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PTR record

Maps an address back to a name, using the reverse in-addr.arpa and ip6.arpa zones.

Last updated July 27, 2026

Zone-file example

; type 12, defined in RFC 1035 — note the reversed octets
10.113.0.203.in-addr.arpa.    3600   IN   PTR   mail.example.com.

; IPv6 reverse: every nibble, reversed, under ip6.arpa
0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. 3600 IN PTR mail.example.com.

Typical uses

  • Reverse DNS for outbound mail servers, which many receivers require
  • Making traceroute, logs, and monitoring output readable
  • Satisfying audit or compliance checks that expect forward-confirmed reverse DNS
  • Service discovery in DNS-SD, which uses PTR for a different purpose entirely

When it breaks, check

  • You can only set PTR if you control the reverse zone — usually your hosting or ISP does
  • Receivers check forward-confirmed reverse DNS: the PTR name must resolve back to the same address
  • IPv4 octets and IPv6 nibbles are reversed in the query name; getting the order wrong is the usual mistake
  • A missing PTR on a mail sender is one of the most common causes of silent delivery failure

What the PTR record does

There is no magic reverse index in DNS. To look up 203.0.113.10, a resolver reverses the octets and queries 10.113.0.203.in-addr.arpa for a PTR record. IPv6 does the same thing one hexadecimal nibble at a time, which produces the famously unreadable names under ip6.arpa.

The consequence is that reverse DNS is delegated with the address space, not with your domain. Whoever was allocated the block controls the reverse zone, which is why setting a PTR is a support ticket or a cloud console setting rather than a zone-file edit.

Zone-file and wire format

The owner name is the reversed address plus the arpa suffix, and the RDATA is a single domain name. There is no requirement that the target name resolve back to the same address, which is exactly why receivers check it themselves. Multiple PTR records for one address are legal but confuse many consumers, so publish one.

; IPv4: 203.0.113.10 -> reverse the four octets
10.113.0.203.in-addr.arpa.   3600   IN   PTR   mail.example.com.

; and the forward record must agree, or FCrDNS checks fail
mail.example.com.            3600   IN   A     203.0.113.10

; IPv6: 2001:db8::10 -> every nibble, reversed
0.1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2.ip6.arpa. 3600 IN PTR mail.example.com.

Common uses

Outbound mail is the use that actually costs you money when it is missing. Most large receivers apply forward-confirmed reverse DNS: they take the connecting address, look up its PTR, resolve that name forward, and require it to come back to the same address. Fail the check and your mail is deprioritised or dropped, often without a bounce. Beyond mail, PTR records make logs and traceroutes legible, and DNS-SD (RFC 6763) reuses the PTR type for service enumeration, which is unrelated to address reversal and a common source of confusion when reading packet captures.

Troubleshooting

Check the round trip, not just the record. Query the PTR for the address, then resolve the answer forward and confirm you land back on the same address. If either half is missing, mail receivers will treat the sender as suspect.

If you cannot create the record at all, you do not control the reverse zone. For cloud instances the setting is usually on the network interface or elastic IP; for colocated address space it is a request to whoever holds the allocation. Note that many cloud providers refuse to set PTR records for outbound mail unless you ask explicitly.

# reverse lookup the easy way
dig -x 203.0.113.10 +short

# the query dig actually sends
dig 10.113.0.203.in-addr.arpa PTR +noall +answer

# forward-confirmed reverse DNS: these two must agree
name=$(dig -x 203.0.113.10 +short | sed 's/\.$//')
dig "$name" A +short

# IPv6 works the same way
dig -x 2001:db8::10 +short

The defining RFC

PTR is type 12, defined in RFC 1035 §3.3.12, with the IPv4 in-addr.arpa construction in RFC 1035 §3.5. IPv6 reverse mapping under ip6.arpa is RFC 3596 §2.5. Guidance on reverse-mapping operations, including why delegation follows address allocation, is RFC 1912 §2.1, and the DNS-SD reuse of PTR is RFC 6763. Test a reverse lookup with the DNS lookup tool.

Reference and tooling

Every record type and its assigned numeric value is listed in theIANA DNS Parameters registry. To query a live zone from the browser, use theDNS lookup tool. If the record you are chasing is TLS-related, the TLS inspector andCertificate Transparency lookup are usually the next two stops.

Other record types