The IMS: IP Multimedia Concepts And Services, Second Edition

16.2. The Naming Authority Pointer (NAPTR) DNS RR

NAPTR RRs are used to replace compact, regular expressions with a replacement field that may well be a pointer to another rule. [RFC2915] defines the NAPTR; however, it does not define why or how replacement fields are used. It is up to the applications using NAPTR to do this. The DNS-type code for NAPTR is 35.

The format of a NAPTR record is as follows (Table 16.1 describes each field in detail):

Domain TTL Class Type Order Preference Flag Service Regexp Replacement

Table 16.1. NAPTR RR fields.

Field	Description
Domain	The key for an entry (the domain name of the RR)
TTL	Time to live
Class	Class of record (NAPTR records are of class IN, or Internet)
Type	DNS-type code (for NAPTR records it is 35)
Order	The order in which NAPTR records need to be processed. This allows NAPTR records to specify a complete rule in an incremental fashion. Records are processed from lower order numbers to higher order numbers
Preference	Specifies the order in which NAPTR records with the same "order" values need to be processed. Records are processed from lower preference numbers to higher preference numbers
Flags	Indicate what happens next after this look-up. The "S" flag indicates that the next look-up should be an SRV look-up. The replacement field carries an SRV reference. The "U" flag indicates that the next step is not a DNS look-up. The regexp (see below) carries a URI. The "A" flag indicates that the next step is a DNS A, AAAA or an A6 look-up. The "P" flag ...