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A domain name system based list (DNSBL) is an anti-spam technique used to track IP addresses that send spam. DNSBL is a database of IP addresses that includes the addresses of known spammers, open relays, proxy servers and computers compromised by crackers and malicious code, also known as zombie computers. Emails sent from addresses included in the DNSBL are rejected from all sites that use the list.
A domain name system based list is also known as a domain name system blacklist, block list or DNS-based blackhole list.
Several DNSBL lists are published on the Internet at http://www.dnsbl.info. These lists are maintained by different individuals and groups or organizations. DNSBL lists are based on the Internet’s domain name system (DNS), which converts IP addresses into domain names. This make DNSBL lists easy to query over the Internet. Anyone can check IP addresses associated with email spam and spamming. This also gives administrators easy access to the lists, allowing them to configure mail servers to reject messages from addresses on a DNSBL.
Built-in spam filtering for mail servers is based on DNSBL lists. If email messages appear to be spam-based on the mail server’s DNSBL lists, the messages will be automatically flagged as spam or rejected. An end-user can also add a domain to the mail server’s DNSBL list by clicking a “spam” label in the email client. The sender may get a message such as “denied access due to DNSBL listing.”
Often, a DNSBL may lead email that is not spam to be filtered as such. This is known as a false positive. A DNS resource record (RR) can be used to interrogate DNSBL and organize a reverse mapping zone file. Real spam and domain names are put into the DNSBL, while legitimate domain names are put into a DNS white list (DNSWL), which contains information on companies, organizations and individuals believed to be legitimate. These lists are also maintained by individuals and organizations.