A DNS-based Blackhole List (DNSBL) or Real-time Blackhole List (RBL) is an effort to stop email spamming.
In 2003, a number of DNSBLs came under denial-of-service attacks.
Since no party has admitted to these attacks nor been discovered responsible, their purpose is a matter of speculation.
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.
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".
However, many observers believe the attacks are perpetrated by spammers in order to interfere with the DNSBLs' operation or hound them into shutting down.
In August 2003, the firm Osirusoft, an operator of several DNSBLs including one based on the SPEWS data set, shut down its lists after suffering weeks of near-continuous attack.
Technical specifications for DNSBLs came relatively late in RFC5782.
A URI DNSBL is a DNSBL that lists the domain names and sometimes also IP addresses which are found in the "clickable" links contained in the body of spams, but generally not found inside legitimate messages.
The term "blackhole" refers to a networking black hole, an expression for a link on a network that drops incoming traffic instead of forwarding it normally.