RFC 1725
Network Working Group J. Myers
Request for Comments: 1725 Carnegie Mellon
Obsoletes: RFC 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:
- 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 command
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- added warning regarding length of shared secrets with APOP
- 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
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(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
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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
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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.
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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
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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.
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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
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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:
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
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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
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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
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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:
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.
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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.
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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:
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).
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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
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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:
C:
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:
S: .
C: DELE 1
S: +OK message 1 deleted
C: RETR 2
S: +OK 200 octets
S:
S: .
C: DELE 2
S: +OK message 2 deleted
C: QUIT
S: +OK dewey POP3 server signing off (maildrop empty)
C:
S:
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
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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.
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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
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