A Name Authority Pointer (NAPTR) is a type of

resource record The Domain Name System (DNS) is a hierarchical and distributed name service that provides a naming system for computers, services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various informatio ...

in the

Domain Name System The Domain Name System (DNS) is a hierarchical and distributed name service that provides a naming system for computers, services, and other resources on the Internet or other Internet Protocol (IP) networks. It associates various information ...

of the Internet. NAPTR records are most commonly used for applications in

Internet telephony Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...

, for example, in the mapping of servers and user addresses in the

Session Initiation Protocol The Session Initiation Protocol (SIP) is a signaling protocol used for initiating, maintaining, and terminating communication sessions that include voice, video and messaging applications. SIP is used in Internet telephony, in private IP telepho ...

(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 identifier A Uniform Resource Identifier (URI), formerly Universal Resource Identifier, is a unique sequence of characters that identifies an abstract or physical resource, such as resources on a webpage, mail address, phone number, books, real-world obje ...

s (URIs). The DNS type code for the NAPTR record is 35.

Rationale

Uniform Resource Names (

URNs An urn is a vase, often with a cover, with a typically narrowed neck above a rounded body and a footed pedestal. Describing a vessel as an "urn", as opposed to a vase or other terms, generally reflects its use rather than any particular shape ...

) are a subset of Uniform Resource Identifiers (

URI Uri may refer to: Places * Canton of Uri, a canton in Switzerland * Úri, a village and commune in Hungary * Uri, Iran, a village in East Azerbaijan Province * Uri, Jammu and Kashmir, a town in India * Uri (island), off Malakula Island in V ...

s) 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 (

URL A uniform resource locator (URL), colloquially known as an address on the Web, is a reference to a resource that specifies its location on a computer network and a mechanism for retrieving it. A URL is a specific type of Uniform Resource Identi ...

s) are often used to describe such resources, such as a computer

hostname In computer networking, a hostname (archaically nodename) is a label that is assigned to a device connected to a computer network and that is used to identify the device in various forms of electronic communication, such as the World Wide Web. Hos ...

, 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 In the Internet, a domain name is a string that identifies a realm of administrative autonomy, authority, or control. Domain names are often used to identify services provided through the Internet, such as websites, email services, and more. D ...

and suggest to clients the protocols available for communication with the mapped resource. Each NAPTR record contains a service name, a set of flags, a

regular expression A regular expression (shortened as regex or regexp), sometimes referred to as rational expression, is a sequence of characters that specifies a match pattern in text. Usually such patterns are used by string-searching algorithms for "find" ...

rule, an order value, a preference and a replacement pattern. Multiple records can be chained together in a cascade to rewrite URIs in

deterministic Determinism is the metaphysical view that all events within the universe (or multiverse) can occur only in one possible way. Deterministic theories throughout the history of philosophy have developed from diverse and sometimes overlapping mo ...

ways. These cascading rules have been standardized in .

Example

A common use of NAPTR records is by the

, 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: 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 Telephone number mapping is a system of unifying the international telephone number system of the public switched telephone network with the Internet addressing and identification name spaces. Internationally, telephone numbers are systematicall ...

string indicating that the record can be used in telephone number-to-SIP-URI queries. The client applies the regular expression , which replaces its entire URI ''tel:+1-800-555-1234'' with ''sip:customer-service@example.com''. 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: 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

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 Voice over Internet Protocol (VoIP), also known as IP telephony, is a set of technologies used primarily for voice communication sessions over Internet Protocol (IP) networks, such as the Internet. VoIP enables voice calls to be transmitted as ...

services and products that require or support NAPTR records. NAPTR records are also used by non-SIP services. Notable roaming services using NAPTR for route signposting are listed below:

References

{{reflist, 30em DNS record types