Ubuntu Manpage: popa3d - Post Office Protocol (POP3) server

NAME

       popa3d - Post Office Protocol (POP3) server

SYNOPSIS

       popa3d [-D] [-V]

DESCRIPTION

       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.

OPTIONS

       -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.

COMMANDS

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.

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.

       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.

AUTHORS

       Solar Designer <solar at openwall.com>

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