Section: System Administration (8)
Updated: 2 March 2003Index
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 remoteusers.Users can connect at any time to check their mailbox and fetch themail that has accumulated.The advantage of this "pull" approach is that any user with a simplePOP3-capable mail reader program can receive mail, eschewing the needfor a full-fledged Mail Transfer Agent (MTA) and a permanent networkconnection.
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 thepassword database. Additionally,popa3ddoes not permit null passwords and will refuse to serve mail for root(UID 0) users.
- Standalone server mode.In this mode,popa3dwill become a daemon, accepting connections on the pop3 port (110/tcp)and forking child processes to handle them.This has lower overhead than startingpopa3dfrom an inetd equivalent (whichpopa3dassumes by default) and is thus useful on busy servers to reduce load.In this modepopa3dalso does quite a few checks to significantly reduce the impact ofconnection 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 itselfto the server during the AUTHORIZATION state.The following commands are supported in the AUTHORIZATION state (allcommand names are case-insensitive).
- USER name
- Authenticate as username.
- PASS string
- Authenticate using passwordstring.
- Quit; do not enter UPDATE state.
When authorization is successful, the server enters the TRANSACTIONstate.The client can now list and retrieve messages or mark messages fordeletion.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 topnlines of message body.
- UIDL [msg]
- List message number and digest.
When the client issues theQUITcommand 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 aboutthe security of your individual user accounts, should only be usedeither in trusted networks or tunneled over encrypted channels.
There exist extensions to the protocol that are supposed to fix thisproblem.popa3ddoes not support them yet, partly because this isn't going to fullyfix the problem. In fact, APOP and the weaker defined SASL mechanismssuch as CRAM-MD5 may potentially be even less secure than transmissionof plaintext passwords because of the requirement that plaintextequivalents be stored on the server.
Solar Designer <solar at openwall.com>
This manual page is based heavily on the one Camiel Dobbelaar wrotefor OpenBSD.
Official Internet Protocol StandardSTD 53
,also known asRFC 1939
- SEE ALSO
This document was created byman2html,using the manual pages.