Reverse DNS (PTR) Lookup

About Reverse DNS Lookup

The PTR record for an address — what the address calls itself — and the check that makes it worth anything.

Anybody can point a PTR at any name they like. The owner of the address controls it, and the owner of the *name* does not, so a PTR saying mail.google.com proves nothing on its own. What proves something is the round trip: the name resolving back to the same address. That is forward-confirmed reverse DNS, and it is exactly what mail servers check.

The name a PTR lives at is easy to build wrong. IPv4 reverses its octets under in-addr.arpa. IPv6 reverses its *nibbles* — every hex digit separately — under ip6.arpa, which means the address has to be expanded to its full thirty-two digits first. Get either wrong and you get a name that simply does not exist, which looks identical to an address with no PTR at all.

For a mail server this matters directly: many receivers refuse mail from an address with no reverse name, and some require the forward confirmation as well. For everything else it is diagnostic — it tells you whose machine an address is.

Where it helps

• Builds the .arpa name correctly for both families, including expanding an IPv6 address to its nibbles.
• Checks the forward direction as well, which is the only thing that makes a PTR mean anything.
• Says plainly when a name does not confirm, rather than reporting it as a result.
• Explains why a mail server cares, which is the commonest reason anybody asks.

Limits

• Only the owner of an address can set its PTR, so if the answer is wrong, the fix is with whoever gave you the address — usually your hosting or connectivity provider.
• A home connection almost always has a generic PTR from the ISP, and that is normal rather than a fault.
• It says what the name is, not whether any particular receiver is happy with it — policies differ.
• Nothing here checks whether the address is blocklisted, which is a different question with a different tool.

Frequently asked questions

Why does my mail get rejected for having no reverse DNS?

Because sending mail from an address with no PTR is overwhelmingly a sign of a compromised machine rather than a mail server, so many receivers refuse it outright. Ask whoever provides the address to set a PTR to your server's name, and make sure that name resolves back to the address.

What is forward-confirmed reverse DNS?

The round trip. The address has a PTR naming a host; that host resolves back to the same address. It matters because the two halves are controlled by different people — anybody can point their PTR at your name, and only you can make your name point at their address.

Why did an IPv6 lookup find nothing when the address is definitely in use?

Most likely because no PTR was published — IPv6 reverse DNS is far less consistently set up than IPv4. It is also the case that the name is built from every hex digit of the fully expanded address, so a hand-built query is easy to get wrong in a way that looks like an empty result.

Related tools

• Reverse IP Lookup — What an address says about itself: its name, its certificate, its network
• IP Lookup — Resolve addresses and reverse DNS for a host or IP
• IP Blacklist Checker — Whether an address is on the blocklists that actually stop mail
• DNS Lookup — Resolve A, AAAA, MX, TXT, NS, CNAME, SOA and CAA records
• Port Checker — Test whether TCP ports accept connections
• Nmap Port Scanner — Scan a host's common TCP ports, Nmap style, and see what each open service says it is

More Network & DNS tools

• DNS Lookup — Resolve A, AAAA, MX, TXT, NS, CNAME, SOA and CAA records
• IP Lookup — Resolve addresses and reverse DNS for a host or IP
• Port Checker — Test whether TCP ports accept connections
• Nmap Port Scanner — Scan a host's common TCP ports, Nmap style, and see what each open service says it is
• Ping (TCP) — Measure connection latency with repeated TCP probes
• What Is My IP — Your public address, and what a browser reveals about you
• WHOIS — Registry records for a domain, an IP address or an AS number
• DNS Propagation Checker — The same name asked of resolvers worldwide, and of the zone's own servers
• Email Authentication Checker — SPF, DKIM, DMARC and MX for a domain, read together
• Email Header Analyzer — The delivery path, the authentication results, and what a forgery looks like
• DKIM Lookup — A domain's DKIM keys, for a selector or by trying the common ones, and whether each is worth anything
• DMARC Lookup — A domain's DMARC policy, and what it actually asks receivers to do
• IP Blacklist Checker — Whether an address is on the blocklists that actually stop mail
• Reverse IP Lookup — What an address says about itself: its name, its certificate, its network
• Traceroute — Every hop between this server and a host, with the time each one took
• Broadband Speed Test — Download, upload, latency and jitter, measured from this tab
• HTTP Headers — Grade a site's security headers from A+ to F, see every response header, and get the fix for your server
• Redirect Checker — Follow a URL's redirects hop by hop, and flag loops, downgrades and temporary redirects doing a permanent job
• IP Address Converter — Write an IPv4 or IPv6 address as an integer, hex, binary, expanded or mapped form, and see what kind it is
• MAC Address Lookup — Find the manufacturer behind a MAC address, and see what kind of address it is
• MAC to IP Converter — Build the IPv6 address a MAC forms with SLAAC, recover the MAC from one, or find a MAC's IP in an ARP table
• IP to MAC Lookup — Find the MAC address behind an IP: computed for multicast and EUI-64 addresses, or read from an ARP table
• DNSSEC Checker — Whether a domain's DNSSEC validates, and if not, whether it is the DS, a key, an algorithm or an expired signature