3Com Switch 8800 Configuration Guide Chapter 14 DNS Configuration14-1Chapter 14 DNS Configuration14.1 Introduction to DNSUsed in the TCP/IP application, Domain Name System (DNS) is a distributed databasewhich provides the translation between domain name and the IP address. In this way,the user can use domain names that are easy to memorize and meaningful, and neverneeds to keep obscure IP addresses in mind.There are two kinds of domain name resolutions: static domain name resolution anddynamic domain name resolution, which supplement each other in real application. Onresolving a domain name, use the static resolution first. If it fails, use the dynamicresolution method. You can put some common domain names into the static domainname resolution table to raise the domain name resolution efficiency greatly.14.1.1 Static Domain Name ResolutionStatic domain resolution is to establish maps between domain name and the IP addressmanually. When you perform some applications using domain names, the system canobtain the IP address of the specified domain name by searching the static domainname resolution table.14.1.2 Dynamic Domain Name ResolutionDynamic domain name resolution is implemented by inquiring the domain name server.As a DNS client, the switch sends an inquiry request to the domain name server, andthe domain name server searches the related IP address of the domain name in its owndatabase and sends it back to the switch. If the domain name server judges that thedomain name does not belong to the local domain, it forwards the request to the upperlevel domain name resolution server till the resolution is finished.Dynamic domain name resolution supports the buffer function. It stores eachsuccessful domain name/IP address mapping that is resolved dynamically in thedynamic domain name buffer. When the same domain name is searched next time, itcan be read directly from the buffer, without requesting the domain name server. Theaged mapping in the buffer is deleted after a certain period of time to ensure theupdated contents can be got from the domain name server timely. The aging time is setby the domain name server and obtained by the switch from the protocol packet.Dynamic domain name resolution supports the domain name suffix list function. Youcan set some domain name suffixes beforehand and input part of the domain namefield during the domain name resolution, then the system adds different suffixes to theinput domain name automatically for resolution. For example, if a user wants to searchthe domain name “3Com.com”, he can configure the “com” in the suffix list and input
