popa3d - Post Office Protocol (POP3) server

popa3d [-D] [-V]

popa3d is a Post Office Protocol version 3 (POP3) server.

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.

Note that POP3 can only be used to retrieve mail, not to send it. To send mail, the SMTP protocol is commonly used.

For access to a mailbox through POP3, the username must be in the password database. Additionally, popa3d does not permit null passwords and will refuse to serve mail for root (UID 0) users.

-D

Standalone server mode. In this mode, 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 popa3d from an inetd equivalent (which popa3d assumes by default) and is thus useful on busy servers to reduce load. In this mode popa3d also does quite a few checks to significantly reduce the impact of connection flood attacks.

-V

Print version information and exit.

A normal POP3 session progresses through three states: AUTHORIZATION, TRANSACTION, and UPDATE.

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).

USER name

Authenticate as user name.

PASS string

Authenticate using password string.

QUIT

Quit; do not enter UPDATE state.

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.

DELE msg

Mark message for deletion.

LAST

Show highest message number accessed (obsolete).

LIST [msg]

List message number and size.

NOOP

Do nothing.

QUIT

Quit; enter UPDATE state.

RETR msg

Retrieve message.

RSET

Clear deletion marks.

STAT

Return total number of messages and total size.

TOP msg n

Show top n lines of message body.

UIDL [msg]

List message number and digest.

When the client issues the 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.

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.

There exist extensions to the protocol that are supposed to fix this problem. 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.

Solar Designer <solar at openwall.com>

This manual page is based heavily on the one Camiel Dobbelaar wrote for OpenBSD.

Official Internet Protocol Standard STD 53, also known as RFC 1939.

http://www.openwall.com/popa3d/