In essence, DNS is simply a database that links meaningful names
(known ashost names), such as http://www.microsoft.com, to a specific IP
address, such as 192.168.124.1. Simply linking addresses to names is just the
beginning, though, because DNS has many more features in addition to
host-name-to-address mapping.
The key features of host name to IP mapping are as follows:
·
Mappings of addresses to names and vice versa (known as records)
are stored in a database.
·
The DNS database is distributed.
·
A DNS database also stores additional records.
Although DNS is a
database, most importantly it's a distributed database. Each DNS server
contains only a small portion of the host name to IP address mappings (relative
to the number of records for the entire Internet). Each DNS server is
configured with a special record that tells the DNS server where (the IP
address of another DNS server) it will perform a lookup for records it doesn't
have in its portion of the DNS database. Because of this arrangement, each DNS
server maintains only a small portion of the total DNS host to IP address
mappings. The collection of host-name-to-IP-address mappings contained with the
DNS database is also known as a namespace. Essentially, when looking for a name
in DNS, the DNS client first checks a top-level DNS server database. That
server tells the client which DNS server hosts the next part of the DNS name,
and the client then queries that server. This lookup-and-handoff process
continues until the client finds the DNS server that hosts the DNS record in
question, and that server provides the IP address.
In addition to the basic IP-address-to-host-name mapping records
stored by the DNS database, records are also maintained by DNS for other
purposes. DNS contains a number of record types that facilitate other
applications. The Mail Exchanger (MX) record, for example, provides mail
servers with the information required to forward e-mail messages to the
recipient's e-mail server. Another type of record, the service (SVC) record, is
used by Microsoft Active Directory to locate network services.
Seeing
the DNS difference
By itself, DNS doesn't appear to do much, and on top of that,
DNS can seem a bit intimidating because it has number of different features and
record types. One key to understanding the importance of DNS is realizing how
other processes and applications depend on the services DNS provides. By
understanding how DNS provides the underlying services used by various
applications, you can get a clearer picture of why DNS exists and how it works.
Many common applications use DNS services, including
·
World Wide Web (WWW)
·
E-mail
·
Other applications, such as instant messaging
The World Wide Web depends on DNS for user-friendly navigation.
You could get to a Web site by entering the IP address of a site in your Web
browser, but remembering lots of arbitrary numbers isn't easy for most folks.
It's much easier to remember a DNS name for a Web site that reflects its
content, such as http://www.yahoo.com or http://www.microsoft.com. It's fair to
say that without DNS, the Web wouldn't have become quite the phenomenon that it
is now.
Serving
the e-mail connection
E-mail is one of the more popular applications that use DNS.
Although the Web simply uses DNS for linking names to IP addresses for Web
sites, e-mail servers also require some specialized records above and beyond
what is required for basic host name to IP addresses. For example, when an
e-mail message is sent from your e-mail client (such as Microsoft Outlook or
Netscape Messenger), it can be sent either directly to the target domain
(Microsoft.com if the note was sent to user@microsoft.com) or to another e-mail
server that is providing a relay service. If your e-mail application specifies
an outgoing (SMTP) mail server that is not the final destination server for the
message, you're making use of the relay process.
An e-mail address is made up of two parts: a recipient and a
host. In the address postmaster@domain.tld, postmaster is the recipient, the user who will
receive the message. This is irrelevant to the SMTP process, though, because
the mail transfer agent (MTA) is responsible for making sure that the message
gets into the mailbox of the recipient.
The host, domain.tld, is of much more interest. In this
case, domain.tld refers not to a host in the traditional sense of an A record
but rather to a mail server known as a mail
exchanger (MX). This server
is responsible for accepting all mail for domain.tld, denoted by a special
record — an MX record — in DNS.
Beyond the Web and e-mail are many applications that either rely
on or can use DNS services. These applications can include databases,
multi-tier Web applications built by using middleware or an application server,
peer-to-peer sharing programs, instant messaging, and multiplayer games.
In quite a practical sense, any application
that uses the Internet to connect two or more hosts to share information, or
otherwise communicate, is probably relying on DNS services in one form or
another.
