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POPA3D(8)               OpenBSD System Manager's Manual              POPA3D(8)

NAME
     popa3d - Post Office Protocol (POP3) server

SYNOPSIS
     popa3d [-46DV]

DESCRIPTION
     popa3d is a POP3 server.  It offers network access to user mailboxes
     through the POP3 protocol.  The server uses the TCP protocol and listens
     at the port specified in the ``pop3'' service specification; see
     services(5).

     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; see sendmail(8).

     The options are as follows:

     -4      In standalone mode (-D), listen to IPv4 only.

     -6      In standalone mode (-D), listen to IPv6 only.

     -D      With this option set, popa3d will detach and become a daemon, ac-
             cepting connections on the pop3 port and forking child processes
             to handle them.  This has lower overhead than starting popa3d
             from inetd(8) 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      Show version information and exit.

     Alternatively, popa3d can be used through inetd(8).  This requires the
     following entry to be activated in /etc/inetd.conf:

     pop3 stream tcp nowait root /usr/sbin/popa3d popa3d

     or, using tcpd(8) for TCP-wrappers access control:

     pop3 stream tcp nowait root /usr/libexec/tcpd /usr/sbin/popa3d

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

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 in the authorization state.  The following commands are sup-
     ported in the authorization state.  All commands are case-insensitive.

           Command        Description
           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.

           Command        Description
           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 dele-
     tion are now removed.  The server then closes the connection.

FILES
     /var/mail
             User mail spools

     /etc/hosts.allow, /etc/hosts.deny
             TCP-wrappers access controls may be defined here as described in
             hosts_access(5).  Valid if popa3d is started in daemon-mode, or
             if popa3d is running through inetd(8) and is used in combination
             with tcpd(8).

SEE ALSO
     inetd(8), sendmail(8)

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

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

HISTORY
     popa3d first appeared in OpenBSD 3.0.

AUTHORS
     Solar Designer <solarATopenwall.com>

CAVEATS
     POP3 authenticates using cleartext passwords.

OpenBSD 3.6                     August 15, 2001                              2