A NAPTR (Name Authority Pointer) record is an advanced type of DNS record that uses Regular Expressions (regex) to map a domain or string to a specific Uniform Resource Identifier (URI) or hostname. It is most heavily used in Internet Telephony (VoIP) for routing SIP calls.
Troubleshooting SIP or ENUM configurations? Check your active NAPTR rules.
The most famous use case for NAPTR is ENUM (E.164 Number Mapping). ENUM is the system that translates standard telephone numbers into internet addresses (like a SIP URI) so that phone calls can be routed over the internet instead of traditional phone lines.
For example, if you dial +1-555-123-4567, the telecom system reverses the number and looks up the NAPTR records for 7.6.5.4.3.2.1.5.5.5.1.e164.arpa. The NAPTR record uses a regular expression to rewrite that query into a destination URI, like sip:user@example.com.
NAPTR records are complex and consist of several fields:
S means the next lookup is an SRV Record. A means the next lookup is an A Record or AAAA record. U means the output is a final URI.E2U+sip or SIP+D2T)..).While both are used for service discovery, NAPTR is a broader routing tool. NAPTR uses regex to rewrite strings and can point to URIs, SRV records, or A records. An SRV record specifically maps a service to a hostname and port. Often, a NAPTR record is the first step, and its output directs the client to query an SRV record next.
In NAPTR syntax, the record must provide either a Regular Expression (Regexp) OR a Replacement hostname, but rarely both. If the Regexp field is actively rewriting the string to create the target URI, the Replacement field is left empty by inserting a single dot (.).
While telecom and SIP routing (ENUM) are the most common use cases, NAPTR records can theoretically be used by any application that requires complex URI rewriting or dynamic service discovery rules based on DNS.