41-001343-01 Rev 02, Release 3.2.2 6-55Domain Name Server (DNS) Pre-caching SupportThe IP phones now support the use of a local DNS host file to resolve DNS queries, and supportspre-provisioning of DNS SRV records. This feature allows administrators to configure the phoneto download a text file which contains persistent DNS “A record” hostname to IP addressmappings. In addition, support for persistent DNS “SRV records” has been added to permit SRVbased high availability of services.There are two methods used to configure DNS pre-caching on the IP phone:• Configure a unix style “host” file used instead of a DNS “A query” to resolve hostnames to IPaddresses. The host file is downloaded and cached on the IP phone.• Configure DNS “SRV queries” for geographic redundancy and failover. The configured SRVentries are used to pre-load the DNS cache on the IP phone.Both these methods are configurable using the configuration files only, and are primarily intendedfor use when a third party hosting provider delivers SIP services but does not have local access orcontrol of the LAN side DNS infrastructure.Configuring DNS “Host File” Pre-caching from the Configuration ServerThe DNS host file must reside on the same server as the configuration files(aastra.cfg/.tuz, etc.) and the filename to download is specified within the configuration.Use the following parameter to configure the phone to use the host file for host IP address lookups.The following procedure is an example of how to use the “sip dns host file” parameter to configureDNS lookup pre-caching from the configuration server.Note: Time-to-Live (TTL) used in this feature is hard-coded for eachserver and not configurable.Configuration FilesFor the specific parameter you can set in the configuration files, see Appendix A, the section, “DNS HostFile” on page A-249.Configuring “DNS Host” Pre-caching1 Using a text-based editing application, create a blank text file2 Enter the IP addresses of the DNS servers in your local network. For example:1.2.3.4 server15.6.7.8 server29.0.1.2 server3Note: Ensure each line uses a Carriage Return (CR) or Carriage Return + Line Feed (CRLF) toterminate the line.
