From the networking point of view, the most basic services are the ability to
execute commands and to move data around.  The Internet protocols perform these
tasks with a pair of processes: a \fIclient\fP\| which actively asks for
services at one end of the link, and a \fIserver\fP\| at the other end of the
link which responds to requests and perform the requested activity.  These terms
are also used to describe computer systems, but here we're talking about
processes, not systems.  In this chapter, we'll look at the client side of
things: in the next section we'll look at executing programs remotely, on page
.Sref "\*[filecopy]" \&
we'll look at copying files, and on page
.Sref "\*[www]" \&
we'll look at browsing the World-Wide web. 
.P
In the olden days, you would use
.Command telnet
and
.Command rlogin
to log into another machine, and
.Command ftp
and
.Command rcp
to transfer files across the network.  These programs are still with us, but
security concerns make
.Command rlogin
and
.Command rcp
.Command ftp
effectively obsolete, and
.Command telnet
and
.Command ftp
are only of limited use.  For normal situations, you would replace
.Command telnet
and
.Command rlogin
with
.Command ssh ,
a \fIsecure shell\fP.  Similarly, you would use
.Command scp ,
an \fIssh\fP\|-based copy program, instead of
.Command ftp
and
.Command rcp .
.H2 "Running programs on another computer"
Your main interface with the local machine is the \fIshell\fP, which we looked
at in \*[chunixref].  You can access remote machines in pretty much the same way
as local machines: you just need a program to relay your input to the remote
computer and the output from the remote computer back to your machine.
.P
.Command telnet
and
.Command rlogin
both do this, but they suffer from the disadvantage that they transmit data in
clear text without any kind of encryption.  Any sniffer program, for example
.Command tcpdump ,
can intercept the data and use it (passwords for example) to break into one of
the machines.
.Command telnet
and
.Command rlogin
are still of marginal use on trusted local networks.
.Command telnet
has a couple of other uses, so we'll look at it briefly on page
.Sref "\*[telnet]" .
