To get domain name from ip in Reverse DNS Dig

dns

dig www.ksu.edu.tw +short
120.114.100.101

We get ip address :120.114.100.101 ,whose domain name is www.ksu.edu.tw .
To get domain name from ip in Reverse DNS Dig

dig -x 120.114.100.101 +short
www.ksu.edu.tw.

We get domain name : www.ksu.edu.tw ,whose ip address is 120.114.100.101. 

dig  smtp.mail.yahoo.com  +short
smtp.mail.global.gm0.yahoodns.net.
106.10.150.171

We get ip address :106.10.150.171 ,whose domain name is smtp.mail.yahoo.com. 

dig  -x 106.10.150.171  +short
vcs-s-myc.mail.vip.sg3.yahoo.com.

why the result is not smtp.mail.yahoo.com as 106.10.150.171's domain name?

Best Answer

The simpliest answer: because it's done so

You can create ptr to put to any dns name.

Do not need to follow the pattern:

A record --A-resolving--> ip address
ip address --PTR-resolving--> A record

You can do so, but do not have to. You just need to have a PTR record.

The full answer:

Some mail servers (public especially) have more than one domain. But you cannot create several PTR record to single ip*.

* see comments below

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