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I manage a few DNS servers (Bind9, Windows Server 2008 R2) that are used internally. I have always taken the time to make sure that the correct corresponding PTR records were configured for Reverse DNS Lookups, because it 1) seemed more complete to me and 2) I'm under the impression it is "best practice" to do so.

Other than for email services are there any other services that require having Reverse DNS Lookups available? What services are they?

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3 Answers 3

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PTR records are for reverse DNS and were originally designed for troubleshooting purposes.

Back in the good ol' days, normally there was a 1:1 mapping between a hostname and an IP address. With the invention of HTTP/1.1 and Host headers, virtual hosting support in mail servers, web servers etc. this is normally not as black and white.

However, being able to do a simple "host 1.2.3.4" on your IP address of choice and know what the hostname of that device is and subsequently derive what function it carriers out within your network is important. Especially for network related services (routers, mail servers, DNS servers etc.)

The main original reason for this is troubleshooting, it became used in the fight against spam as spammers tended to use poorly configured networks or home machines without reverse DNS records.

Lack of PTR records generally signifies a poorly configured network. Add in PTR records, as there are pieces of software that sometimes assume they exist and they simply make your life easier.

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Cfengine is one example of a service with requires proper A, PTR and hostname set. It checks the 3 of them to make sure hosts are who they pretend to be.

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Nearly anything that is logging connections or making access control decisions based on DNS has an option to do a reverse DNS lookup - Webservers, FTP and SSH daemons being the most commonly used examples. It is not as much of a requirement in the default configuration, but it can be made one by the administrator.

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