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.