rfc1460.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

                 greatly enhanced.

                 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



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RFC 1460                          POP3                         June 1993


                 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.

               Possible Responses:
                   +OK maildrop locked and ready
                   -ERR permission denied
               Examples:
                   S: +OK POP3 server ready <[e-mail]>
                   C: APOP mrose c4c9334bac560ecc979e58001b3e22fb
                   S: +OK maildrop has 1 message (369 octets)

                 In this example, the shared secret is the string "tanstaaf".
                 Hence, the MD5 algorithm is applied to the string

                    <[e-mail]>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
           LAST
           RSET




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           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

       POP3 Replies:
           +OK
           -ERR

       Note that with the exception of the STAT command, 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 <[e-mail]>
    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>




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RFC 1460                          POP3                         June 1993


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 byte 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 client
   can calculate the size of each message in octets when it parses the
   maildrop into messages.  For example, if the POP3 server host
   internally represents end-of-line as a single character, then the
   POP3 server simply counts 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. The POP and the Split-UA model

   The underlying paradigm in which the POP3 functions is that of a
   split-UA model.  The POP3 client host, being a remote PC based
   workstation, acts solely as a client to the message transport system.
   It does not provide delivery/authentication services to others.
   Hence, it is acting as a UA, on behalf of the person using the
   workstation.  Furthermore, the workstation uses SMTP to enter mail
   into the MTS.

   In this sense, we have two UA functions which interface to the
   message transport system: Posting (SMTP) and Retrieval (POP3).  The
   entity which supports this type of environment is called a split-UA
   (since the user agent is split between two hosts which must
   interoperate to provide these functions).

                 ASIDE:  Others might term this a remote-UA instead.
                 There are arguments supporting the use of both terms.

   This memo has explicitly referenced TCP as the underlying transport
   agent for the POP3.  This need not be the case.  In the MZnet split-
   UA, for example, personal micro-computer systems are used which do
   not have IP-style networking capability [MZnet].  To connect to the
   POP3 server host, a PC establishes a terminal connection using some
   simple protocol (PhoneNet).  A program on the PC drives the
   connection, first establishing a login session as a normal user.  The
   login shell for this pseudo-user is a program which drives the other
   half of the terminal protocol and communicates with one of two
   servers.  Although MZnet can support several PCs, a single pseudo-
   user login is present on the server host.  The user-id and password



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   for this pseudo-user login is known to all members of MZnet.  Hence,
   the first action of the login shell, after starting the terminal
   protocol, is to demand a USER/PASS authorization pair from the PC.
   This second level of authorization is used to ascertain who is
   interacting with the MTS.  Although the server host is deemed to
   support a "trusted" MTS entity, PCs in MZnet are not.  Naturally, the
   USER/PASS authorization pair for a PC is known only to the owner of
   the PC (in theory, at least).

   After successfully verifying the identity of the client, a modified
   SMTP server is started, and the PC posts mail with the server host.
   After the QUIT command is given to the SMTP server and it terminates,
   a modified POP3 server is started, and the PC retrieves mail from the
   server host.  After the QUIT command is given to the POP3 server and
   it terminates, the login shell for the pseudo-user terminates the
   terminal protocol and logs the job out.  The PC then closes the
   terminal connection to the server host.

   The SMTP server used by MZnet is modified in the sense that it knows
   that it's talking to a user agent and not a "trusted" entity in the
   message transport system.  Hence, it does performs the validation
   activities normally performed by an entity in the MTS when it accepts
   a message from a UA.

   The POP3 server used by MZnet is modified in the sense that it does
   not require a USER/PASS combination before entering the TRANSACTION
   state.  The reason for this (of course) is that the PC has already
   identified itself during the second-level authorization step
   described above.

                 NOTE: Truth in advertising laws require that the author
                 of this memo state that MZnet has not actually been
                 fully implemented.  The concepts presented and proven
                 by the project led to the notion of the MZnet
                 split-slot model.  This notion has inspired the
                 split-UA concept described in this memo, led to the
                 author's interest in the POP, and heavily influenced
                 the the description of the POP3 herein.

   In fact, some UAs present in the Internet already support the notion
   of posting directly to an SMTP server and retrieving mail directly
   from a POP3 server, even if the POP3 server and client resided on the
   same host!

                 ASIDE: this discussion raises an issue which this memo
                 purposedly avoids: how does SMTP know that it's talking
                 to a "trusted" MTS entity?




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12. References

   [MZnet]   Stefferud, E., Sweet, J., and T. Domae, "MZnet: Mail
             Service for Personal Micro-Computer Systems,:
             Proceedings, IFIP 6.5 International Conference on
             Computer Message Systems, Nottingham, U.K., May 1984.

   [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", MIT
             Laboratory for Computer Science, April 1992.

13. 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.

   Otherwise, security issues are not discussed in this memo.

14. Acknowledgements

   The POP family has a long and checkered history.  Although primarily
   a minor revision to [RFC1225], 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.

15. Author's Address

   Marshall T. Rose
   Dover Beach Consulting, Inc.
   Mountain View, CA  94043-2186

   Phone: [phone]
   Fax:   [phone]

   EMail: [e-mail]
   X.500: rose, dbc, us



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