A DNSBL (DNS-based blackhole list/block list/DNS-based blacklist) contains the Internet and IP addresses of networks and computers that are linked to known spam generators. These lists are used to block messages originating from known spam senders. Although the DNSBL primarily relies on IP addresses, it also contains plain language domain names that make it easier to read the listing to see what domain names have been responsible for sending enough spam to make it onto the black list. There is no single DNSBL, however, as a number of organizations and networks maintain their own black listing based on individualized criteria and thresholds.

How does the DNSBL Work?

Electronic spam is defined as the receipt of unsolicited bulk email messages sent to a large number of recipients. Most email servers have built-in spam filtering, which automatically rejects or flags email messages that appear as spam based on their DNSBL. End-users can add a domain to an ISP’s DNSBL by clicking a “Spam” label on the email client interface. Network administrators for ISPs and other large organizations may also add domains or IP addresses to their black list based on network spam receipt. Once domains or IPs are added to the network DNSBL, they are blocked from communicating with computers or individuals on the respective network. Some network administrators have also started using a “white list” (DNSWL), which has a list of known “good” domains or IP addresses and prevents network spam filters from removing legitimate email.

When was the DNSBL Created?

The first DNSBL was used in 1997. It was referred to as the “Real Time Blackhole List” or the “RBL” and was used to aid in blocking email spam. ISPs and websites could use the list to determine known spam senders. The networks and ISPs using the list would drop incoming traffic from these addresses instead of forwarding the traffic to the recipients.

What are the Uses of the DNSBL?

The DNSBL’s primary use is to prevent network resources from being wasted on known spam email traffic. In order for a DNSBL to effectively function, it must have a domain name for the list, a server to post it on, and addresses to include on the list. There is no universal DNSBL, however, due to the different thresholds that organizations use to label incoming spam on their respective network(s). Although the listings are used to filter spam, some Internet activists view this as censorship.