Administrator’s Guide for SIP-T2 Series/T19(P) E2/T4 Series/T5 Series IP Phones
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The IP phone performs NAPTR query for the NAPTR pointer and transport protocol (UDP, TCP
and TLS), the SRV query on the record returned from the NAPTR for the target domain name
and the port number, and the A query for the IP addresses.
If an explicit port (except 0) is specified, A query will be performed only. If a server port is set to
0 and the transport type is set to DNS-NAPTR, NAPTR and SRV queries will be tried before
falling to A query. If no port is found through the DNS query, 5060 will be used.
The following details the procedures of DNS query for the IP phone to resolve the domain name
(for example, yealink.pbx.com) of working server into the IP address, port and transport
protocol.
NAPTR (Naming Authority Pointer)
First, the IP phone sends NAPTR query to get the NAPTR pointer and transport protocol.
Example of NAPTR records:
order pref flags service regexp replacement
IN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.yealink.pbx.com
IN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.yealink.pbx.com
Parameters are explained in the following table:
Parameter
Description
order
Specify preferential treatment for the specific record. The order is from
lowest to highest, lower order is more preferred.
pref
Specify the preference for processing multiple NAPTR records with the
same order value. Lower value is more preferred.
Flags
The flag “s” means to perform an SRV lookup.
service
Specify the transport protocols:
SIP+D2U: SIP over UDP
SIP+D2T: SIP over TCP
SIP+D2S: SIP over SCTP
SIPS+D2T: SIPS over TCP
regexp
Always empty for SIP services.
replacement
Specify a domain name for the next query.
The IP phone picks the first record because its order of 90 is lower than 100. The pref parameter
is unimportant as there is no other record with order 90. The flag “s” indicates performing the
SRV query next. TCP will be used, targeted to a host determined by an SRV query of
“_sip._tcp.yealink.pbx.com”. If the flag of the NAPTR record returned is empty, the IP phone will
perform NAPTR query again according to the previous NAPTR query result.
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