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.TH POPA3D 8 "2 March 2003" "Openwall Project" "System Administration"
.SH NAME
popa3d \- Post Office Protocol (POP3) server
.SH SYNOPSIS
.B popa3d
.RB [ -D ]
.RB [ -V ]
.SH DESCRIPTION
.B popa3d
is a Post Office Protocol version 3 (POP3) server.
.PP
A POP3 server operates on local mailboxes on behalf of its remote
users.
Users can connect at any time to check their mailbox and fetch the
mail that has accumulated.
The advantage of this "pull" approach is that any user with a simple
POP3-capable mail reader program can receive mail, eschewing the need
for a full-fledged Mail Transfer Agent (MTA) and a permanent network
connection.
.PP
Note that POP3 can only be used to retrieve mail, not to send it.
To send mail, the SMTP protocol is commonly used.
.PP
For access to a mailbox through POP3, the username must be in the
password database. Additionally,
.B popa3d
does not permit null passwords and will refuse to serve mail for root
(UID 0) users.
.SH OPTIONS
.TP
.B -D
Standalone server mode.
In this mode,
.B popa3d
will become a daemon, accepting connections on the pop3 port (110/tcp)
and forking child processes to handle them.
This has lower overhead than starting
.B popa3d
from an inetd equivalent (which
.B popa3d
assumes by default) and is thus useful on busy servers to reduce load.
In this mode
.B popa3d
also does quite a few checks to significantly reduce the impact of
connection flood attacks.
.TP
.B -V
Print version information and exit.
.SH COMMANDS
A normal POP3 session progresses through three states: AUTHORIZATION,
TRANSACTION, and UPDATE.
.PP
After the TCP connection opens, the client must authenticate itself
to the server during the AUTHORIZATION state.
The following commands are supported in the AUTHORIZATION state (all
command names are case-insensitive).
.TP
.BI USER " name"
Authenticate as user
.IR name .
.TP
.BI PASS " string"
Authenticate using password
.IR string .
.TP
.B QUIT
Quit; do not enter UPDATE state.
.PP
When authorization is successful, the server enters the TRANSACTION
state.
The client can now list and retrieve messages or mark messages for
deletion.
The following commands are supported in the TRANSACTION state.
.TP
.BI DELE " msg"
Mark message for deletion.
.TP
.B LAST
Show highest message number accessed (obsolete).
.TP
\fBLIST\fR [\fImsg\fR]
List message number and size.
.TP
.B NOOP
Do nothing.
.TP
.B QUIT
Quit; enter UPDATE state.
.TP
.BI RETR " msg"
Retrieve message.
.TP
.B RSET
Clear deletion marks.
.TP
.B STAT
Return total number of messages and total size.
.TP
.BI TOP " msg n"
Show top
.I n
lines of message body.
.TP
\fBUIDL\fR [\fImsg\fR]
List message number and digest.
.PP
When the client issues the
.B QUIT
command in the TRANSACTION state, the server enters the UPDATE state.
All messages that were marked for deletion are now removed.
The server then closes the connection.
.SH BUGS
POP3 transmits passwords in plaintext and thus, if you care about
the security of your individual user accounts, should only be used
either in trusted networks or tunneled over encrypted channels.
.PP
There exist extensions to the protocol that are supposed to fix this
problem.
.B popa3d
does not support them yet, partly because this isn't going to fully
fix the problem. In fact, APOP and the weaker defined SASL mechanisms
such as CRAM-MD5 may potentially be even less secure than transmission
of plaintext passwords because of the requirement that plaintext
equivalents be stored on the server.
.SH AUTHORS
Solar Designer <solar at openwall.com>
.PP
This manual page is based heavily on the one Camiel Dobbelaar wrote
for OpenBSD.
.SH SEE ALSO
Official Internet Protocol Standard
.IR "STD 53" ,
also known as
.IR "RFC 1939" .
.PP
http://www.openwall.com/popa3d/
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