DNS o o o 12 1 DNS resource

DNS

o o o 12. 1 DNS resource records 12. 2 The naming authority pointer (NAPTR) DNS RR 12. 4 Service records (SRVs)

12. 1 DNS resource records o Domain Name Service (DNS) n 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) n each entry is referred to as a resource record (RR)

o o The alphanumeric names (domain names) are hierarchical in nature where country, company, department and even a host (machine) name can be identified n each step in the hierarchy is identified as a zone The domain name below identifies a machine named pc 27 in the engineering department of an Australian company called Foobar pc 2 7. engineering. foobar. com. au

o 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 n In IPv 6 these records are referred to as AAAA records

12. 2 The naming authority pointer (NAPTR) DNS RR o o 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 o o This example uses the SIP service A NAPTR RR looks like

o 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) o o o An entity must know the exact address of a server to contact for a service With the introduction of SRV RRs in [RFC 2782], 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

o The format of an SRV record is as follows _Service. _Proto. Name TTL Class SRV Priority Weight Port Target

12. 4. 1 SRV example o Continuing with the example, the SRV RR looks like

o 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 server 2. example. com