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Martin
Asked: 2011-05-19 01:35:16 +0800 CST

How are DNS PTR records managed?

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As I understand it, an IP address-owner can set up a DNS PTR record for his IP address to map the IP addresses to its host names. There are some things about this I don’t understand. I’ve managed other DNS records often using web based interfaces, but never seen PTR records exposed in those.

A DNS record of type CNAME or A is stored in the DNS zone for a specific domain such as example.com. Are the PTR records pointing at example.com typically stored in the same zone or are they stored in separate zones? If the PTR record is stored in the same zone as as example.com, how does other servers know to query this zone for the PTR record?

What stops me from adding a PTR record saying that <ip-address-of-my-server> resolves to, say, gmail.com? If I manage the PTR records for my IP address, shouldn't I be able to add anything?

domain-name-system reverse-dns ptr-record

2 Answers

  1. or are they stoerd in separate zones?

    Separate zones, one per old C network (last byte in the octet).

    What stops me from adding a PTR record saying that resolves to say gmail.com?

    Nothing. But as this is not used exceptt for nice pings or some email validity checks, you achieved nothing. people will still go to gmail when they type in gmail.com. All people now see is gmail.com in a traceroute, nothing else.

    The one real use for this is smtp - the HELO string given in SMTP should match the PTR record name given. Basically the server must say it is who the ptr record says it is. Note that it can still accept emails for other domains.

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  2. If you have a zonefile for example.com with:

    www.example.com  300  IN  A  16.24.32.40

    Then you would have a zonefile for 32.24.16.in-addr.arpa with:

    40.32.24.16.in-addr.arpa  300  IN  PTR  www.example.com
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