3Com Router 3000 Ethernet FamilyConfiguration Guide Chapter 3 DNS Configuration3Com Corporation3-3Figure 3-1 DNS system componentsFigure 3-1 illustrates the process of DNS resolving:1) The user program queries the resolver for a domain name or IP address.2) Upon receipt of the query, the resolver first looks up the local cache. If therequested map entry is found, it directly replies. If not, it assembles a query packetappropriate to the query type, that is, whether IP address or domain name isneeded. This packet can take TCP or UDP format, but in this program UDP isadopted.3) Then, based on its DNS configuration, the resolver sends the query packet to port53 on the default DNS server (the foreign name server in this scenario).4) After receiving the response, the resolver resolves the response packet andreplies to the user.In this scenario, resolver and its cache are called DNS client and as a whole function toaccept and respond to the DNS queries of user programs. Normally, the user programand resolver are on the same host whereas the foreign name server can be located onthat same host or more likely on a different one.3.3.2 Configuring the DNS ClientFollowing are the DNS client configuration tasks:z Enable DNS resolvingz Configure IP address of the DNS serverz Configure the DNS domain name searching listWhere, you must enable DNS resolving and provide IP address of the DNS server. Youneed only to enable DNS resolving, however, if the interfaces on the device use IPaddresses assigned by a DHCP client and the address of the DNS server and thedomain name are included in the information that the DHCP server issues to thedevice.I. Enabling DNS ResolvingTo use the DNS client function, enable DNS resolving on the device first.
