What is ENUM and how does it use NAPTR?

ENUM (E.164 Number Mapping) uses NAPTR records to map telephone numbers to URIs. A phone number like +1-555-123-4567 is reversed and looked up in DNS (7.6.5.4.3.2.1.5.5.5.1.e164.arpa) where NAPTR records provide SIP or other contact URIs.

What do NAPTR flags mean?

NAPTR flags control processing: 'U' means terminal (regexp produces final URI), 'S' means follow up with SRV lookup, 'A' means follow up with A/AAAA lookup, and empty means continue with NAPTR lookup at the replacement domain.

When should I use NAPTR vs SRV records?

Use NAPTR when you need regex-based URI rewriting, chained lookups, or ENUM support. Use SRV for simpler service discovery where you just need hostname, port, and priority/weight without URI transformation.

NAPTR Record Service

Q: What is a NAPTR record?

A NAPTR (Naming Authority Pointer) record enables complex service discovery and URI rewriting using regular expressions. It's commonly used in ENUM (telephone number to URI mapping), SIP, and telecommunications applications.

The NAPTR record (Naming Authority Pointer) enables complex service discovery and URI rewriting using regular expressions. It's commonly used in ENUM (telephone number to URI mapping), SIP, and other telecommunications applications.

Look Up NAPTR Records

Check NAPTR records for any domain using our free DNS lookup tool.

Look Up NAPTR Records →

What Is a NAPTR Record?

NAPTR records allow DNS to be used for more sophisticated lookups than simple name-to-address resolution. They can:

Rewrite strings using regular expressions
Chain lookups through multiple steps
Point to SRV records or other services
Support ENUM (E.164 to URI mapping)

NAPTR Record Format

Example NAPTR Record

example.com.    NAPTR    100 10 "U" "E2U+sip" "!^.*$!sip:[email protected]!" .

Order: 100, Preference: 10, Flags: U (terminal), Service: E2U+sip

NAPTR Record Fields

Field	Description	Example
Order	Processing order (lower first)	100
Preference	Preference within same order	10
Flags	Processing flags	"U", "S", "A", ""
Service	Service type	"E2U+sip", "SIP+D2U"
Regexp	Substitution expression	"!^.*$!sip:user@host!"
Replacement	Next lookup domain	. (none) or hostname

NAPTR Flags

Flag	Meaning
U	Terminal - regexp produces final URI
S	Next lookup is SRV record
A	Next lookup is A/AAAA record
""	Continue with NAPTR lookup
P	Protocol specific (non-terminal)

Common NAPTR Use Cases

ENUM (Phone Number to SIP)

; Phone: +1-555-123-4567 → 7.6.5.4.3.2.1.5.5.5.1.e164.arpa
7.6.5.4.3.2.1.5.5.5.1.e164.arpa.    NAPTR    100 10 "U" "E2U+sip" "!^.*$!sip:[email protected]!" .

SIP Service Discovery

example.com.    NAPTR    10 0 "S" "SIP+D2U" "" _sip._udp.example.com.
example.com.    NAPTR    20 0 "S" "SIP+D2T" "" _sip._tcp.example.com.

Email Service

example.com.    NAPTR    100 10 "U" "E2U+email" "!^.*$!mailto:[email protected]!" .

NAPTR Processing

Query NAPTR records for the domain
Sort by Order, then Preference
Apply regexp or follow replacement
Based on flags, either finish or continue
If flag is "S", query SRV; if "A", query A record

NAPTR Best Practices

Order carefully — Lower order numbers are processed first.
Test regexps — Validate regular expressions before deploying.
Use appropriate flags — Ensure correct terminal/non-terminal behavior.
Consider SRV — For simpler service discovery, SRV may suffice.
Document thoroughly — NAPTR can be complex; document your setup.

NAPTR vs SRV

Aspect	NAPTR	SRV
Complexity	More complex	Simpler
Regexp support	Yes	No
URI output	Yes	No (hostname only)
Use case	ENUM, complex routing	Simple service discovery

Troubleshooting NAPTR

Common issues and solutions:

Regexp not matching — Verify regex syntax and escaping.
Wrong order — Check Order and Preference values.
Missing follow-up records — If using "S" flag, ensure SRV exists.
Application not using NAPTR — Verify application supports NAPTR lookups.

Check Your NAPTR Records

Use our DNS Record Finder to look up NAPTR records for any domain.