POP3 Commands - RFC 1939 - Post Office Protocol - Version 3
Network Working
Group
J. Myers
Request for Comments:
1939
Carnegie Mellon
STD:
53 M.
Rose
Obsoletes:
1725
Dover Beach Consulting, Inc.
Category: Standards
Track
May 1996
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.
Table of
Contents
1. Introduction
................................................ 2
2. A Short Digression
.......................................... 2
3.
Basic Operation
............................................. 3
4.
The AUTHORIZATION State
..................................... 4
QUIT Command
................................................ 5
5. The TRANSACTION State
....................................... 5
STAT Command
................................................ 6
LIST Command
................................................ 6
RETR Command
................................................ 8
DELE Command
................................................ 8
NOOP Command
................................................ 9
RSET Command
................................................ 9
6. The UPDATE State ............................................
10
QUIT Command
................................................ 10
7.
Optional POP3 Commands ...................................... 11
TOP Command
................................................. 11
UIDL Command
................................................ 12
USER Command
................................................ 13
PASS Command
................................................ 14
APOP Command
................................................ 15
8.
Scaling and Operational Considerations ...................... 16
9. POP3 Command Summary
........................................ 18
10. Example
POP3 Session ....................................... 19
11. Message Format .............................................
19
12. References
................................................. 20
13.
Security Considerations .................................... 20
14. Acknowledgements
........................................... 20
15.
Authors' Addresses ......................................... 21
Appendix A. Differences from RFC 1725
.......................... 22
Appendix B. Command
Index ...................................... 23
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 protocol
is
used to allow a workstation to retrieve mail that the server is
holding for it.
POP3 is not intended to provide
extensive manipulation operations of
mail on the server;
normally, mail is downloaded and then deleted. A
more
advanced (and complex) protocol, IMAP4, is discussed in
[RFC1730].
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 and sends all mail to it. This relay host could
be, but need not be, the POP3 server host for the
client host. Of
course, the relay host must
accept mail for delivery to arbitrary
recipient
addresses, that functionality is not required of all
SMTP servers.
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
(respectively) until the
connection is closed or aborted.
Commands in the POP3
consist of a case-insensitive 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. Responses may be up to 512 characters
long,
including the terminating CRLF. There are currently two status
indicators: positive ("+OK") and negative
("-ERR"). Servers MUST
send the
"+OK" and "-ERR" in upper case.
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 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 server MUST respond to an unrecognized, unimplemented, or
syntactically invalid command by responding with a negative
status
indicator. A server MUST respond to a command
issued when the
session is in an incorrect state by responding
with a negative status
indicator. There is no general
method for a client to distinguish
between a server which does
not implement an optional command and a
server which is
unwilling or unable to process the command.
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
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 positive
response. An example might be:
S: +OK POP3 server ready
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. Both
mechanisms are
described later in this document. Additional
authentication mechanisms are described in [RFC1734]. While there
is
no single authentication mechanism that is required of all
POP3
servers, a POP3 server must of course support at least
one
authentication mechanism.
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
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
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 nth message
in a maildrop is assigned a
message-number of "n". In POP3 commands
and responses, all message-numbers and message sizes are expressed in
base-10 (i.e., decimal).
Here is the summary
for the QUIT command when used in the
AUTHORIZATION state:
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
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 are
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.
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. If there are no
messages in the maildrop, then the
POP3 server responds
with no
scan listings--it issues a positive response
followed by a line containing a
termination octet and a
CRLF
pair.
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. Methods
for calculating the exact
size
of the message are described in the "Message Format"
section below. 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
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
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
Restrictions: none
Discussion:
The POP3 server removes all
messages marked as deleted
from
the maildrop and replies as to the status of this
operation. If there is an
error, such as a resource
shortage, encountered while removing messages, the
maildrop may result in having some
or none of the messages
marked
as deleted be removed. In no case may the server
remove any messages not marked as
deleted.
Whether the
removal was successful or not, the server
then releases any exclusive-access
lock on the maildrop
and closes
the TCP connection.
Possible
Responses:
+OK
-ERR some deleted messages not
removed
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
of lines (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
line
separating the headers from the body, and then the
number of lines of the 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 (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 "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.
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
one to 70 characters in the range 0x21
to 0x7E, which uniquely identifies a
message within a
maildrop and which
persists across sessions. This
persistence is required even if a session
ends without
entering the UPDATE
state. 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.
While
it is generally preferable for server implementations
to store arbitrarily assigned unique-ids
in the maildrop,
this
specification is intended to permit unique-ids to be
calculated as a hash of the
message. Clients should be able
to
handle a situation where two identical copies of a
message in a maildrop have the same
unique-id.
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
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
Discussion:
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.
The server may return a positive
response even though no
such
mailbox exists. The server may return a negative
response if mailbox exists, but
does not permit plaintext
password authentication.
Possible
Responses:
+OK name is a valid
mailbox
-ERR never heard of
mailbox name
Examples:
C: USER frated
S: -ERR sorry, no mailbox for
frated here
...
C: USER mrose
S: +OK mrose is a real hoopy
frood
PASS string
Arguments:
a server/mailbox-specific password
(required)
Restrictions:
may only be given in the
AUTHORIZATION state immediately
after a successful USER command
Discussion:
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.
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.
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: -ERR maildrop already locked
...
C: USER mrose
S: +OK mrose is a real hoopy
frood
C: PASS secret
S: +OK mrose's maildrop has 2
messages (320 octets)
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 or after an unsuccessful
USER or PASS command
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.
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).
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. Scaling and Operational Considerations
Since
some of the optional features described above were added to the
POP3 protocol, experience has accumulated in using them in
large-
scale commercial post office operations where most of
the users are
unrelated to each other. In these
situations and others, users and
vendors of POP3 clients have
discovered that the combination of using
the UIDL command and
not issuing the DELE command can provide a weak
version of the
"maildrop as semi-permanent repository" functionality
normally associated with IMAP. Of course the other
capabilities of
IMAP, such as polling an existing
connection for newly arrived
messages and supporting multiple
folders on the server, are not
present in POP3.
When these facilities are used in this way by casual users, there
has
been a tendency for already-read messages to accumulate on
the server
without bound. This is clearly an undesirable
behavior pattern from
the standpoint of the server
operator. This situation is aggravated
by the fact that
the limited capabilities of the POP3 do not permit
efficient
handling of maildrops which have hundreds or thousands of
messages.
Consequently, it is recommended that operators
of large-scale multi-
user servers, especially ones in which
the user's only access to the
maildrop is via POP3, consider
such options as:
* Imposing a per-user maildrop
storage quota or the like.
A disadvantage to
this option is that accumulation of messages may
result in the user's inability to receive new ones
into the
maildrop. Sites which choose this
option should be sure to inform
users of impending
or current exhaustion of quota, perhaps by
inserting an appropriate message into the user's
maildrop.
* Enforce a site policy regarding mail
retention on the server.
Sites are free to
establish local policy regarding the storage and
retention of messages on the server, both read and
unread. For
example, a site might delete
unread messages from the server after
60 days and
delete read messages after 7 days. Such message
deletions are outside the scope of the POP3 protocol
and are not
considered a protocol violation.
Server operators enforcing message deletion
policies should take
care to make all users aware
of the policies in force.
Clients must not
assume that a site policy will automate message
deletions, and should continue to explicitly delete
messages using
the DELE command when
appropriate.
It should be noted that
enforcing site message deletion policies
may be
confusing to the user community, since their POP3 client
may contain configuration options to leave mail on the
server
which will not in fact be supported by the
server.
One special case of a site policy is
that messages may only be
downloaded once from the
server, and are deleted after this has
been
accomplished. This could be implemented in POP3 server
software by the following mechanism:
"following a POP3 login by a
client which
was ended by a QUIT, delete all messages downloaded
during the session with the RETR
command". It is important not to
delete messages in the event of abnormal connection
termination
(ie, if no QUIT was received from the
client) because the client
may not have
successfully received or stored the messages.
Servers implementing a download-and-delete policy may
also wish to
disable or limit the optional TOP
command, since it could be used
as an alternate
mechanism to download entire messages.
9. 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
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
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.
10. 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>
11. 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
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 (and must not)
be counted twice, since the POP3 client will
remove all
byte-stuffed termination characters when it receives a
multi-line response.
12. 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.
[RFC1730]
Crispin, M., "Internet Message Access Protocol - Version
4", RFC 1730, University of Washington,
December 1994.
[RFC1734] Myers, J., "POP3
AUTHentication command", RFC 1734,
Carnegie Mellon, December 1994.
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 should not allow both methods of access
for a given user;
that is, for a given mailbox name, either
the USER/PASS command
sequence or the 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.
14. 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.
15. 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
Appendix A. Differences from RFC 1725
This memo
is a revision to RFC 1725, a Draft Standard. It makes the
following changes from that document:
-
clarifies that command keywords are case insensitive.
- specifies that servers must send
"+OK" and "-ERR" in
upper case.
-
specifies that the initial greeting is a positive response,
instead of any string which should be a positive
response.
- clarifies behavior for
unimplemented commands.
- makes the USER and
PASS commands optional.
- clarified the set
of possible responses to the USER command.
-
reverses the order of the examples in the USER and PASS
commands, to reduce confusion.
- clarifies that the PASS command may only be given
immediately
after a successful USER
command.
- clarified the persistence
requirements of UIDs and added some
implementation notes.
- specifies a UID length limitation of one to 70
octets.
- specifies a status indicator length
limitation
of 512 octets, including the
CRLF.
- clarifies that LIST with no arguments
on an empty mailbox
returns success.
- adds a reference from the LIST command to the
Message Format
section
- clarifies the behavior of QUIT upon failure
- clarifies the security section to not imply the use
of the
USER command with the APOP command.
- adds references to RFCs 1730 and 1734
- clarifies the method by which a UA may enter mail
into the
transport system.
- clarifies that the second argument to the TOP
command is a
number of lines.
- changes the suggestion in the Security
Considerations section
for a server to not
accept both PASS and APOP for a given user
from a "must" to a
"should".
- adds a section
on scaling and operational considerations
Appendix B. Command
Index
APOP
....................................................... 15
DELE
....................................................... 8
LIST
....................................................... 6
NOOP
....................................................... 9
PASS
....................................................... 14
QUIT
....................................................... 5
QUIT
....................................................... 10
RETR
....................................................... 8
RSET
....................................................... 9
STAT
....................................................... 6
TOP
........................................................ 11
UIDL
....................................................... 12
USER
....................................................... 13
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