Advanced Operational Features41-001160-04 Rev 02, Release 2.5 6-47Configuring Advanced Operational FeaturesConfigurable DNS QueriesThe domain name system (DNS) is the way that Internet domain names arelocated and translated into Internet Protocol addresses. A domain name is ameaningful and easy-to-remember identifier for an Internet address.The lists of domain names and IP addresses are distributed throughout the Internetin a hierarchy of authority within a database of records. There is usually a DNSserver within close proximity to your geographic location that maps the domainnames 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 tovarious deployment environments: eNone, eSRV, and eNAPTR&SRV. (See thefollowing table for a description of each method). When the IP Phone accesses theInternet, it sends out a DNS query to the proxy to lookup the IP address and theport, and then waits for a response from the proxy.You can configure the phone to use any one of these methods by entering theapplicable value in the configuration files:.ConfigurationFile ValueDNS ServerMethod Used Description0 A only The phone sends “A” (Host IP Address) lookup for the IP addressand uses the default port number of 5060.1 SRV & A The phone sends “SRV” (Service Location Record) lookup to getthe port number. Most often, the IP address is included in theresponse from the DNS server to avoid extra queries. If there is noIP 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) lookupto get the “SRV” pointer and service type (such as "aastra.comSIP+2DT .... _sip.tcp.aastra.net", which means the service prefersto use TCP and "_sip.tcp.aastra.net" for the SRV query instead ofthe default "_sip._tcp.aastra.com"). If the NAPTR record isreturned 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 portnumber. If there is no IP address in the SRV response then itsends out and “A” lookup to get it.
