GCE custom hostname reverse lookup in private DNS zones - dns

Question

I want to create an instance in Google Cloud Engine with a custom (private) hostname. For that reason, when creating the instance from the Console (or from an SDK) I supply the hostname, or example instance0.custom.hostname.
The instance is created and the search domain is set correctly in /etc/resolv.conf For Ubuntu in particular I have to set the hostname with hostnamectl but it is irrelevant to the question.
Forward DNS lookups work as normal for instance0.custom.hostname. The problem comes when I do a reverse lookup for the private IP address of the instace. In that case the answer I get is the GCE "long" name instead of my custom hostname.
How can I make the reverse lookup reply with my custom name instead of the GCE?
I know in Azure you can use a Private DNS Zone with VM auto-registration to handle the "custom hostnames". I tried using a private zone with Google Cloud DNS (PTR records) but with no luck.

Answer 1

After some serious digging I found a solution and tested it.
Reverse DNS works even without a "regular" DNS records for your custom.hostname domain.
To get reverse dns working lets assume your VM's in 10.128.0.0/24 network.
Their IP's are 24,27,54,55 as in my example.
I created a private dns zone and named it "my-reverse-dns-zone" - the name is just for information and can be anything.
"DNS name" field however is very important. Since my network address starts with 10 I want all the instances that are created in that network segment to be subject to reverse dns. So the DNS name has to be 10.in-addr.arpa in this case. If you're using 192.168.... or 172.16.... then adjust everything accordingly.
If you wanted just 10.128.0 then you can put 0.128.10.in-addr.arpa. Then you select the VPC networks zone has to be visible in and voila:
Then you add the PTR records that will allow this to work. I'm setting all TTL's to 1 minute to shorten the wait :)
After accepting wait a minute (literally) and test it:
dig -x 10.128.0.24
; <<>> DiG 9.11.5-P4-5.1+deb10u6-Debian <<>> -x 10.128.0.24
;; global options: +cmd
;; Got answer:
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 35229
;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL: 1
;; OPT PSEUDOSECTION:
; EDNS: version: 0, flags:; udp: 512
;; QUESTION SECTION:
;24.0.128.10.in-addr.arpa. IN PTR
;; ANSWER SECTION:
24.0.128.10.in-addr.arpa. 60 IN PTR instance0.custom.hostname.
;; Query time: 6 msec
;; SERVER: 169.254.169.254#53(169.254.169.254)
;; WHEN: Mon Jan 31 13:35:57 UTC 2022
;; MSG SIZE rcvd: 92
Done !
You can even put some completely other domain for one of the IP's. Have a look at my zone configuration:
dig -x 10.128.0.55 | grep PTR
;55.0.128.10.in-addr.arpa. IN PTR
55.0.128.10.in-addr.arpa. 60 IN PTR b2.example.com.
There's a similar question & answer here.
To have a better (technical) understanding of how this works have a look at PTR records in private zones documenation and about PTR records and how they work in the internal GCP's DNS.