POPA3D(8)						       System Administration							 POPA3D(8)

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.

SEE ALSO

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

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

Openwall Project						   2 March 2003 							 POPA3D(8)