A Name Authority Pointer (NAPTR) is a type of resource record in the Domain Name System of the Internet.[1][2]
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.[1]
Rationale
editUniform 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.[3] 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 RFC 2915 and 3403.
Example
editA 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:
$ORIGIN 4.3.2.1.5.5.5.0.0.8.1.e164.arpa.
IN NAPTR 100 10 "U" "E2U+sip" "!^.*$!sip:customer-service@example.com!" .
IN NAPTR 102 10 "U" "E2U+email" "!^.*$!mailto:information@example.com!" .
The first record has an order value of 100, which is lower than 102 and therefore has precedence. Its preference of 10 is unimportant as there are no other rules with order 100. The service name E2U+sip is an ENUM string indicating that the record can be used in telephone number-to-SIP-URI queries.[4] The client applies the regular expression !^.*$!sip:customer-service@example.com!, 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:
$ORIGIN example.com.
IN NAPTR 100 10 "S" "SIP+D2U" "!^.*$!sip:customer-service@example.com!" _sip._udp.example.com.
IN NAPTR 102 10 "S" "SIP+D2T" "!^.*$!sip:customer-service@example.com!" _sip._tcp.example.com.
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
edit| Vendor | Product | NAPTR support? |
|---|---|---|
| ISC | BIND | Yes |
| CZ.NIC | Knot DNS | Yes |
| Cisco Systems | CNR | Yes |
| Daniel J. Bernstein | djbdns | generic record, or patch |
| BlueCat Networks | Integrity | Yes |
| EfficientIP | SOLIDserver | Yes |
| Google Cloud DNS | Yes[5] | |
| Infoblox | Infoblox Trinzic Appliance | Yes |
| Microsoft | Windows Server 2003 DNS Server | No |
| Microsoft | Windows Server 2008 R2 DNS Server | Yes |
| Microsoft | Azure DNS | No |
| NS1 | mDNS and DDI | Yes |
| PowerDNS/Open-Xchange | PowerDNS | Yes |
| NLnet Labs | NSD | Yes |
| Amazon Web Services | Amazon Route 53 | Yes |
| Sam Trenholme | MaraDNS | version 1.4 on[6] |
| Unixservice, LLC. | unxsBind | Yes |
| Simon Kelley | Dnsmasq | Yes |
| F5 Networks | F5 Networks BIG-IP DNS | Yes |
| OVH | DNS | Yes |
| DNS.com | 51DNS DNS | No |
| Citrix Systems | NetScaler GSLB | Yes |
| Cloudflare | Cloudflare Authoritative DNS | 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
editBelow are some Voice over IP services and products that require or support NAPTR records.
| Organisation | Service/Product | NAPTR Support? |
|---|---|---|
| Snom | Snom VoIP Phones | Yes |
| Yealink | Yealink Phones | 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 | Yes | For routing non-ccTLD realms[7] |
| Wireless Broadband Alliance | OpenRoaming | Yes | by design[8] |
References
edit- ^ a b Mealling, M; Daniel, R (September 2000). The Naming Authority Pointer (NAPTR) DNS Resource Record. Network Working Group. doi:10.17487/RFC2915. RFC 2915. Proposed Standard. Obsoleted by RFC 3401, 3402, 3403, 3404. Updates RFC 2168.
- ^ Mealling, M (October 2002). Dynamic Delegation Discovery System (DDDS), Part Three: The Domain Name System (DNS) Database. Network Working Group. doi:10.17487/RFC3403. RFC 3403. Proposed Standard. Obsoletes RFC 2915, 2168.
- ^ Sollins, K (January 1998). Architectural Principles of Uniform Resource Name Resolution. Network Working Group. doi:10.17487/RFC2276. RFC 2276. Informational. Updated by RFC 3401.
- ^ van der Berg, Rudolf (2010-01-13). "ENUM: Dragging telephone numbers into the Internet Age". Ars Technica.
- ^ "Records format (JSON)". CloudDNS Documentation. Retrieved 2018-04-25.
- ^ "Updating MaraDNS". Retrieved 2009-01-17.
- ^ "Advisory: NAPTR records - Improving Efficiency of International Authentication through utilisation of RadSec at National Level". Jisc community. 3 October 2012.
- ^ "What is OpenRoaming (Radiator Software), Slide 9" (PDF). 3 October 2022.