Basic Understanding of DNS Blacklists

 It is one of the most efficient ways and spam filtering techniques widely used on mail servers to block unwanted emails for anti-spam countermeasures.

DNS Blacklist are used to publish lists of Internet Protocol (IP) addresses of known spammers, abuses, open relays, proxy servers through the DNS (a.k.a. Domain Name System, Domain Name Server or Domain Name Service) that can be queried by software on the Internet. Once it's configured, it can automatically reject or flag email messages as spam, if the sender's IP address is listed on one or more DNS Blacklists.

Each DNS Blacklist has its own criteria to remove any of the listed IP addresses in their database but in most cases have the same way of interrogating the list. As a subscriber to the DNS Blacklists, it is advised to examine their rules and policies before using them.

How Does It Work?

It uses the DNS to transmit information to check if mails should be accepted or rejected.

For example, server A send mail to server B, server B analyze the server A IP address information and execute query against a list of chosen domain name system blacklist servers. Let's say if server A IP address is "122.252.246.234" append the domain name to become "234.246.252.122.dnsbl.example.com" with octets listed in reverse order. If it resolves, the IP address has been blacklisted or is not, then it isn't blacklisted.

What is the Disadvantage?

Majority of DNS Blacklist services available out there either not bother with maintaining, server goes down frequently or have been retired. You are advised to take a turn on various search engines to find and test out whether or not it will function properly before putting any of the DNS query zone into your list. Also note that some of them contain a higher false positive (email messages believed to be legitimate being filtered as spam) rate than others that may have resulted in innocent IP addresses being blocked accidentally. This may lead to a situation where your mail server will reject mails you would like to receive. The other problem is that you must take the right decisions to accept or reject mail from the sender.

The main purpose of using DNS Blacklist filters is to prevent network resources wasted on unsolicited junk mail traffic. In short, DNS Blacklist can help keep you away from spammers.


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