D. J. Bernstein

The rblsmtpd program

rblsmtpd blocks mail from RBL-listed sites. It works with any SMTP server that can run under tcpserver.


     rblsmtpd opts prog
opts is a series of getopt-style options. prog consists of one or more arguments.

Normally rblsmtpd runs prog. prog is expected to carry out an SMTP conversation to receive incoming mail messages.

However, rblsmtpd does not invoke prog if it is told to block mail from this client. Instead it carries out its own limited SMTP conversation, temporarily rejecting all attempts to send a message. Meanwhile it prints one line on descriptor 2 to log its activity.

rblsmtpd drops the limited SMTP conversation after 60 seconds, even if the client has not quit by then.


Blocked clients

If the $RBLSMTPD environment variable is set and is nonempty, rblsmtpd blocks mail. It uses $RBLSMTPD as an error message for the client. Normally rblsmtpd runs under tcpserver; you can use tcprules to set $RBLSMTPD for selected clients.

If $RBLSMTPD is set and is empty, rblsmtpd does not block mail.

If $RBLSMTPD is not set, rblsmtpd looks up $TCPREMOTEIP in the RBL, and blocks mail if $TCPREMOTEIP is listed. tcpserver sets up $TCPREMOTEIP as the IP address of the remote host.


You may supply any number of -r and -a options. rblsmtpd tries each source in turn until it finds one that lists or anti-lists $TCPREMOTEIP.

If you do not supply any -r options, rblsmtpd tries an RBL source of rbl.maps.vix.com. This will be changed in subsequent versions.

RBL sources

If you want to run your own RBL source or anti-RBL source for rblsmtpd, you can use rbldns from the djbdns package.

I've heard about the following public RBL sources:

I've given up on the following RBL sources for various reasons: relays.mail-abuse.org stopped working with rblsmtpd in August 2000, because all the TXT records were removed. ``They were eliminated because the zone file is growing rather large,'' the maintainers said. This problem wouldn't occur with rbldns, because rbldns databases are much smaller than zone files. However, the people who run MAPS also have financial interests in BIND, and they refuse to use rbldns.

Temporary errors

Normally, if $RBLSMTPD is set, rblsmtpd uses a 451 error code in its limited SMTP conversation. This tells legitimate clients to try again later. It gives innocent relay operators a chance to see the problem, prohibit relaying, get off the RBL, and get the mail delivered.

However, if $RBLSMTPD begins with a hyphen, rblsmtpd removes the hyphen and uses a 553 error code. This tells legitimate clients to bounce the message immediately.

There are several error-handling options for RBL lookups:


Thanks to Andrew Richards for his comments on this documentation.