popa3d - Post Office Protocol (POP3) server
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
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.
- 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.
- Print version information and exit.
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; 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.
- Show highest message number accessed (obsolete).
- LIST [msg]
- List message number and size.
- Do nothing.
- Quit; enter UPDATE state.
- RETR msg
- Retrieve message.
- Clear deletion marks.
- 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.
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.
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.
Solar Designer <solar at openwall.com>
This manual page is based heavily on the one Camiel Dobbelaar wrote for OpenBSD.
Official Internet Protocol Standard STD 53
, also known as RFC