12-4
Domain Name System (DNS) Services
Overview
This system diffuses domain records throughout the Internet. Hosts anywhere
on the Internet can still reach each other because name servers can query each
other for the hostnames they cannot translate.
Authoritative and Caching Name Servers
Most name servers function as an authoritative server for one or several zones
and as a caching server for all other zones. A name server’s host table includes
entries for all hosts in the zones on which it is authoritative. When a client
requests the IP address for one of these hosts, the authoritative server can
immediately provide it. The server caches the most recently requested entries
for hosts in other zones. It has received these entries from other servers
through a query process.
DNS Queries
When a server receives a request to translate a hostname that is not in its host
table or cache, it runs its resolver and queries its root server. Root servers
know the addresses for the top-level name servers, which in turn know the
addresses for the name servers of their first-level domains. These servers
provide IP addresses for hosts in their domain. (See Figure 12-1 for an example
of a DNS query.)
Caching addresses speeds up the query process. Clients are constantly
requesting .com addresses. A name server will hold the top-level name server’s
address in its cache, instead of having to query its root server for it each time
a client requests an address ending in .com.
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