This document is derived from RFC 3403 and covers the basics for NATPR record. For advanced use please read RFC 3403.

The Naming Authority Pointer (NAPTR) DNS Resource Record (RR) specified here was originally produced by the URN Working Group as a way to encode rule-sets in DNS so that the delegated sections of a Uniform Resource Identifiers (URI) could be decomposed in such a way that they could be changed and re-delegated over time. The result was a Resource Record that included a regular expression that would be used by a client program to rewrite a string into a domain name.

Regular expressions were chosen for their compactness to expressivity ratio allowing for a great deal of information to be encoded in a rather small DNS packet.

Over time this process was generalized for other Applications and Rule Databases. This document defines a Rules Database absent any particular Application as there may be several Applications all taking advantage of this particular Rules Database.

The packet format of the NAPTR RR is given below. The DNS type code for NAPTR is 35.

order pref flags service  regexp                               replacement
100   10   ""    ""       "!^urn:cid:.+@([^\.]+\.)(.*)$!\2!i"  .

The master file format follows the standard rules in RFC-1035. Order and preference, being 16-bit unsigned integers, shall be an integer between 0 and 65535. The Flags and Services and Regexp fields are all quoted s. Since the Regexp field can contain numerous backslashes and thus should be treated with care.

E164 Example

The ENUM Working Group in the IETF has specified a service that allows a telephone number to be mapped to a URI. The Application Unique String for the ENUM Application is the E.164 telephone number with the dashes removed. The First Well Known Rule is to remove all characters from the the telephone number and then use the entire number as the first Key. For example, the phone number "770-555-1212" represented as an E.164 number would be "+1- 770-555-1212". Converted to the Key it would be "17705551212".

The ENUM Application at present only uses this Database. It specifies that, in order to convert the first Key into a form valid for this Database, periods are inserted between each digit, the entire Key is inverted and then "e164.arpa" is appended to the end. The above telephone number would then read "2.1.2.1.5.5.5.0.7.7.1.e164.arpa.". This domain-name is then used to retrieve Rewrite Rules as NAPTR records.

For this example telephone number we might get back the following NAPTR records:

$ORIGIN 2.1.2.1.5.5.5.0.7.7.1.e164.arpa.
IN NAPTR 100 10 "u" "sip+E2U"  "!^.*$!sip:information@foo.se!i"     .
IN NAPTR 102 10 "u" "smtp+E2U" "!^.*$!mailto:information@foo.se!i"  .

Both the ENUM and URI Resolution Applications use the 'u' flag. This flag states that the Rule is terminal and that the output is a URI which contains the information needed to contact that telephone service. ENUM also uses the same format for its Service Parameters. These state that the available protocols used to access that telephone's service are either the Session Initiation Protocol or SMTP mail.