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I've been trying to set up rDNS so my IP could get resolved to my hostname in a program which has a built in rDNS lookup feature. I had done everything I was suggested (ask my ISP to make a PTR record for me, set up A & SRV records at my registrant's dashboard, though my ISP told me that on their side there is an another PTR record which they couldn't remove because it is necessary, but they told me that it's not likely to cause any problem since I have linked the record on my side to the particular port which that app use).

As soon as I had fixed the problem with registrant's records I've got it working, the program had been resolving my IP to the right hostname, as well as the other connected clients, but the next day, when I intentionally restarted the rDNS resolve plugin, somehow it began to only resolve me as the hostname assigned to me by the ISP (like myID01.provider.com instead of mydomain.com) even though all the connections go through the only port specified in the SRV record on my dashboard.

No mistake — I've tested it with a VPN connection, so there's no way that the server found out that he's in the same network as the client.

Here are the PTR records present on ISP's side:

my.ip.in-addr.arpa domain name pointer mydomain.com.
my.ip.in-addr.arpa domain name pointer myID01.provider.com.

Here are the records present on my domain's side

What could be the problem?

Henry
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1 Answers1

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If you have multiple PTR records for the same IP in a reverse zone, you can expect clients to arbitrarily choose which one they will use. It's not a good idea, it only causes results that vary from client to client as well as over time.

This bit does not really make any sense:

they told me that it's not likely to cause any problem since I have linked the record on my side to the particular port which that app use

A reverse lookup for IP 192.0.2.7 would be:

7.2.0.192.in-addr.arpa. IN PTR

as you can see there is no sign of any port numbers in there, and if the answer has multiple entries there is nothing to tell the client which entry to pick.

Håkan Lindqvist
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  • I seem to get it now. So there is absolutely no way to specify which records should be used unless I remove all the other entries or add more «wanted» entries so the chances of the client picking up that `myID01.provider.com.` would be lower, right? Is it completely random or it's chosen by some algorithm? Thanks. – Henry Apr 29 '20 at 11:12
  • @Henry It's undefined, may be implementation specific. The correct solution is to only have the record that you actually want. – Håkan Lindqvist Apr 29 '20 at 12:47