Reverse DNS Lookup

Server assisted New DNS & Domains

Look up the PTR record of a public IP address under in-addr.arpa or ip6.arpa, then resolve each returned name forward (A or AAAA) to see whether it points back to the address — the forward-confirmed reverse DNS that mail servers check. The queries go from the Mutqan server at the Cloudflare edge to Cloudflare or Google public DNS.

Privacy: The name you look up is sent from the Mutqan server to the public DNS resolvers named in the results. Nothing is stored.

What is reverse DNS?

Reverse DNS maps an IP address back to a host name with a PTR record stored under in-addr.arpa (IPv4) or ip6.arpa (IPv6). The record is published by whoever holds the address block, not by the domain owner, so it is only trustworthy when the name it gives also resolves forward to the same address (forward-confirmed reverse DNS).

How to use Reverse DNS Lookup

  1. Enter one public IPv4 or IPv6 address (brackets around IPv6 are fine).
  2. Choose Cloudflare or Google as the resolver if you want to compare them.
  3. Press Look up PTR and read the PTR name and its TTL.
  4. Check the "Matches the IP" column: ✓ means the name resolves back to the same address (forward-confirmed); ✗ means it does not.

Reverse DNS Lookup features

  • PTR lookup for IPv4 (in-addr.arpa) and IPv6 (ip6.arpa) addresses — the reverse name is built for you
  • Forward confirmation: each PTR name is resolved (A or AAAA) and compared with the address you entered
  • Clear outcome when there is no PTR: NXDOMAIN, an empty answer or a resolver failure are told apart
  • Classless reverse delegation (RFC 2317 CNAME aliases) recognised and explained
  • Notes on several PTR names for one address and on generic ISP pool names that spell out the address
  • Private, reserved and documentation addresses are explained locally and never sent to a resolver

Inputs and outputs

Inputs

  • IP address — one public IPv4 or IPv6 address; CIDR ranges, host names and private addresses are not accepted.
  • Resolver — Cloudflare 1.1.1.1 (default) or Google Public DNS, used for the PTR and the forward lookups.

Outputs

  • Summary — the reverse name that was queried, the response code, the number of PTR records and how many names were forward-confirmed.
  • PTR records — each name with its TTL, the addresses it resolves to and whether they include the address you entered.
  • Findings — no PTR, NODATA, classless delegation, failed forward lookups, several PTRs and generic pool names.

Reverse DNS Lookup example

PTR of Google Public DNS

Input:

8.8.8.8

Output:

Reverse name: 8.8.8.8.in-addr.arpa · Response code: NOERROR · PTR records: 1 · Forward-confirmed: 1/1
dns.google   TTL 15331   forward A: 8.8.8.8, 8.8.4.4   ✓
Findings: Forward-confirmed: dns.google resolves back to 8.8.8.8 (FCrDNS).

How to read the Reverse DNS Lookup results

✓ in "Matches the IP": the PTR name resolves back to the address — forward-confirmed reverse DNS, which mail receivers expect from a sending server.

✗: the name exists but points elsewhere or has no address of that family. The PTR may be stale after a server move, or the name was never given an A/AAAA record.

NXDOMAIN for the reverse name: no PTR is published. For a mail server, ask the provider holding the address to set one that matches your server's host name.

A name that spells the address (for example 203-0-113-7.dsl.example.net) is typical of residential and dynamic ranges; it says nothing certain about the address, but receivers may score it lower.

When to use Reverse DNS Lookup

  • Preparing a mail server: check that its sending IP has a PTR matching the server's name before the first message goes out.
  • Reading server logs: put a name to an unknown client or crawler address and see whether that name confirms forward.
  • After moving a site or VPS to a new address: confirm the provider updated the reverse record.
  • Verifying that a crawler claiming to be a search engine really resolves to that engine's domain and back.

Troubleshooting

  • "This address is private, reserved or otherwise not routable": addresses such as 10.x, 192.168.x or 127.0.0.1 have no public reverse DNS; query your own network's DNS server for them.
  • The PTR is correct at your provider but the tool shows the old name: the resolver may still hold the earlier answer until its TTL ends — compare both resolvers or wait for the TTL shown.
  • "no A record" (or "no AAAA record") for a name: the PTR points to a host name with no address in DNS. Create the A/AAAA record or ask the provider to change the PTR. "not looked up: not a public host name" means the PTR names a local or internal host such as localhost, which a public address should never do; "lookup failed" means the resolver did not answer, so try again.
  • Only a CNAME comes back: the reverse zone uses classless delegation, and the alias target must hold the PTR; the provider managing that target zone has to publish it.

Frequently asked questions about Reverse DNS Lookup

Who sets the PTR record of an IP address?

Whoever holds the address block: your hosting provider, cloud platform or ISP. Reverse zones are delegated along with the addresses, so you cannot add a PTR in your own domain's DNS; ask the provider, or use its control panel where it offers one.

What is forward-confirmed reverse DNS (FCrDNS)?

The PTR name of the address resolves (A for IPv4, AAAA for IPv6) back to the same address. Many mail servers require it from senders, and it is the reason the tool runs the forward lookup for each name it finds.

Why is there no PTR record for my address?

Many providers publish none, or only a generic name, until you ask. The tool shows NXDOMAIN in that case. Private addresses such as 192.168.1.10 are refused outright: public DNS has nothing for them.

Does a PTR name prove who owns or uses an address?

No. The holder of the reverse zone can write any name there, including someone else's domain. Only a matching forward lookup shows that the named domain also points at the address.

Technical notes

IPv4 addresses are reversed octet by octet under in-addr.arpa (RFC 1035 §3.5), IPv6 addresses nibble by nibble under ip6.arpa (RFC 3596 §2.5). The PTR record there names a host; the tool then asks for that host's A or AAAA records, which is the PTR/A consistency RFC 1912 §2.1 asks operators to keep.

Reverse zones follow the address allocation, not the domain: a regional registry delegates them to the network holding the block, which may delegate further — with CNAME records for blocks smaller than /24 (RFC 2317). One run makes at most four queries: the PTR and up to three forward lookups.

Limitations and methodology

The tool reports what the public resolver returns from the Cloudflare edge; it cannot see reverse records that exist only on internal DNS servers.

At most three PTR names per address are checked forward, and only in the address family of the input (A for IPv4, AAAA for IPv6).

A PTR name is written by whoever holds the reverse zone; it is not proof of identity or ownership.

Security and privacy

The address you enter and the PTR names found are sent from the Mutqan server to the public resolver you chose. The address is never contacted, and nothing is stored.

References