DNSBL

A DNS-based Blackhole List, or DNSBL, is a means by which an Internet site may publish a list of IP addresses, in a format which can be easily queried by computer programs on the Internet. As the name suggests, it is built on top of the Internet DNS or Domain Name System. DNSBLs are chiefly used to publish lists of addresses linked to spamming. Most mail server software can be configured to reject or flag messages which have been sent from a site listed on one or more such lists.

The first DNSBL was the Real-time Blackhole List (RBL), created in 1997 by Paul Vixie as part of his Mail Abuse Prevention System (MAPS). Vixie, an influential Internet programmer and administrator, encouraged the authors of sendmail and other mail software to implement RBL clients. These allowed the mail software to query the RBL and reject mail from listed sites. However, the purpose of the RBL was not simply to block spam -- it was to educate Internet service providers and other Internet sites about spam and related problems, such as open SMTP relays. Before an address would be listed on the RBL, volunteers and MAPS staff would attempt repeatedly to contact the persons responsible for it and get its problems corrected.

Soon after the advent of the RBL, others started developing their own lists with different policies. One of the first was Alan Brown's Open Relay Behavior-modification System (ORBS). This used automated testing to discover and list mail servers running as open mail relays -- exploitable by spammers to carry their spam. ORBS was controversial at the time because many people felt running an open relay was acceptable, and that scanning the Internet for open mail servers could be abusive.

Major events: ORBS controversy, lawsuits, RBL commercialization, ORBS breakup, ORBZ, SBL, SPEWS, RHSBLs