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Classless Reverse DNS

Posted on 2013-06-25
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Last Modified: 2013-08-27
I have a /25 block of IP Addresses from my ISP.  Since its not a full class C I have to setup Classless Reverse DNS.  I have read a hundred articles, but am still confused and reverse DNS is not working.  I like to control my own reverse DNS, so the ISP did point my /25 Block to my DNS servers.  I still failing reverse DNS checks.  Can anyone assist me with setting up classless reverse DNS?  Thanks.
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Question by:denver218
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4 Comments
 
LVL 22

Expert Comment

by:Olaf De Ceuster
ID: 39277083
I have always been under the impression that PTR records need to be done by the ISP, because it needs to be done by whoever generates the IP in the first place.
Hope that helps,
Olaf
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Expert Comment

by:Chris Dent
ID: 39277361
How did they implement the classless delegation?

Typically an ISP would create CNAME records for each IP address in the /25 block, something like this:
1   IN CNAME   1.0/25.x.y.z.in-addr.arpa.
2   IN CNAME   2.0/25.x.y.z.in-addr.arpa.

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Then you would need to create a zone called 0/25.x.y.z.in-addr.arpa and populate it with a PTR record based on the aliases above.

The key here is understanding exactly how they've delegated it to you, then implementing your system based on that.

You can always look-up the addresses and see what gives. For example:

nslookup -q=cname <oneofyourIPs>

Let me know how you get on?

Chris
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Steve earned 500 total points
ID: 39278176
rDNS in internet IPs is controlled by the ISP they are registered to and many ISPs have no idea how to set up a delegation so its often more hassle than its worth.

Just give them the FQDNs and ask them to add them. Its pretty straightforward for most ISP even if you have a fairly big block of IPs. (how on earth did you manage to get that many?? My ISP moans if I ask for an extra 5 IPs !!)

Why do you want to host the rDNS yourself? are your FQDNs going to change regularly?

If you want to check your own rDNS:

nslookup
set type=ptr
<enter ipaddress>
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Author Closing Comment

by:denver218
ID: 39443649
Thanks
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