Advanced Operational Features
41-001160-03, Rev 00, Releaes 2.4
6-49
Configuring Advanced Operational Features
Configurable DNS Queries
The domain name system (DNS) is the way that Internet domain names are
located and translated into Internet Protocol addresses. A domain name is a
meaningful and easy-to-remember identifier for an Internet address.
The lists of domain names and IP addresses are distributed throughout the Internet
in a hierarchy of authority within a database of records. There is usually a DNS
server within close proximity to your geographic location that maps the domain
names in your Internet requests or forwards them to other servers in the Internet.
The IP Phones support three methods of DNS lookups allowing it to adapt to
various deployment environments:
eNone
,
eSRV
, and
eNAPTR&SRV
. (See the
following table for a description of each method). When the IP Phone accesses the
Internet, it sends out a DNS query to the proxy to lookup the IP address and the
port, and then waits for a response from the proxy.
You can configure the phone to use any one of these methods by entering the
applicable value in the configuration files:.
Configuration
File Value
DNS Server
Method Used
Description
0
A only
The phone sends “
A
” (Host IP Address) lookup for the IP address
and uses the default port number of 5060.
1
SRV & A
The phone sends “
SRV”
(Service Location Record) lookup to get
the port number. Most often, the IP address is included in the
response from the DNS server to avoid extra queries. If there is no
IP address returned in the response, the phone sends out the “
A
”
DNS lookup to find the IP address.
2
NAPTR & SRV & A
First, the phone sends "
NAPTR
" (Naming Authority Pointer) lookup
to get the “
SRV
” pointer and service type (such as "aastra.com
SIP+2DT .... _sip.tcp.aastra.net", which means the service prefers
to use TCP and "_sip.tcp.aastra.net" for the SRV query instead of
the default "_sip._tcp.aastra.com"). If the NAPTR record is
returned empty then the default value is used, so in the same case,
the phone will use "_sip._udp.aastra.com" for the next step lookup.
Next, the phone does SRV lookup to get the IP address and port
number. If there is no IP address in the SRV response then it
sends out and “
A
” lookup to get it.
Draft 1
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