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Reverse DNS

A reverse DNS look up is a DNS query for the domain name that is associated with a specific IP address. It's the opposite of the more commonly used DNS lookup where the DNS system is queried to return an IP address. So, for example, a DNS lookup for returns Google's IP address, while a reverse DNS lookup on the given IP address will return Google’s domain name.

A reverse DNS lookup queries DNS servers for a PTR, or pointer record. If the server does not have a PTR record, the lookup will fail. PTR records store IP addresses with their segments reversed and they append ‘’ to the end of the IP address. IPv6, the latest version of the Internet protocol, stores PTR records by appending ‘’. 

The standards of the Internet Engineering Task Force (the IETF), suggests that every domain name should be capable of a reverse DNS lookup, but because they're not important to the normal functioning of the Internet they are not technically required. For this reason, DNS lookups are not adopted by all sites, although they may be important for some.     

Why Is Reverse DNS Important?

Because one of the uses of reverse DNS lookup is by email servers to verify that the sending server is not a malicious spammer, it's important for businesses to make sure that their server has a PTR record. This is because, if it doesn't, the email server won't find the reverse DNS record and the spam filter will reject the email. This is especially important for businesses that use email as an integral part of their business operations like marketing firms or B2B businesses. Even if it does have a PTR record, businesses should still follow the appropriate warming techniques to ensure the most effective email delivery.     

What Is Reverse DNS Used For?

As stated, reverse DNS lookups are typically used by email servers to find spam emails. This is because the IP address alone makes it difficult to differentiate legitimate mail servers from spam mail servers. Reverse DNS lookups can be helpful here because they can provide clues that the mail server is a spam server. 

They’re also used by businesses to find out who is visiting their website. Website logs usually contain IP addresses that aren't very useful for tracking exactly who is visiting a website. The reverse DNS look up process can then be used to find the host name of the visitor to a website and give businesses a better idea of who their users are. As an alternative, logging software that employs reverse lookups, can be used to provide users with human-readable domains in their log data.    

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