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Network Working Group                                           J. MyersRequest for Comments: 1725                               Carnegie MellonObsoletes:1460                                                  M. RoseCategory: Standards Track                   Dover Beach Consulting, Inc.                                                           November 1994Post Office Protocol - Version 3Status 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 toRFC 1460, a Draft Standard.  It makes the   following changes from that document:      - removed text regarding "split-UA model", which didn't add        anything to the understanding of POP      - clarified syntax of commands, keywords, and arguments      - clarified behavior on broken connection      - explicitly permitted an inactivity autologout timer      - clarified the requirements of the "exclusive-access lock"      - removed implementation-specific wording regarding the parsing of        the maildrop      - allowed servers to close the connection after a failed        authentication command      - removed the LAST command      - fixed typo in example of TOP command      - clarified that the second argument to the TOP command is non-        negative      - added the optional UIDL commandMyers & Rose                                                    [Page 1]

RFC 1725                          POP3                     November 1994      - added warning regarding length of shared secrets with APOP      - added additional warnings to the security considerations section1. 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 responsesMyers & 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 enterMyers & 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 theMyers & 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 off5. 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 stateMyers & 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 stateMyers & 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: noneMyers & 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: noneMyers & 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 blankMyers & 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                c4c9334bac560ecc979e58001b3e22fb8. 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      -ERRMyers & 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 countsMyers & 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,RFC821, 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 toRFC 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 199414. 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.usMyers & Rose                                                   [Page 18]