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 0×21 to 0×7E,
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:00WBw1Ph7×7
S: .
…
C: UIDL 2
S: +OK 2 QhdPYR:00WBw1Ph7×7
…
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
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
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
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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.