Domain Name System

Keeping track of names and information: the domain system
As we indicated earlier, the network software generally needs a 32-bit Internet address in
order to open a connection or send a datagram. However users prefer to deal with
computer names rather than
numbers. Thus there is a database that allows the software to look up a name and find the
corresponding number. When the Internet was small, this was easy. Each system would
have a file that listed all of the
other systems, giving both their name and number. There are now too many computers
for this approach to be practical. Thus these files have been replaced by a set of name
servers that keep track of host
names and the corresponding Internet addresses. (In fact these servers are somewhat
more general than that. This is just one kind of information stored in the domain system.)
Note that a set of interlocking servers are used, rather than a single central one. There are
now so many different institutions connected to the Internet that it would be impractical
for them to notify a central
authority whenever they installed or moved a computer. Thus naming authority is
delegated to individual institutions. The name servers form a tree, corresponding to
institutional structure. The names
themselves follow a similar structure.
A typical example is the name BORAX.LCS.MIT.EDU. This is a computer at the
Laboratory for Computer Science (LCS) at MIT. In order to find its Internet address, you
might potentially have to consult 4
different servers. First, you would ask a central server (called the root) where the EDU
server is. EDU is a server that keeps track of educational institutions. The root server
would give you the names and
Internet addresses of several servers for EDU. (There are several servers at each level, to
allow for the possibly that one might be down.) You would then ask EDU where the
server for MIT is. Again, it
would give you names and Internet addresses of several servers for MIT. Generally, not
all of those servers would be at MIT, to allow for the possibility of a general power
failure at MIT. Then you would ask
MIT where the server for LCS is, and finally you would ask one of the LCS servers about
BORAX. The final result would be the Internet address for BORAX.LCS.MIT.EDU.
Each of these levels is referred to as
a "domain". The entire name, BORAX.LCS.MIT.EDU, is called a "domain name". (So
are the names of the higher-level domains, such as LCS.MIT.EDU, MIT.EDU, and
EDU.)
Fortunately, you don't really have to go through all of this most of the time. First of all,
the root name servers also happen to be the name servers for the top-level domains such
as EDU. Thus a single
query to a root server will get you to MIT. Second, software generally remembers
answers that it got before. So once we look up a name at LCS.MIT.EDU, our software
remembers where to find servers for
LCS.MIT.EDU, MIT.EDU, and EDU. It also remembers the translation of
BORAX.LCS.MIT.EDU. Each of these pieces of information has a "time to live"
associated with it. Typically this is a few days. After that,
the information expires and has to be looked up again. This allows institutions to change
things.
 
The domain system is not limited to finding out Internet addresses. Each domain name is
a node in a database. The node can have records that define a number of different
properties. Examples are
Internet address, computer type, and a list of services provided by a computer. A program
can ask for a specific piece of information, or all information about a given name. It is
possible for a node in the
database to be marked as an "alias" (or nickname) for another node. It is also possible to
use the domain system to store information about users, mailing lists, or other objects.
There is an Internet standard defining the operation of these databases, as well as the
protocols used to make queries of them. Every network utility has to be able to make
such queries, since this is now the official way to evaluate host names. Generally utilities
will talk to a server on their own system. This server will take care of contacting the other
servers for them. This keeps down the amount of code that has to be in each application
program.
The domain system is particularly important for handling computer mail. There are entry
types to define what computer handles mail for a given name, to specify where an
individual is to receive mail, and to
define mailing lists. (See RFC's 882, 883, and 973 for specifications of the domain
system. RFC 974 defines the use of the domain system in sending mail.)