It is one of the most efficient ways and widely used spam filtering techniques in mail servers to block spam emails for antispam countermeasures.
DNS Blacklist are used to publish lists of Internet Protocol (IP) addresses of known spammers, abuses, open relays, proxy servers through the DNS (aka Domain Name System, Domain Name Server or Domain Name Service) that can be queried by software on the Internet. Once configured, it can automatically reject or mark email messages as spam, if the sender’s IP address is on one or more DNS blacklists.
Each DNS blacklist has its own criteria for removing any of the IP addresses listed in their database, but in most cases they have the same way of interrogating the list. As a subscriber to DNS blacklists, it is recommended that you review their rules and policies before using them.
How does it work?
It uses the DNS to transmit information to verify if the mails should be accepted or rejected.
For example, server A sends mail to server B, server B parses server A’s IP address information and runs a query against a list of chosen domain name system blacklist servers. Let’s say if Server A’s IP address is “122.252.246.234”, add the domain name so that it becomes “234.246.252.122.dnsbl.example.com” with the octets numbered in reverse order. If it resolves whether the IP address has been blacklisted or not, then it is not blacklisted.
What is the downside?
Most of the DNS blacklisting services available don’t bother to maintain them, the server goes down frequently, or they have been taken down. It is recommended that you try various search engines to find and test whether or not they will work correctly before including any of the DNS query zones in your list. Also note that some of them contain a higher rate of false positives (email messages that are believed to be legitimate and are filtered as spam) than others which may have resulted in the accidental blocking of innocent IP addresses. This can lead to a situation where your mail server rejects the mails you would like to receive. The other problem is that you have to make the right decisions to accept or reject the mail from the sender.
The main purpose of using DNS Blacklist filters is to prevent network resources from being wasted on unsolicited spam traffic. In short, DNS Blacklist can help you stay away from spammers.
