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I had configured a DNS server.But when nslookup with the IP address of the DNS server. It shows something like that"x.x.x.x.in-addr.arpa name=dns.domain.com". But when i nslookup with the dns.domain.com output is ok.

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1 Answer 1

The output of nslookup is to be expected.

When you look up an IP address for which a PTR record is set up, the result is that same IP address, with the octets printed in reverse order, ending with in-addr.arpa.

Finally, you get the corresponding fully qualified domain name set in the PTR record.

If a PTR record is not set, you will get NXDOMAIN as a lookup result for that IP address.

Example of a result of a reverse lookup with a PTR record

I do a lookup of twitter.com

kenny@computer ~ $ nslookup twitter.com
Server:     127.0.1.1
Address:    127.0.1.1#53

Non-authoritative answer:
Name:   twitter.com
Address: 199.59.148.10
Name:   twitter.com
Address: 199.59.148.82
Name:   twitter.com
Address: 199.59.150.39

I now do a reverse lookup of the first result of my previous lookup.

kenny@computer ~ $ nslookup 199.59.148.10
Server:     127.0.1.1
Address:    127.0.1.1#53

Non-authoritative answer:
10.148.59.199.in-addr.arpa  name = r-199-59-148-10.twttr.com.

Authoritative answers can be found from:

Ok, so it turns out that the folks at twitter set up a PTR record for that IP address (note that the octets of the IP in the output are reversed, beginning with 10 and ending with 199) that points to r-199-59-148-10.twttr.com.

Example of a result of a reverse lookup without a PTR record
Let's do the same thing for this website, stackoverflow.com

kenny@computer ~ $ nslookup stackoverflow.com
Server:     127.0.1.1
Address:    127.0.1.1#53

Non-authoritative answer:
Name:   stackoverflow.com
Address: 69.59.197.21  

The result is only one IP address, let's do a reverse lookup on that.

kenny@computer ~ $ nslookup 69.59.197.21
Server:     127.0.1.1
Address:    127.0.1.1#53

** server can't find 21.197.59.69.in-addr.arpa.: NXDOMAIN

Again, the octets in the output have been reversed and appended with .in-addr.arpa. The result this time is not a domain name, but NXDOMAIN. No PTR record is set.

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