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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?

showkey
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  • Most probably an issue with the DNS issue you have set up in resolv.conf. There is no issue when I try it and there is no 106.10.150.171 reported. You can conform this with "dig 8.8.8.8 smtp.mail.yahoo.com +short" – Raman Sailopal Sep 29 '17 at 12:23

1 Answers1

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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

Egor Vasilyev
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    You can have multiple records for one IP address. Probably useless (as the few clients using PTR records will not expect multiple records) but totally legit. – Patrick Mevzek Sep 30 '17 at 01:51
  • @Patrick Mevzek you are right. But simplest answer for best understanding - one ptr-record for single ip address. Multiple PTRs are technically possible - this is true. But your ISP may prohibit the creation of multiple ptrs on single address (my real experience). Thanx for your comment! – Egor Vasilyev Sep 30 '17 at 09:24