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In my business, my physical sever has an external IP of 123.123.123.123. Performing a reverse lookup on that IP will give you mail.myserver.com. I had to ask my web host to create that PTR record for me.

In my Google Cloud, I have a VM, say the IP is 200.200.200.200. In the Cloud DNS, I can add a PTR record, but the options look like this:

screen shot

The redacted part is myserver.com.

So what does this mean? I create a reverse lookup that isn't reverse at all????

When using my favourite query tool (mxtoolbox.com), the PTR does not change from the default. The command PTR:200.200.200.200 gives 123.456.789.101.bc.googleusercontent.com

I've read that VMs in Compute Engine can't have PTRs on their IPs, so what does this PTR option do???

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This is used for delegation. You would need to have your web host create a CNAME record that looks very much like a traditional PTR record (i.e. 200.200.200.200.in-addr.arpa.), but points to a PTR record in Google (i.e. the name you specify in the first input). The PTR resource record at Google would then point to an A record at Google as well (i.e. the name you specify in the second input). Then when someone tries to resolve your IP address to a PTR record, through redirection it resolves to the appropriate A record hosted by Google as expected.

PTR (pointer) records map the IP address of a host to the canonical (true) domain name for a host (address-to-name mapping). Known as reverse DNS lookup, the IP address is written in reverse and appended with the Address and Routing Parameter Area (arpa) top-level domain.

PTR records are used as a security device and anti-spam measure; mail and other types of servers can do reverse DNS lookups to verify the identities of hosts.

Usually, you will not manage PTR through Google Domains, because they need to be set by the owner of your IP address block (generally your ISP). Different IP block owners have different procedures for you to request these records. Please contact your provider for more information.

Google Domains supports PTR records that reside in the DNS zone corresponding to the your domain, for the purposes of having your ISP create CNAME records that delegate to us the responsibility for reverse lookups of those specific addresses.

If your provider delegates a PTR record to you, your provider will create a CNAME record that points to a PTR record that you manage through Google Domains. For example, suppose your server's A record looks like this: www A 1h 111.222.33.4

To delegate the PTR record to you, your provider must set the following CNAME. Note that the order of the 4 numbers comprising the IP address have been reversed: 4.33.222.111.in-addr.arpa. CNAME 1h ptr_www.example.com.

In Google Domains, you would set the following PTR record: ptr_www
PTR 1h www.example.com.

Once these records are set, requests to reverse-lookup the IP address 111.222.33.4 will first go to your provider's record for 4.33.222.111.in-addr.arpa., which redirects to your record for ptr_www.example.com., which tells the requester that 111.222.33.4 corresponds to www.example.com.

As a similar example for IPv6 addresses, if your server's AAAA record looks like this: www AAAA 1h 202:db80:1:2:3:4:567:89ab

then its fully-specified IPv6 address is 0202:db80:0001:0002:0003:0004:0567:89ab. To obtain the CNAME record that your provider must set, reverse this address (digit-wise, ignoring the colons), putting dots between every digit and appending .ip6.arpa. (including the trailing dot): b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.8.b.d.2.0.2.0.ip6.arpa. CNAME 1h ptr_www.example.com.

(Source: https://support.google.com/domains/answer/3251147?hl=en)

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