MN700004 Rev 01 39037. DNS ResolverIntroductionThe Domain Name System (DNS) is the means by which Internet domain names are locatedand translated into Internet Protocol addresses. A domain name is a meaningful and easy-to-remember "handle" for an Internet address.Because maintaining a central list of domain name/IP address correspondences would beimpractical, the lists of domain names and IP addresses are distributed throughout the Internetin a hierarchy of authority. There is probably a DNS server within close geographic proximityto your access provider that maps the domain names in your Internet requests or forwardsthem to other servers in the Internet.Figure 37-1 is an example for the DNS operation. The client enters a domain name(www.domainname.com) into his browser. The browser contacts the Client's ISP for the IPaddress of the domain name. The ISP first tries to answer by itself using "cached" data. If theanswer is found it is returned. Since the ISP isn't in charge of the DNS, and is just acting as a"DNS relay", the answer is marked "non-authoritative". If the answer is not found or if it istoo old (past the TTL), the ISP DNS contacts the nameservers for the domain directly for theanswer. If the nameservers are not known, the ISP looks for the information at the 'rootservers', or 'registry servers'. For com/net/org, these start with a.gtld-servers.net.Feature OverviewYou can define up to three DNS servers. If a query to the primary address fails to be resolvedafter three attempts, the next gateway address is queried. This process continues for eachdefined gateway address until the query is resolved or when all the queries have failed. Theorder in which the default gateway addresses are polled is the same as the order in which youenter them.DNS is a distributed database with which you can map host names to IP addresses through theDNS protocol from a DNS server. When you configure DNS on the switch, you can substitutethe host name for the IP address with the ping and traceroute commands in Privileged(Enable) mode.The BiNOS supports a cache mechanism for names that are already resolved. If a resolvequery arrives at the switch, the DNS Resolver first searches the requested name in that cache.If it does not find any match, the DNS Resolver delivers the query to the DNS server. TheTTLs (time to live) of those cache entries are extracted from the Resource Record (RR) of theserver's response.To use DNS, you must have a DNS name server present on your network.
