37. Dns Resolver; Introduction; Feature Overview - Nokia ESB26 User Manual

37. DNS Resolver

Introduction

The Domain Name System (DNS) is the means by which Internet domain names are located
and 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 be
impractical, the lists of domain names and IP addresses are distributed throughout the Internet
in a hierarchy of authority. There is probably a DNS server within close geographic proximity
to your access provider that maps the domain names in your Internet requests or forwards
them 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 IP
address of the domain name. The ISP first tries to answer by itself using "cached" data. If the
answer 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 is
too old (past the TTL), the ISP DNS contacts the nameservers for the domain directly for the
answer. If the nameservers are not known, the ISP looks for the information at the 'root
servers', or 'registry servers'. For com/net/org, these start with a.gtld-servers.net.

Feature Overview

You can define up to three DNS servers. If a query to the primary address fails to be resolved
after three attempts, the next gateway address is queried. This process continues for each
defined gateway address until the query is resolved or when all the queries have failed. The
order in which the default gateway addresses are polled is the same as the order in which you
enter them.
DNS is a distributed database with which you can map host names to IP addresses through the
DNS protocol from a DNS server. When you configure DNS on the switch, you can substitute
the 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 resolve
query 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. The
TTLs (time to live) of those cache entries are extracted from the Resource Record (RR) of the
server's response.
To use DNS, you must have a DNS name server present on your network.
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