GENWiki

Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


rfc:rfc1725

Network Working Group J. Myers Request for Comments: 1725 Carnegie Mellon Obsoletes: 1460 M. Rose Category: Standards Track Dover Beach Consulting, Inc.

                                                         November 1994
                  Post Office Protocol - Version 3

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Overview

 This memo is a revision to RFC 1460, a Draft Standard.  It makes the
 following changes from that document:
  1. removed text regarding "split-UA model", which didn't add

anything to the understanding of POP

  1. clarified syntax of commands, keywords, and arguments
  1. clarified behavior on broken connection
  1. explicitly permitted an inactivity autologout timer
  1. clarified the requirements of the "exclusive-access lock"
  1. removed implementation-specific wording regarding the parsing of

the maildrop

  1. allowed servers to close the connection after a failed

authentication command

  1. removed the LAST command
  1. fixed typo in example of TOP command
  1. clarified that the second argument to the TOP command is non-

negative

  1. added the optional UIDL command

Myers & Rose [Page 1] RFC 1725 POP3 November 1994

  1. added warning regarding length of shared secrets with APOP
  1. added additional warnings to the security considerations section

1. Introduction

 On certain types of smaller nodes in the Internet it is often
 impractical to maintain a message transport system (MTS).  For
 example, a workstation may not have sufficient resources (cycles,
 disk space) in order to permit a SMTP server [RFC821] and associated
 local mail delivery system to be kept resident and continuously
 running.  Similarly, it may be expensive (or impossible) to keep a
 personal computer interconnected to an IP-style network for long
 amounts of time (the node is lacking the resource known as
 "connectivity").
 Despite this, it is often very useful to be able to manage mail on
 these smaller nodes, and they often support a user agent (UA) to aid
 the tasks of mail handling.  To solve this problem, a node which can
 support an MTS entity offers a maildrop service to these less endowed
 nodes.  The Post Office Protocol - Version 3 (POP3) is intended to
 permit a workstation to dynamically access a maildrop on a server
 host in a useful fashion.  Usually, this means that the POP3 is used
 to allow a workstation to retrieve mail that the server is holding
 for it.
 For the remainder of this memo, the term "client host" refers to a
 host making use of the POP3 service, while the term "server host"
 refers to a host which offers the POP3 service.

2. A Short Digression

 This memo does not specify how a client host enters mail into the
 transport system, although a method consistent with the philosophy of
 this memo is presented here:
    When the user agent on a client host wishes to enter a message
    into the transport system, it establishes an SMTP connection to
    its relay host (this relay host could be, but need not be, the
    POP3 server host for the client host).

3. Basic Operation

 Initially, the server host starts the POP3 service by listening on
 TCP port 110.  When a client host wishes to make use of the service,
 it establishes a TCP connection with the server host.  When the
 connection is established, the POP3 server sends a greeting.  The
 client and POP3 server then exchange commands and responses

Myers & Rose [Page 2] RFC 1725 POP3 November 1994

 (respectively) until the connection is closed or aborted.
 Commands in the POP3 consist of a keyword, possibly followed by one
 or more arguments.  All commands are terminated by a CRLF pair.
 Keywords and arguments consist of printable ASCII characters.
 Keywords and arguments are each separated by a single SPACE
 character.  Keywords are three or four characters long. Each argument
 may be up to 40 characters long.
 Responses in the POP3 consist of a status indicator and a keyword
 possibly followed by additional information.  All responses are
 terminated by a CRLF pair.  There are currently two status
 indicators: positive ("+OK") and negative ("-ERR").
 Responses to certain commands are multi-line.  In these cases, which
 are clearly indicated below, after sending the first line of the
 response and a CRLF, any additional lines are sent, each terminated
 by a CRLF pair.  When all lines of the response have been sent, a
 final line is sent, consisting of a termination octet (decimal code
 046, ".") and a CRLF pair.  If any line of the multi-line response
 begins with the termination octet, the line is "byte-stuffed" by
 pre-pending the termination octet to that line of the response.
 Hence a multi-line response is terminated with the five octets
 "CRLF.CRLF".  When examining a multi-line response, the client checks
 to see if the line begins with the termination octet.  If so and if
 octets other than CRLF follow, the the first octet of the line (the
 termination octet) is stripped away.  If so and if CRLF immediately
 follows the termination character, then the response from the POP
 server is ended and the line containing ".CRLF" is not considered
 part of the multi-line response.
 A POP3 session progresses through a number of states during its
 lifetime.  Once the TCP connection has been opened and the POP3
 server has sent the greeting, the session enters the AUTHORIZATION
 state.  In this state, the client must identify itself to the POP3
 server.  Once the client has successfully done this, the server
 acquires resources associated with the client's maildrop, and the
 session enters the TRANSACTION state.  In this state, the client
 requests actions on the part of the POP3 server.  When the client has
 issued the QUIT command, the session enters the UPDATE state.  In
 this state, the POP3 server releases any resources acquired during
 the TRANSACTION state and says goodbye.  The TCP connection is then
 closed.
 A POP3 server MAY have an inactivity autologout timer.  Such a timer
 MUST be of at least 10 minutes' duration.  The receipt of any command
 from the client during that interval should suffice to reset the
 autologout timer.  When the timer expires, the session does NOT enter

Myers & Rose [Page 3] RFC 1725 POP3 November 1994

 the UPDATE state--the server should close the TCP connection without
 removing any messages or sending any response to the client.

4. The AUTHORIZATION State

 Once the TCP connection has been opened by a POP3 client, the POP3
 server issues a one line greeting.  This can be any string terminated
 by CRLF.  An example might be:
    S:  +OK POP3 server ready
 Note that this greeting is a POP3 reply.  The POP3 server should
 always give a positive response as the greeting.
 The POP3 session is now in the AUTHORIZATION state.  The client must
 now identify and authenticate itself to the POP3 server.  Two
 possible mechanisms for doing this are described in this document,
 the USER and PASS command combination and the APOP command.  The APOP
 command is described later in this document.
 To authenticate using the USER and PASS command combination, the
 client must first issue the USER command.  If the POP3 server
 responds with a positive status indicator ("+OK"), then the client
 may issue either the PASS command to complete the authentication, or
 the QUIT command to terminate the POP3 session.  If the POP3 server
 responds with a negative status indicator ("-ERR") to the USER
 command, then the client may either issue a new authentication
 command or may issue the QUIT command.
 When the client issues the PASS command, the POP3 server uses the
 argument pair from the USER and PASS commands to determine if the
 client should be given access to the appropriate maildrop.
 Once the POP3 server has determined through the use of any
 authentication command that the client should be given access to the
 appropriate maildrop, the POP3 server then acquires an exclusive-
 access lock on the maildrop, as necessary to prevent messages from
 being modified or removed before the session enters the UPDATE state.
 If the lock is successfully acquired, the POP3 server responds with a
 positive status indicator.  The POP3 session now enters the
 TRANSACTION state, with no messages marked as deleted.  If the the
 maildrop cannot be opened for some reason (for example, a lock can
 not be acquired, the client is denied access to the appropriate
 maildrop, or the maildrop cannot be parsed), the POP3 server responds
 with a negative status indicator.  (If a lock was acquired but the
 POP3 server intends to respond with a negative status indicator, the
 POP3 server must release the lock prior to rejecting the command.)
 After returning a negative status indicator, the server may close the

Myers & Rose [Page 4] RFC 1725 POP3 November 1994

 connection.  If the server does not close the connection, the client
 may either issue a new authentication command and start again, or the
 client may issue the QUIT command.
 After the POP3 server has opened the maildrop, it assigns a message-
 number to each message, and notes the size of each message in octets.
 The first message in the maildrop is assigned a message-number of
 "1", the second is assigned "2", and so on, so that the n'th message
 in a maildrop is assigned a message-number of "n".  In POP3 commands
 and responses, all message-number's and message sizes are expressed
 in base-10 (i.e., decimal).
 Here are summaries for the three POP3 commands discussed thus far:
    USER name
       Arguments:
           a string identifying a mailbox (required), which is of
           significance ONLY to the server
       Restrictions:
           may only be given in the AUTHORIZATION state after the POP3
           greeting or after an unsuccessful USER or PASS command
       Possible Responses:
           +OK name is a valid mailbox
           -ERR never heard of mailbox name
       Examples:
           C: USER mrose
           S: +OK mrose is a real hoopy frood
              ...
           C: USER frated
           S: -ERR sorry, no mailbox for frated here
    PASS string
       Arguments:
           a server/mailbox-specific password (required)
       Restrictions:
           may only be given in the AUTHORIZATION state after a
           successful USER command
       Discussion:
           Since the PASS command has exactly one argument, a POP3
           server may treat spaces in the argument as part of the
           password, instead of as argument separators.

Myers & Rose [Page 5] RFC 1725 POP3 November 1994

       Possible Responses:
           +OK maildrop locked and ready
           -ERR invalid password
           -ERR unable to lock maildrop
       Examples:
           C: USER mrose
           S: +OK mrose is a real hoopy frood
           C: PASS secret
           S: +OK mrose's maildrop has 2 messages (320 octets)
             ...
           C: USER mrose
           S: +OK mrose is a real hoopy frood
           C: PASS secret
           S: -ERR maildrop already locked
    QUIT
       Arguments: none
       Restrictions: none
       Possible Responses:
           +OK
       Examples:
           C: QUIT
           S: +OK dewey POP3 server signing off

5. The TRANSACTION State

 Once the client has successfully identified itself to the POP3 server
 and the POP3 server has locked and opened the appropriate maildrop,
 the POP3 session is now in the TRANSACTION state.  The client may now
 issue any of the following POP3 commands repeatedly.  After each
 command, the POP3 server issues a response.  Eventually, the client
 issues the QUIT command and the POP3 session enters the UPDATE state.
 Here are the POP3 commands valid in the TRANSACTION state:
    STAT
       Arguments: none
       Restrictions:
           may only be given in the TRANSACTION state

Myers & Rose [Page 6] RFC 1725 POP3 November 1994

       Discussion:
           The POP3 server issues a positive response with a line
           containing information for the maildrop.  This line is
           called a "drop listing" for that maildrop.
           In order to simplify parsing, all POP3 servers required to
           use a certain format for drop listings.  The positive
           response consists of "+OK" followed by a single space, the
           number of messages in the maildrop, a single space, and the
           size of the maildrop in octets.  This memo makes no
           requirement on what follows the maildrop size.  Minimal
           implementations should just end that line of the response
           with a CRLF pair.  More advanced implementations may
           include other information.
              NOTE: This memo STRONGLY discourages implementations
              from supplying additional information in the drop
              listing.  Other, optional, facilities are discussed
              later on which permit the client to parse the messages
              in the maildrop.
           Note that messages marked as deleted are not counted in
           either total.
       Possible Responses:
           +OK nn mm
       Examples:
           C: STAT
           S: +OK 2 320
    LIST [msg]
       Arguments:
           a message-number (optional), which, if present, may NOT
           refer to a message marked as deleted
       Restrictions:
           may only be given in the TRANSACTION state
       Discussion:
           If an argument was given and the POP3 server issues a
           positive response with a line containing information for
           that message.  This line is called a "scan listing" for
           that message.
           If no argument was given and the POP3 server issues a
           positive response, then the response given is multi-line.

Myers & Rose [Page 7] RFC 1725 POP3 November 1994

           After the initial +OK, for each message in the maildrop,
           the POP3 server responds with a line containing information
           for that message.  This line is also called a "scan
           listing" for that message.
           In order to simplify parsing, all POP3 servers are required
           to use a certain format for scan listings.  A scan listing
           consists of the message-number of the message, followed by
           a single space and the exact size of the message in octets.
           This memo makes no requirement on what follows the message
           size in the scan listing.  Minimal implementations should
           just end that line of the response with a CRLF pair.  More
           advanced implementations may include other information, as
           parsed from the message.
              NOTE: This memo STRONGLY discourages implementations
              from supplying additional information in the scan
              listing.  Other, optional, facilities are discussed
              later on which permit the client to parse the messages
              in the maildrop.
           Note that messages marked as deleted are not listed.
       Possible Responses:
           +OK scan listing follows
           -ERR no such message
       Examples:
           C: LIST
           S: +OK 2 messages (320 octets)
           S: 1 120
           S: 2 200
           S: .
             ...
           C: LIST 2
           S: +OK 2 200
             ...
           C: LIST 3
           S: -ERR no such message, only 2 messages in maildrop
    RETR msg
       Arguments:
           a message-number (required) which may not refer to a
           message marked as deleted
       Restrictions:
           may only be given in the TRANSACTION state

Myers & Rose [Page 8] RFC 1725 POP3 November 1994

       Discussion:
           If the POP3 server issues a positive response, then the
           response given is multi-line.  After the initial +OK, the
           POP3 server sends the message corresponding to the given
           message-number, being careful to byte-stuff the termination
           character (as with all multi-line responses).
       Possible Responses:
           +OK message follows
           -ERR no such message
       Examples:
           C: RETR 1
           S: +OK 120 octets
           S: <the POP3 server sends the entire message here>
           S: .
    DELE msg
       Arguments:
           a message-number (required) which may not refer to a
           message marked as deleted
       Restrictions:
           may only be given in the TRANSACTION state
       Discussion:
           The POP3 server marks the message as deleted.  Any future
           reference to the message-number associated with the message
           in a POP3 command generates an error.  The POP3 server does
           not actually delete the message until the POP3 session
           enters the UPDATE state.
       Possible Responses:
           +OK message deleted
           -ERR no such message
       Examples:
           C: DELE 1
           S: +OK message 1 deleted
              ...
           C: DELE 2
           S: -ERR message 2 already deleted
    NOOP
       Arguments: none

Myers & Rose [Page 9] RFC 1725 POP3 November 1994

       Restrictions:
           may only be given in the TRANSACTION state
       Discussion:
           The POP3 server does nothing, it merely replies with a
           positive response.
       Possible Responses:
           +OK
       Examples:
           C: NOOP
           S: +OK
    RSET
       Arguments: none
       Restrictions:
           may only be given in the TRANSACTION state
       Discussion:
           If any messages have been marked as deleted by the POP3
           server, they are unmarked.  The POP3 server then replies
           with a positive response.
       Possible Responses:
           +OK
       Examples:
           C: RSET
           S: +OK maildrop has 2 messages (320 octets)

6. The UPDATE State

 When the client issues the QUIT command from the TRANSACTION state,
 the POP3 session enters the UPDATE state.  (Note that if the client
 issues the QUIT command from the AUTHORIZATION state, the POP3
 session terminates but does NOT enter the UPDATE state.)
 If a session terminates for some reason other than a client-issued
 QUIT command, the POP3 session does NOT enter the UPDATE state and
 MUST not remove any messages from the maildrop.
    QUIT
       Arguments: none

Myers & Rose [Page 10] RFC 1725 POP3 November 1994

       Restrictions: none
       Discussion:
           The POP3 server removes all messages marked as deleted from
           the maildrop.  It then releases any exclusive-access lock
           on the maildrop and replies as to the status of these
           operations.  The TCP connection is then closed.
       Possible Responses:
           +OK
       Examples:
           C: QUIT
           S: +OK dewey POP3 server signing off (maildrop empty)
              ...
           C: QUIT
           S: +OK dewey POP3 server signing off (2 messages left)
              ...

7. Optional POP3 Commands

 The POP3 commands discussed above must be supported by all minimal
 implementations of POP3 servers.
 The optional POP3 commands described below permit a POP3 client
 greater freedom in message handling, while preserving a simple POP3
 server implementation.
    NOTE: This memo STRONGLY encourages implementations to support
    these commands in lieu of developing augmented drop and scan
    listings.  In short, the philosophy of this memo is to put
    intelligence in the part of the POP3 client and not the POP3
    server.
    TOP msg n
       Arguments:
           a message-number (required) which may NOT refer to to a
           message marked as deleted, and a non-negative number
           (required)
       Restrictions:
           may only be given in the TRANSACTION state
       Discussion:
           If the POP3 server issues a positive response, then the
           response given is multi-line.  After the initial +OK, the
           POP3 server sends the headers of the message, the blank

Myers & Rose [Page 11] RFC 1725 POP3 November 1994

           line separating the headers from the body, and then the
           number of lines indicated message's body, being careful to
           byte-stuff the termination character (as with all multi-
           line responses).
           Note that if the number of lines requested by the POP3
           client is greater than than the number of lines in the
           body, then the POP3 server sends the entire message.
       Possible Responses:
           +OK top of message follows
           -ERR no such message
       Examples:
           C: TOP 1 10
           S: +OK
           S: <the POP3 server sends the headers of the
              message, a blank line, and the first 10 lines
              of the body of the message>
           S: .
              ...
           C: TOP 100 3
           S: -ERR no such message
    UIDL [msg]
    Arguments:
        a message-number (optionally)  If a message-number is given,
        it may NOT refer to a message marked as deleted.
    Restrictions:
        may only be given in the TRANSACTION state.
    Discussion:
        If an argument was given and the POP3 server issues a positive
        response with a line containing information for that message.
        This line is called a "unique-id listing" for that message.
        If no argument was given and the POP3 server issues a positive
        response, then the response given is multi-line.  After the
        initial +OK, for each message in the maildrop, the POP3 server
        responds with a line containing information for that message.
        This line is called a "unique-id listing" for that message.
        In order to simplify parsing, all POP3 servers are required to
        use a certain format for unique-id listings.  A unique-id
        listing consists of the message-number of the message,
        followed by a single space and the unique-id of the message.

Myers & Rose [Page 12] RFC 1725 POP3 November 1994

        No information follows the unique-id in the unique-id listing.
        The unique-id of a message is an arbitrary server-determined
        string, consisting of characters in the range 0x21 to 0x7E,
        which uniquely identifies a message within a maildrop and
        which persists across sessions. The server should never reuse
        an unique-id in a given maildrop, for as long as the entity
        using the unique-id exists.
        Note that messages marked as deleted are not listed.
    Possible Responses:
        +OK unique-id listing follows
        -ERR no such message
    Examples:
        C: UIDL
        S: +OK
        S: 1 whqtswO00WBw418f9t5JxYwZ
        S: 2 QhdPYR:00WBw1Ph7x7
        S: .
           ...
        C: UIDL 2
        S: +OK 2 QhdPYR:00WBw1Ph7x7
           ...
        C: UIDL 3
        S: -ERR no such message, only 2 messages in maildrop
    APOP name digest
       Arguments:
           a string identifying a mailbox and a MD5 digest string
           (both required)
       Restrictions:
           may only be given in the AUTHORIZATION state after the POP3
           greeting
       Discussion:
           Normally, each POP3 session starts with a USER/PASS
           exchange.  This results in a server/user-id specific
           password being sent in the clear on the network.  For
           intermittent use of POP3, this may not introduce a sizable
           risk.  However, many POP3 client implementations connect to
           the POP3 server on a regular basis -- to check for new
           mail.  Further the interval of session initiation may be on
           the order of five minutes.  Hence, the risk of password
           capture is greatly enhanced.

Myers & Rose [Page 13] RFC 1725 POP3 November 1994

           An alternate method of authentication is required which
           provides for both origin authentication and replay
           protection, but which does not involve sending a password
           in the clear over the network.  The APOP command provides
           this functionality.
           A POP3 server which implements the APOP command will
           include a timestamp in its banner greeting.  The syntax of
           the timestamp corresponds to the `msg-id' in [RFC822], and
           MUST be different each time the POP3 server issues a banner
           greeting.  For example, on a UNIX implementation in which a
           separate UNIX process is used for each instance of a POP3
           server, the syntax of the timestamp might be:
              <process-ID.clock@hostname>
           where `process-ID' is the decimal value of the process's
           PID, clock is the decimal value of the system clock, and
           hostname is the fully-qualified domain-name corresponding
           to the host where the POP3 server is running.
           The POP3 client makes note of this timestamp, and then
           issues the APOP command.  The `name' parameter has
           identical semantics to the `name' parameter of the USER
           command. The `digest' parameter is calculated by applying
           the MD5 algorithm [RFC1321] to a string consisting of the
           timestamp (including angle-brackets) followed by a shared
           secret.  This shared secret is a string known only to the
           POP3 client and server.  Great care should be taken to
           prevent unauthorized disclosure of the secret, as knowledge
           of the secret will allow any entity to successfully
           masquerade as the named user.  The `digest' parameter
           itself is a 16-octet value which is sent in hexadecimal
           format, using lower-case ASCII characters.
           When the POP3 server receives the APOP command, it verifies
           the digest provided.  If the digest is correct, the POP3
           server issues a positive response, and the POP3 session
           enters the TRANSACTION state.  Otherwise, a negative
           response is issued and the POP3 session remains in the
           AUTHORIZATION state.
           Note that as the length of the shared secret increases, so
           does the difficulty of deriving it.  As such, shared
           secrets should be long strings (considerably longer than
           the 8-character example shown below).

Myers & Rose [Page 14] RFC 1725 POP3 November 1994

       Possible Responses:
           +OK maildrop locked and ready
           -ERR permission denied
       Examples:
           S: +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
           C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
           S: +OK maildrop has 1 message (369 octets)
           In this example, the shared  secret  is  the  string  `tan-
           staaf'.  Hence, the MD5 algorithm is applied to the string
              <1896.697170952@dbc.mtview.ca.us>tanstaaf
           which produces a digest value of
              c4c9334bac560ecc979e58001b3e22fb

8. POP3 Command Summary

 Minimal POP3 Commands:
    USER name               valid in the AUTHORIZATION state
    PASS string
    QUIT
    STAT                    valid in the TRANSACTION state
    LIST [msg]
    RETR msg
    DELE msg
    NOOP
    RSET
    QUIT                    valid in the UPDATE state
 Optional POP3 Commands:
    APOP name digest        valid in the AUTHORIZATION state
    TOP msg n               valid in the TRANSACTION state
    UIDL [msg]
 POP3 Replies:
    +OK
    -ERR

Myers & Rose [Page 15] RFC 1725 POP3 November 1994

 Note that with the exception of the STAT, LIST, and UIDL commands,
 the reply given by the POP3 server to any command is significant only
 to "+OK" and "-ERR".  Any text occurring after this reply may be
 ignored by the client.

9. Example POP3 Session

 S: <wait for connection on TCP port 110>
 C: <open connection>
 S:    +OK POP3 server ready <1896.697170952@dbc.mtview.ca.us>
 C:    APOP mrose c4c9334bac560ecc979e58001b3e22fb
 S:    +OK mrose's maildrop has 2 messages (320 octets)
 C:    STAT
 S:    +OK 2 320
 C:    LIST
 S:    +OK 2 messages (320 octets)
 S:    1 120
 S:    2 200
 S:    .
 C:    RETR 1
 S:    +OK 120 octets
 S:    <the POP3 server sends message 1>
 S:    .
 C:    DELE 1
 S:    +OK message 1 deleted
 C:    RETR 2
 S:    +OK 200 octets
 S:    <the POP3 server sends message 2>
 S:    .
 C:    DELE 2
 S:    +OK message 2 deleted
 C:    QUIT
 S:    +OK dewey POP3 server signing off (maildrop empty)
 C:  <close connection>
 S:  <wait for next connection>

10. Message Format

 All messages transmitted during a POP3 session are assumed to conform
 to the standard for the format of Internet text messages [RFC822].
 It is important to note that the octet count for a message on the
 server host may differ from the octet count assigned to that message
 due to local conventions for designating end-of-line.  Usually,
 during the AUTHORIZATION state of the POP3 session, the POP3 server
 can calculate the size of each message in octets when it opens the
 maildrop.  For example, if the POP3 server host internally represents
 end-of-line as a single character, then the POP3 server simply counts

Myers & Rose [Page 16] RFC 1725 POP3 November 1994

 each occurrence of this character in a message as two octets.  Note
 that lines in the message which start with the termination octet need
 not be counted twice, since the POP3 client will remove all byte-
 stuffed termination characters when it receives a multi-line
 response.

11. References

 [RFC821] Postel, J., "Simple Mail Transfer Protocol", STD 10, RFC
     821, USC/Information Sciences Institute, August 1982.
 [RFC822] Crocker, D., "Standard for the Format of ARPA-Internet Text
     Messages", STD 11, RFC 822, University of Delaware, August 1982.
 [RFC1321] Rivest, R. "The MD5 Message-Digest Algorithm", RFC 1321,
     MIT Laboratory for Computer Science, April, 1992.

12. Security Considerations

 It is conjectured that use of the APOP command provides origin
 identification and replay protection for a POP3 session.
 Accordingly, a POP3 server which implements both the PASS and APOP
 commands must not allow both methods of access for a given user; that
 is, for a given "USER name" either the PASS or APOP command is
 allowed, but not both.
 Further, note that as the length of the shared secret increases, so
 does the difficulty of deriving it.
 Servers that answer -ERR to the USER command are giving potential
 attackers clues about which names are valid
 Use of the PASS command sends passwords in the clear over the
 network.
 Use of the RETR and TOP commands sends mail in the clear over the
 network.
 Otherwise, security issues are not discussed in this memo.

13. Acknowledgements

 The POP family has a long and checkered history.  Although primarily
 a minor revision to RFC 1460, POP3 is based on the ideas presented in
 RFCs 918, 937, and 1081.
 In addition, Alfred Grimstad, Keith McCloghrie, and Neil Ostroff
 provided significant comments on the APOP command.

Myers & Rose [Page 17] RFC 1725 POP3 November 1994

14. Authors' Addresses

 John G. Myers
 Carnegie-Mellon University
 5000 Forbes Ave
 Pittsburgh, PA 15213
 EMail: jgm+@cmu.edu
 Marshall T. Rose
 Dover Beach Consulting, Inc.
 420 Whisman Court
 Mountain View, CA  94043-2186
 EMail: mrose@dbc.mtview.ca.us

Myers & Rose [Page 18]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1725.txt · Last modified: 1994/11/21 22:30 by 127.0.0.1

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki