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Table of Contents
Locating SIP Services
It is very convenient to be able to use one's personal address, i.e. john@company.com, for both email and SIP communications (IP telephony, presence, video conferencing etc), just by changing the application prefix.
For example: email:john@company.com and sip:john@company.com
DNS is the mechanism for determining the IP address, port and transport protocol of the host to which a SIP request is sent.
DNS provides two different record types that controls SIP requests: SRV and NAPTR. The SRV record allows DNS administrators to use several servers for a single domain. The NAPTR record is a way for the called domain to specify which protocol it prefers a SIP request to use.
Setting up SRV Records
(Information for DNS administrators.)
The SRV records have a format which follows the following convention:
“_service._protocol.name TTL Class RecordType Priority Weight Port Target”
Example:
“_sip._udp.company.com 432000 IN SRV 10 10 5060 sip.company.com”
| Service: _sip | The service is SIP. Can also be _sips for TLS encrypted communication. |
| Protocol: _udp | The protocol is UDP. Other values: _tcp or _sctp. |
| Name: | |
| Cache lifetime 43200 | 43,200 seconds = 12 hours. |
| Class IN | The class is always IN. |
| Record type SRV | The record type is SRV. |
| Priority 10 | When multiple SRV records are used the priority determines the proxy query order. Lower values are queried first. |
| Weight 10 | When multiple SRV records are used the weight determines proportionally how often a proxy is queried. Higher values are queried more often. 20 = queried twice as often as 10. 30 = queried three times as often as 10. |
| Port 5060 | The default SIP port is 5060. |
| Target sip.company.com | The domain name of the proxy server is sip.company.com |
The SRV records are specified in RFC2782.
Setting up NAPTR Records
(Information for DNS administrators.)
The NAPTR records have a format which follows the following convention:
“Order Preference Flags Services Regexp Replacement”
Examples:
company.com. 2508 IN NAPTR 20 50 “S” “SIP+D2T” ”” _sip._tcp.company.com.
company.com. 2508 IN NAPTR 30 50 “S” “SIP+D2U” ”” _sip._udp.company.com.
| Order: 20 | The order in which the NAPTR records must be processed. |
| Preference: 50 | Specifies the order in which NAPTR records with equal Order values should be processed. |
| Flags: “S” | The flags control aspects of the rewriting and interpretation of the fields in the record. |
| Services: “SIP+D2T” | This specifies the Service Parameters applicable to this delegation path. |
| Regexp: “” | This contains a substitution expression that is applied to the original string held by the client in order to construct the next domain name to lookup. |
| Replacement: _sip._tcp.company.com | This is the next domain-name to query for depending on the potential values found in the flags field. |
The NAPTR records are specified in RFC3403.
