Linux – using nsupdate to delete PTR record by hostname

You can't create a PTR record for the ip address in your domain name DNS zone. The PTR record needs to be created in the rDNS zone by the owner of the netblock that encompasses your ip address. You'll need to speak to whomever you get the ip address from and ask them to create a PTR record for you.

Matching the PTR and A records makes it possible to verify the claim made in the PTR record by automated means.

If the A record isn't provided, one must go to the whois records to verify whether the PTR record accurately represents the entity in control of the IP address, a tedious manual process that's difficult to automate and is often wrong or out of date.

This is important for security reasons in many contexts. One that I'm familiar with and will give you an example for is:

Let's say you run a web site and post unique content, but you have discovered your content is being copied to other web sites, and worse, they're ranking higher than you in the search engines!

After hours of staring at your logs wondering how in the world somebody slipped a bot past your defenses, you finally notice hundreds of requests from Googlebot. But when you eventually look up one of the IP addresses, you find it registered to Bulletproof Ukraine Web Hosting and not Google. You thought you were getting indexed but instead you got played.

How do you solve this problem? Easy, you compare the PTR record to the A record. Google even recommends this approach.

This can be automated in many Web programming languages (PHP is a notable exception; you cannot do this reliably in PHP) so that a Web app can check the IP address, see that the PTR is *.google.com and then uses the A record to confirm that *.google.com matches the same IP address. If there's a mismatch somewhere, you have discovered a fake Googlebot.