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NAME    [Toc]    [Back]

     popa3d - Post Office Protocol (POP3) server

SYNOPSIS    [Toc]    [Back]

     popa3d [-46DV]

DESCRIPTION    [Toc]    [Back]

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

     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, accepting
  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
             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

     For  access to a mailbox through the POP3 service, 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.

COMMANDS    [Toc]    [Back]

     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 supported
  in  the authorization state.  All commands are caseinsensitive.

           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
     state.   The  client  can  now list and retrieve messages or
mark messages
     for deletion.  The following commands are supported  in  the

           Command        Description
           DELE msg       mark message for deletion
           LAST            show  highest  message number accessed
           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.

FILES    [Toc]    [Back]

             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
             with tcpd(8).

SEE ALSO    [Toc]    [Back]

     inetd(8), sendmail(8)

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


HISTORY    [Toc]    [Back]

     popa3d first appeared in OpenBSD 3.0.

AUTHORS    [Toc]    [Back]

     Solar Designer <solar@openwall.com>

CAVEATS    [Toc]    [Back]

     POP3 authenticates using cleartext passwords.

OpenBSD     3.6                         August      15,      2001
[ Back ]
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