76 DNS C ONFIGURATIONDNS Overview Domain name system (DNS) is a distributed database system that provides domainname-to-IP address mappings for TCP/IP applications. With DNS, users using IPapplications can directly use meaningful easy-to-remember domain names, whichwill be resolved and mapped to corresponding IP addresses by DNS servers.There are two types of DNS resolution, “Static DNS Resolution” on page 831 and“Dynamic DNS Resolution” on page 831. When a name query is received, thestatic resolution is first performed to check the static DNS list. If the staticresolution fails, the dynamic resolution is performed. Because dynamic resolutionneeds the participating of DNS server and may spend some time, you can putsome commonly used domain names in the static DNS list to increase theresolution efficiency.Static DNS Resolution With static DNS resolution, you can manually configure some name-to-addressmappings in the static DNS list, and the system will search the static list forcorresponding IP addresses when users use domain names with some applications(such as telnet).Dynamic DNS Resolution Resolving procedureThe procedure of dynamic DNS resolution is as follows:1 A user program sends a name query to the resolver in the DNS Client.2 The DNS resolver looks up the local DNS cache for a match. If a match is found, itreturns the corresponding IP address to the user program. If not, it sends a queryto the DNS Server.3 The DNS Server looks up its database for a match. If no match is found, it sends aquery to its parent DNS Server. If the parent DNS Server does not have theinformation, it sends the query to another server. This process continues until aresult (either successful or failed) is found. Finally, the resolution result is returnedto the DNS Client.4 The DNS Client performs the next operation according to the result.
