DNS

12.1 DNS resource records 12.2 The naming authority pointer (NAPTR) DNS RR 12.4 Service records (SRVs)

12.1 DNS resource records

Domain Name Service (DNS) a distributed database holding the alphanumeric

names and their corresponding IP addresses (and more) of every registered system on a Transmission Control Protocol (TCP)/IP network, such as the Internet or the IP Multimedia Subsystem (IMS)

each entry is referred to as a resource record (RR)

The alphanumeric names (domain names) are hierarchical in nature where country, company, department and even a host (machine) name can be identified each step in the hierarchy is identified as a zone

The domain name below identifies a machine named pc27 in the engineering department of an Australian company called Foobar

pc2 7.engineering.foobar.com.au

An entry in a DNS server that maps a domain name to an IP address is referred to as an address record, or A record In IPv6 these records are referred to as AAAA recor

ds

12.2 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

The DNS-type code for NAPTR is 35 Table 12.1 describes each field in detail The format of a NAPTR record is as follows

Domain TTL Class Type Order Preference Flag Service Regexp Replacement

12.2.1 NAPTR example

This example uses the SIP service A NAPTR RR looks like

In this example a client of the SIP service that does not support the Transport Layer Security (TLS) performs a NAPTR RR look-up for the "example.com." domain and selects the replacement

_sip._tcp.example.com

12.4 Service records (SRVs)

An entity must know the exact address of a server to contact for a service

With the introduction of SRV RRs in [RFC2782], DNS clients can query a domain for a service or protocol and get back the names of servers

Administrators can use SRV RRs to distribute the load by using several servers to service a single domain

The DNS-type code for SRVs is 33 Table 12.2 describes each field in detail

The format of an SRV record is as follows

_Service._Proto.Name TTL Class SRV Priority Weight Port Target

12.4.1 SRV example

Continuing with the example, the SRV RR looks like

In this example the client of the SIP service in the "examp.com" domain performs an SRV look-up of "_sip._tcp.example.com" followed by an A record look-up for

server2.example.com