying the same name server twice (This test should be done
       using addresses of name servers instead of domain names to
       avoid problems when a name server has multiple domain names or
       errors are present in aliases).
    4. The resolver must be able to try alternate name servers when a
       name server doesn't respond.
    5. The resolver must be able to communicate different failure
       conditions to its client.  These failure conditions include
       unknown domain name, unknown resource for a know domain name,
       and inability to access any of the authoritative name servers
       for a domain.
    6. If the resolver uses datagrams for queries, it must recover
       from lost and duplicate datagrams.
 Resolvers with cache management
    Caching provides a tool for improving the performance of name
    service, but also is a potential source of incorrect results.  For
    example, a database might cache information that is later changed
    in the authoritative name servers.  While this problem can't be
    eliminated without eliminating caching, it can be reduced to an
    infrequent problem through the use of timeouts.
    When name servers return resource records, each record has an
    associated time-to-live (TTL) field.  This field is expressed in
    seconds, and has 16 bits of significance.
    When a resolver caches a returned resource record it must also
    remember the TTL field.  The resolver must discard the record when
    the equivalent amount of time has passed.  If the resolver shares
    a database with a name server, it must decrement the TTL field of
    imported records periodically rather than simply deleting the
    record.  This strategy is necessary to avoid exporting a resource
    record whose TTL field doesn't reflect the amount of time that the
    resource record has been cached.  Of course, the resolver should

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                                Domain Names - Concepts and Facilities
    not decrement the TTL fields of records for which the associated
    name server is an authority.

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RFC 882 November 1983

                                Domain Names - Concepts and Facilities

Appendix 1 - Domain Name Syntax Specification

 The preferred syntax of domain names is given by the following BNF
 rules.  Adherence to this syntax will result in fewer problems with
 many applications that use domain names (e.g., mail, TELNET).  Note
 that some applications described in [14] use domain names containing
 binary information and hence do not follow this syntax.
    <domain> ::=  <subdomain> | " "
    <subdomain> ::=  <label> | <subdomain> "." <label>
    <label> ::= <letter> [ [ <ldh-str> ] <let-dig> ]
    <ldh-str> ::= <let-dig-hyp> | <let-dig-hyp> <ldh-str>
    <let-dig-hyp> ::= <let-dig> | "-"
    <let-dig> ::= <letter> | <digit>
    <letter> ::= any one of the 52 alphabetic characters A through Z
    in upper case and a through z in lower case
    <digit> ::= any one of the ten digits 0 through 9
 Note that while upper and lower case letters are allowed in domain
 names no significance is attached to the case.  That is, two names
 with the same spelling but different case are to be treated as if
 identical.
 The labels must follow the rules for ARPANET host names.  They must
 start with a letter, end with a letter or digit, and have as interior
 characters only letters, digits, and hyphen.  There are also some
 restrictions on the length.  Labels must be 63 characters or less.
 For example, the following strings identify hosts in the ARPA
 Internet:
    F.ISI.ARPA     LINKABIT-DCN5.ARPA     UCL-TAC.ARPA

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                                Domain Names - Concepts and Facilities

REFERENCES and BIBLIOGRAPHY

 [1]  E. Feinler, K. Harrenstien, Z. Su, and V. White, "DOD Internet
      Host Table Specification", RFC 810, Network Information Center,
      SRI International, March 1982.
 [2]  J. Postel, "Computer Mail Meeting Notes", RFC 805,
      USC/Information Sciences Institute, February 1982.
 [3]  Z. Su, and J. Postel, "The Domain Naming Convention for Internet
      User Applications", RFC 819, Network Information Center, SRI
      International, August 1982.
 [4]  Z. Su, "A Distributed System for Internet Name Service",
      RFC 830, Network Information Center, SRI International,
      October 1982.
 [5]  K. Harrenstien, and V. White, "NICNAME/WHOIS", RFC 812, Network
      Information Center, SRI International, March 1982.
 [6]  M. Solomon, L. Landweber, and D. Neuhengen, "The CSNET Name
      Server", Computer Networks, vol 6, nr 3, July 1982.
 [7]  K. Harrenstien, "NAME/FINGER", RFC 742, Network Information
      Center, SRI International, December 1977.
 [8]  J. Postel, "Internet Name Server", IEN 116, USC/Information
      Sciences Institute, August 1979.
 [9]  K. Harrenstien, V. White, and E. Feinler, "Hostnames Server",
      RFC 811, Network Information Center, SRI International,
      March 1982.
 [10] J. Postel, "Transmission Control Protocol", RFC 793,
      USC/Information Sciences Institute, September 1981.
 [11] J. Postel, "User Datagram Protocol", RFC 768, USC/Information
      Sciences Institute, August 1980.
 [12] J. Postel, "Simple Mail Transfer Protocol", RFC 821,
      USC/Information Sciences Institute, August 1980.
 [13] J. Reynolds, and J. Postel, "Assigned Numbers", RFC 870,
      USC/Information Sciences Institute, October 1983.
 [14] P. Mockapetris, "Domain Names - Implementation and
      Specification", RFC 883, USC/Information Sciences Institute,
      November 1983.

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