2. Uses
• translates domain names (computer hostnames) to IP
addresses.
• lists mail exchange servers accepting e-mail for each
domain.
• an essential component of contemporary Internet use.
• Originally, each computer on the network retrieved a file
called HOSTS which mapped an address (such as
192.0.34.166) to a name (such as www.example.net.)
• The Hosts file still exists on most modern operating
systems, and allows users to specify an IP address to
use for a hostname without checking the DNS.
• Modifying the hosts file is done on each and every PC
(tiresome!!)
3. How the DNS works
• The domain name space consists of a tree of
domain names. Each node or leaf in the tree has
one or more resource records, which hold
information associated with the domain name.
• The tree sub-divides into zones. A zone consists
of a collection of connected nodes authoritatively
served by an authoritative DNS nameserver.
• A resolver (DNS server) looks up the
information associated with nodes. A resolver
knows how to communicate with name servers
by sending DNS requests, and heeding DNS
responses.
4. • A domain name usually consists of two or more
parts (technically labels), separated by dots. For
example wikipedia.org.
• The rightmost label conveys the top-level
domain (for example, the address
en.wikipedia.org has the top-level domain org).
• Each domain or subdomain has one or more
authoritative DNS servers that publish
information about that domain and the name
servers of any domains "beneath" it.
5. The address resolution
mechanism
• DNS resolution process for ahost.inadomain.com
1. the local system is pre-configured with the known addresses of
the root servers in a file of root hints, which need to be updated
periodically by the local administrator from a reliable source to
be kept up to date with the changes which occur over time.
2. query one of the root servers to find the server authoritative for
the next level down (so in the case of our simple hostname, a
root server would be asked for the address of a server with
detailed knowledge of the com top level domain).
3. querying this second server for the address of a DNS server
with detailed knowledge of the second-level domain
(inadomain.com in our example).
4. repeating the previous step to progress down the name, until
the final step which would, rather than generating the address of
the next DNS server, return the final address sought.
6.
7. • DNS says, “Go ask the people who manage it, and they
will tell you.”
Improvement
• results of a query are cached for a period of time after a
successful answer (the server providing the answer
initially dictates the period of validity, which may vary
from just seconds to days or even weeks).
8. Types of Name Servers
• Name servers that hold all information on hosts within a
zone are called authoritative for this zone, and
sometimes are referred to as master name servers. Any
query for a host within this zone will end up at one of
these master name servers.
• To distribute work loads, master name servers have
secondary servers, which transfer the zone data from the
primary (master) server at regular intervals.
– provides backup
• A name server that is not authoritative for any domain
which is able to conduct DNS queries for the applications
running on the local network and cache the information,
is called a caching-only server.
9. Reverse Lookups
• Finding the IP address belonging to a host is
certainly the most common use for the Domain
Name System, but sometimes you'll want to find
the canonical hostname corresponding to an
address.
• Finding this hostname is called reverse
mapping, and is used by several network
services to verify a client's identity.
• When using a single hosts file, reverse lookups
simply involve searching the file for a host that
owns the IP address in question.
10. • Detailed tutorial on the net at
http://www.learntcpip.com/DNS/License_DNS.htm
