Real-time blackhole list

A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to stop email spamming. It is a "blacklist" of locations on the Internet reputed to send email spam. The locations consist of IP addresses which are most often used to publish the addresses of computers or networks 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 term "Blackhole List" is sometimes interchanged with the term "blacklist" and "blocklist".

A DNSBL is a software mechanism, rather than a specific list or policy. There are dozens of DNSBLs in existence, which use a wide array of criteria for listing and delisting of addresses. These may include listing the addresses of zombie computers or other machines being used to send spam, ISPs who willingly host spammers, or those which have sent spam to a honeypot system.

Since the creation of the first DNSBL in 1997, the operation and policies of these lists have been frequently controversial, both in Internet advocacy and occasionally in lawsuits. Many email systems operators and users consider DNSBLs a valuable tool to share information about sources of spam, but others including some prominent Internet activists have objected to them as a form of censorship. In addition, a small number of DNSBL operators have been the target of lawsuits filed by spammers seeking to have the lists shut down.

The first DNSBL was the Real-time Blackhole List (RBL), created in 1997, at first as a feed by Paul Vixie, and then as a DNSBL by Eric Ziegast as part of Vixie's Mail Abuse Prevention System (MAPS); Dave Rand at Abovenet was its first subscriber. The very first version of the RBL was not published as a DNSBL, but rather a list of networks transmitted via BGP to routers owned by subscribers so that network operators could drop all TCP/IP traffic for machines used to send spam or host spam supporting services, such as a website. The inventor of the technique later commonly called a DNSBL was Eric Ziegast while employed at Vixie Enterprises.

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