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<H1><A NAME="SECTION00920000000000000000">Logins via the network</A></H1>
<P>
Two computers in the same network are usually linked via a single
physical cable. When they communicate over the network, the programs
in each computer that take part in the communication are linked
via a <b>virtual connection</b>, a sort of imaginary cable.
As far as the programs at either end of the virtual connection
are concerned, they have a monopoly on their own cable. However,
since the cable is not real, only imaginary, the operating systems
of both computers can have several virtual connections share
the same physical cable. This way, using just a single cable,
several programs can communicate without having to know of or
care about the other communications. It is even possible to
have several computers use the same cable; the virtual
connections exist between two computers, and the other computers
ignore those connections that they don't take part in.
<P>
That's a complicated and over-abstracted description of the
reality. It might, however, be good enough to understand
the important reason why network logins are somewhat different
from normal logins. The virtual connections are established
when there are two programs on different computers that
wish to communicate. Since it is in principle possible to
login from any computer in a network to any other computer,
there is a huge number of potential virtual communications.
Because of this, it is not practical to start a <tt>getty</tt><A NAME="2636"> </A>
for each potential login.
<P>
There is a single process inetd (corresponding
to <tt>getty</tt><A NAME="2638"> </A>) that handles <em>all</em> network logins.
When it notices an incoming network login (i.e., it notices
that it gets a new virtual connection to some other computer),
it starts a new process to handle that single login. The
original process remains and continues to listen for new
logins.
<P>
To make things a bit more complicated, there is more than
one communication protocol for network logins. The two
most important ones are <tt>telnet</tt><A NAME="2640"> </A> and <tt>rlogin</tt><A NAME="2642"> </A>. In
addition to logins, there are many other virtual connections
that may be made (for FTP, Gopher, HTTP, and other network services).
It would be ineffective to have a separate process listening
for a particular type of connection, so instead there is only
one listener that can recognize the type of the connection and
can start the correct type of program to provide the service.
This single listener is called <tt>inetd</tt><A NAME="2644"> </A>; see the
``Linux Network Administrators' Guide'' for more information.
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<P><ADDRESS>
<I>Lars Wirzenius <BR>
Sat Nov 15 02:32:11 EET 1997</I>
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