Verify Authoritative Name Servers for a PTR Record Reverse DNS Zone – UltraDNS Support

Overview

This article explains how to check which name servers are authoritative for the reverse DNS zone that contains a PTR record.

A PTR record is used for reverse DNS lookup. Reverse DNS is separate from forward DNS, so hosting a forward DNS zone in UltraDNS does not automatically mean UltraDNS is authoritative for the related IP address.

A PTR record will resolve from UltraDNS only when the applicable reverse DNS zone is delegated to UltraDNS name servers.

How IPv4 Reverse DNS Names Work

IPv4 reverse DNS uses the in-addr.arpa namespace. For an IP address written as a.b.c.d, the full PTR owner name is written in reverse order as d.c.b.a.in-addr.arpa.

For a standard /24 reverse delegation, the reverse DNS zone is usually c.b.a.in-addr.arpa, and the PTR record name inside that zone is usually d.

Example

IP address:

37.131.69.228

Full PTR owner name:

228.69.131.37.in-addr.arpa

Expected standard /24 reverse DNS zone:

69.131.37.in-addr.arpa

PTR record name inside that zone:

Check the Reverse DNS Delegation

Identify the IP address that needs reverse DNS verification.
Convert the IP address to the full PTR owner name.
Example:
```
37.131.69.228 = 228.69.131.37.in-addr.arpa
```
Run a trace lookup for the IP address:
```
dig -x 37.131.69.228 +trace
```
Review the trace output to identify the delegated reverse DNS zone and the authoritative name servers for that zone.
Confirm whether the authoritative name servers are UltraDNS name servers.

Alternative Manual Delegation Check

You can also follow the reverse DNS delegation manually by querying each delegation step.

Query the full PTR owner name against a root server:

dig @a.root-servers.net 228.69.131.37.in-addr.arpa PTR

Review the authority section for the next delegated name servers.
Query one of the name servers returned in the authority section.
Repeat this process until the response identifies the authoritative name servers for the applicable reverse DNS zone.

How to Interpret the Results

If UltraDNS name servers are authoritative for the applicable reverse DNS zone, the PTR record can be hosted in UltraDNS.
If UltraDNS name servers are not authoritative for the applicable reverse DNS zone, the PTR record will not resolve from UltraDNS even if the record exists in an UltraDNS account.
If another provider's name servers are authoritative, contact the organization that controls the IP address block. This is usually the ISP, hosting provider, or IP address provider.
The organization that controls the IP address block must either create the PTR record or delegate the applicable reverse DNS zone to UltraDNS name servers.

Important Notes

Forward DNS and reverse DNS are delegated separately.
Hosting example.com in UltraDNS does not confirm that UltraDNS is authoritative for the reverse DNS zone of an IP address used by example.com.
Do not confuse the full PTR owner name with the reverse DNS zone name.
For 37.131.69.228, the full PTR owner name is 228.69.131.37.in-addr.arpa, but the standard /24 reverse DNS zone is 69.131.37.in-addr.arpa.
A customer may be correct to create the PTR record in the /24 reverse DNS zone if that zone is delegated to UltraDNS.
Some IP address blocks use non-standard or classless reverse DNS delegation. If the trace output does not match the expected /24 reverse DNS zone, use the delegated zone shown in the trace output as the source of truth.

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