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.
The usage of NAPTR and SRV for SIP is specified in RFC3263.

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”


“_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.
Cache lifetime
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.
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”


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.
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.

sip/locating_sip_services.txt · Last modified: 2011/06/21 13:50 by vopatek
CC Attribution-Noncommercial-Share Alike 3.0 Unported
www.chimeric.de Valid CSS Driven by DokuWiki do yourself a favour and use a real browser - get firefox!! Recent changes RSS feed Valid XHTML 1.0