Spam and RBLDNS

1. In order to reduce spamming through our smtp servers, I have installed rbldns and rblsmtpd to do a first level spam filtering on my mail servers. I have been using qmail for all my mail servers.

2. How does this work? Many spammers use open-relays, vulnerable hosts on dial-ups or broadbands to spam. For example, these hosts will need to connect to my smtp server. When rblsmtpd is used, this smtp server will actually query some "blacklist" server to check if the connecting host is a "blacklisted" spamming host. If it is, connection is disallowed.

The querying of the blacklist is based on the DNS protocol. One can use freely available servers like relays.ordb.org. The alternative is the run your own server. rbldns is a simple IP dns server to do this. One of the reason that you may want to run your "blacklist" or "whitelist" server, is that your legitimate mail sender hosts can be easily identified.

3. The following are links to the applications
rbldns
rblsmtpd


4. Here is a patch by John M. Simpson that helps to generate the data files.
rbldns-patch