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i have a VPS hosted with myhosting.com, the domain is hosted in godaddy and the DNS records are managed in AMAZON route 53. i was told by myhosting support that i should create a PTR record on my DNS but i have read that in order to reverse DNS for an IP this record should be created in the ISP records wich sounds have sense.

because if i want to get the domain name via an IP the request will never be forwarded to AMAZON instead it will ask the ISP for it

am i rigth or MyHosting support is correct and i should setup PTR record on Amazon which i already did

THANKS Oscar

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1 Answer

This is common if you have a small subnet delegated from your ISP. It's called RFC 2317 delegation or something like that.

Many ISPs will have you create a PTR record, under your domain (i.e. oscar.com which you have control over), and put a CNAME in their reverse zone (i.e. 0.168.192.in-addr.arpa which the ISP has control over).

For example, for an IP address of 221.222.223.15, the reverse record would be 15.223.222.221.in-addr.arpa. A reverse lookup would look for that record from the owner of the IP address (in your case, myhosting.com or their parent ISP).

The ISP would normally have a record like

    15.223.222.221.in-addr.arpa IN PTR net223ip15.myhosting.com

But in the cname delegation method, they have something like this:

    15.223.222.221.in-addr.arpa IN CNAME 15.oscar.223.222.221.oscar.com

Then you create a record in your zone like this:

    15.oscar.223.222.221.oscar.com IN PTR www.oscar.com

Then folks will look up 221.222.223.15 and follow the CNAME from 15.223.222.221.in-addr.arpa to 15.oscar.223.222.221.oscar.com to www.oscar.com.

I've never had this done for a single IP, but I've had several ISPs do something like this for routed subnets.

Check with myhosting.com to see if they have a preference or specification for the record. But I think this is the general story behind the delegation.

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Do you know how this approach effects email deliverability vs using just plain PTR records without the CNAMEs? –  Drew Mar 6 '12 at 19:31
    
In my experience, it has not impacted it. I've had this sort of delegation from small local ISPs and "tier 1" ISPs/NSPs, and as the CNAME method is an "Internet Standard" more or less (as much as SMTP or the other RFC implementations are standards), abiding by it should not negatively impact your deliverability. Note, if your forward zone's DNS is slow, unreliable, or down... it may very well destroy your deliverability. –  Robert Novak Mar 7 '12 at 23:29
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