How does one query the resolver name of a DNS server?

bitinerant

1/4/24, 4:55 PM

The Shodan search tool, for some DNS servers, shows a Resolver name value for what it finds on UDP port 53. For example, 80.82.117.140 shows Resolver name: server402.xenserve.com and 103.81.27.118 reports Resolver name: smartway-h7.

I have searched the web as well as the man pages for dig and nslookup, as well as searching the output of dig @ip_of_dns_server, but I cannot figure out how to retrieve the resolver name for a DNS server.

How can I query the resolver name myself using Linux command-line tools?

360

1 + 5

domain-name-system

bind

dig

nslookup

Patrick Mevzek

1/4/24, 9:12 PM

PTR record on the IP shows `ns1.402.xenserve.com.` as name. But how is this useful in any way? When clients configure DNS servers they obviously put their IP, not their names, otherwise there is a circular loop.

bitinerant

1/5/24, 12:07 PM

Thank you. However, this doesn't explain where Shodan gets `server402.xenserve.com`. I'm trying to figure out how to configure and verify the resolver name in BIND9 for my own server, since--useful or not--it is apparently publicly visible.

Patrick Mevzek

1/5/24, 1:29 PM

`server402.xenserve.com` does also resolve to same IP, so if someone maintains a database of forward queries results, after some time to learn data, a reverse can be found by just poking in it. Or the `PTR` record changed "recently".

bitinerant

1/5/24, 4:30 PM

I added an example above where the resolver name is not even a valid domain.

Patrick Mevzek

1/5/24, 5:43 PM

`dig -x 103.81.27.118` returns NXDOMAIN from APnic, so whatever name you get comes from "elsewhere" and not the DNS (but there is also no guarantee that a PTR record follows guidelines for a name, nor that that name does in fact resolve, and resolve to the original IP address; for most uses `PTR` records are useless today).

Score:1

Server

bitinerant

1/24/24, 10:00 AM

I finally found it. It is part of the Chaos (CH) class DNS record. For the examples given above, these queries return the information I was looking for:

dig @80.82.117.140 hostname.bind TXT CHAOS +short
dig @103.81.27.118 hostname.bind TXT CHAOS +short

These commands display "server402.xenserve.com" and "smartway-h7" respectively, the exact values that Shodan displays as "Resolver name". This is normally the hostname of the DNS server.

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