Expert Documentation

Chapter 6
6.4.1.7.4 DNS (Domain Name System)
The DNS is a directory system distributed on Internet.
The basic function of a DNS server is to convert domain names into IP addresses. This is
done:
- Through a DNS A request to convert a name into an ipv4 IP address
- Through a DNS AAAA request to convert an name into an ipv6 IP address.
Any SIP entity can use the DNS if the domain part of a URL appears as a name, in order to
convert it into an IP address.
For SIP, the DNS can also be used to resolve protocol type, address and the port number
where requests relating to a given SIP address must be sent. This is done through NAPTR
and DNS SRV requests.
6.4.1.7.5 NAPTR and DNS SRV
An answer to a NAPTR (Naming Authority Pointer) request for a given domain name consists
of one or several NAPTR records. A NAPTR record contains the supported transport protocol
(UDP, TCP, TLS over TCP, ...) and the replacement name to be used for DNS SRV requests.
The example below shows NAPTR records which could be obtained for a NAPTR request for
the domain "mydomain.com".
Example:
Order pref flags service regexp replacement
IN NAPTR 50 50 “s” “SIP+D2T” “” _sip._tcp.mydomain.com
IN NAPTR 90 50 “s” “SIP+D2U” “” _sip._udp.mydomain.com
The records indicate that the server supports TCP and UDP in that order of preference. Order
specifies the order in which the NAPTR records must be processed to ensure the correct
ordering of rules. Pref specifies the order in which NAPTR records with equal Order values
should be processed, low numbers being processed before high numbers.
Then, the system must make a TCP lookup to get SRV records for “_sip._tcp.mydomain.com”.
An SRV RR answer may be:
Priority Weight Port Target
IN SRV 0 1 5060 server1.mydomain.com
IN SRV 0 2 5060 server2.mydomain.com
The records indicate that the system should send its request to server1. If there is no answer,
server2 should be used. Note that the domain name “mydomain” can change between NAPTR
records and SRV records.
Once the protocol, the port and the domain have been resolved, the system should determine
the IP address of the server. The system performs DNS A query (or AAAA for IPV6) related to
“server1.mydomain.com” to get a list of IP addresses.
The system should try the first SRV RR record. If no answer, the next in the list should be
queried until the end of the list.
If no SRV records were found, the system has to perform DNS A query (or AAAA for IPV6) on
the domain name.
If a port is specified in the URI (example : 1234@mydomain.com:5060), then the system has
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