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Is reverse DNS part of the DNS protocol, i.e. part of the official DNS specification?

Is reverse DNS managed by standard DNS servers and a type of request that DNS servers understand and respond to, or is it additionally functionality on the client side where extra work is done to resolve the domain of an IP?

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    Yes. The only client side "extra work" is knowing it's an address and formatting the question accordingly. (d.c.b.a.in-addr.arpa. PTR record)
    – Ricky
    Jul 10 '14 at 17:08
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    Yes (as Ricky has said) however reverse DNS queries are just a different type of record. If you look at A records for example and then PTR records you will see that "forward" and "reverse" DNS requests are just queries for two differnt record types (at a basic level) so there isn't any differnce.
    – jwbensley
    Jul 10 '14 at 17:51
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Reverse DNS is part of the DNS protocol official specification see RFC 1035 Section 6.4 specifies Inverse queries

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  • This answer is vague not massively helpful, you could edit it to be more informative? :)
    – jwbensley
    Jul 10 '14 at 17:52
  • What's vague about it? RFC1035 is "the official DNS specification"
    – Ricky
    Jul 10 '14 at 18:30
  • @RickyBeam I have to agree with jwbensley; any monkey can go on Google and search "DNS RFC". IMO, your comment to the OP was more informative.
    – Ryan Foley
    Jul 10 '14 at 19:52
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    Only that the OP is asking for "reverse DNS" not "inverse queries". These are two different things. Jul 10 '14 at 20:56
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    @hrtednrup Inverse queries are no longer used and have been obsoleted in RFC 3425. Reverse DNS uses PTR records and the in-addr.arpa domain, and is specified in section 3.5 of RFC 1035.
    – Gerben
    Jul 10 '14 at 21:25
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Reverse DNS for IPv4 is specified in RFC1035 Section 3.5. The special in-addr.arpa domain was created for this. For IPv6 the special domain is ip6.arpa as specified in RFC3596. The record type that gets requested for reverse lookup is the PTRrecord.

For IPv4 the IP address is reversed on the "dot" boundaries after every byte:

192.168.0.1 gets transformed into 1.0.168.192.in-addr.arpa.

For IPv6 the address gets reversed after every 4-bit "nibble" (every number/letter) with a dot inserted:

2001:db8::dead:beef gets transformed into

f.e.e.b.d.a.e.d.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa.

For DNS servers there is no extra work involved, every DNS server software understands/supports PTR records. The reverse lookup on the client is done by the application(s) you use. There are standard system calls for reverse lookup on every OS.

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