NAPTR record

A Name Authority Pointer (NAPTR) is a type of resource record in the Domain Name System of the Internet.Template:Ref RFCTemplate:Ref RFC

NAPTR records are most commonly used for applications in Internet telephony, for example, in the mapping of servers and user addresses in the Session Initiation Protocol (SIP). The combination of NAPTR records with Service Records (SRV) allows the chaining of multiple records to form complex rewrite rules which produce new domain labels or uniform resource identifiers (URIs).

The DNS type code for the NAPTR record is 35.Template:Ref RFC

Rationale

Uniform Resource Names (URNs) are a subset of Uniform Resource Identifiers (URIs) used for abstract identifiers, such as a person's name or their telephone number. For URNs to be meaningful, they must be mapped to a concrete resource of some sort. Uniform Resource Locators (URLs) are often used to describe such resources, such as a computer hostname, or a local file.

The NAPTR record aids in the standardization of URNs. NAPTR records map between sets of URNs, URLs and plain domain names and suggest to clients the protocols available for communication with the mapped resource.Template:Ref RFC Each NAPTR record contains a service name, a set of flags, a regular expression rule, an order value, a preference and a replacement pattern. Multiple records can be chained together in a cascade to rewrite URIs in deterministic ways. These cascading rules have been standardized in Template:IETF RFC.

Example

A common use of NAPTR records is by the Session Initiation Protocol, which routes telephony sessions over IP networks. For example, the URI for the US telephone number 1-800-555-1234 might be tel:+1-800-555-1234 and its domain name 4.3.2.1.5.5.5.0.0.8.1.e164.arpa. A SIP client querying that name might receive: Template:Sxhl The first record has an order value of 100, which is lower than 102 and therefore has precedence. Its preference of 10 is unimportant as there are no other rules with order 100. The service name E2U+sip is an ENUM string indicating that the record can be used in telephone number-to-SIP-URI queries.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The client applies the regular expression <syntaxhighlight lang="text" class="" style="" inline="1">!^.*$!sip:[email protected]!</syntaxhighlight>, which replaces its entire URI tel:+1-800-555-1234 with sip:[email protected]. The flag U indicates that the replacement string is a SIP URI, and that no further rules should be applied.

To resolve the SIP URI, the client performs a second NAPTR lookup—on example.com, yielding: Template:Sxhl As in the first example, the client picks the first record because it has the lowest order value. The regular expression rule replaces the query URI, this time with the domain name _sip._udp.example.com. The flag S indicates that the resulting domain name points to a SRV record. The client thus finishes with _sip._udp.example.com, for which it can then fetch an SRV record to initiate a telephony call.

Support

Vendor	Product	NAPTR support?
ISC	BIND	Template:Yes
CZ.NIC	Knot DNS	Template:Yes
Cisco Systems	CNR	Template:Yes
Daniel J. Bernstein	djbdns	Template:Yes
BlueCat Networks	Integrity	Template:Yes
EfficientIP	SOLIDserver	Template:Yes
Google	Google Cloud DNS	Template:Yes<ref>{{#invoke:citation/CS1|citation	CitationClass=web }}</ref>
Infoblox	Infoblox Trinzic Appliance	Template:Yes
Microsoft	Windows Server 2003 DNS Server	Template:No
Microsoft	Windows Server 2008 R2 DNS Server	Template:Yes
Microsoft	Azure DNS	Template:No
NS1	mDNS and DDI	Template:Yes
PowerDNS/Open-Xchange	PowerDNS	Template:Yes
NLnet Labs	NSD	Template:Yes
Amazon Web Services	Amazon Route 53	Template:Yes
Sam Trenholme	MaraDNS	Template:Yes
Unixservice, LLC.	unxsBind	Template:Yes
Simon Kelley	Dnsmasq	Template:Yes
F5 Networks	F5 Networks BIG-IP DNS	Template:Yes
OVH	DNS	Template:Yes
DNS.com	51DNS DNS	Template:No
Citrix Systems	NetScaler GSLB	Template:Yes
Cloudflare	Cloudflare Authoritative DNS	Template:Yes

NAPTR implementations generally also implement EDNS as responses returning multiple NAPTR records are usually larger than the normal 512 byte packet size limit and would otherwise require a less efficient fallback to TCP, rather than UDP, for the transport protocol.

Services

Below are some Voice over IP services and products that require or support NAPTR records.

Organisation	Service/Product	NAPTR Support?
Snom	Snom VoIP Phones	Template:Yes
Template:Ill	Yealink Phones	Template:Yes

NAPTR records are also used by non-SIP services. Notable roaming services using NAPTR for route signposting are listed below:

Organisation	Service/Product	NAPTR required?	Reason
eduroam	eduroam	Template:Yes	citation	CitationClass=web }}</ref>
Wireless Broadband Alliance	OpenRoaming	Template:Yes	citation	CitationClass=web }}</ref>