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(gawkinet.info)Setting Up

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Setting Up a Service
====================

   The preceding programs behaved as clients that connect to a server
somewhere on the Internet and request a particular service. Now we set
up such a service to mimic the behavior of the `daytime' service.  Such
a server does not know in advance who is going to connect to it over
the network. Therefore we cannot insert a name for the host to connect
to in our special file name.

   Start the following program in one window. Notice that the service
does not have the name `daytime', but the number `8888'.  From looking
at `/etc/services', you know that names like `daytime' are just
mnemonics for predetermined 16-bit integers.  Only the system
administrator (`root') could enter our new service into `/etc/services'
with an appropriate name.  Also notice that the service name has to be
entered into a different field of the special file name because we are
setting up a server, not a client:

     BEGIN {
       print strftime() |& "/inet/tcp/8888/0/0"
       close("/inet/tcp/8888/0/0")
     }

   Now open another window on the same machine.  Copy the client
program given as the first example (Note: Establishing a TCP
Connection.)  to a new file and edit it, changing the
name `daytime' to `8888'.  Then start the modified client.  You should
get a reply like this:

     Sat Sep 27 19:08:16 CEST 1997

Both programs explicitly close the connection.

   Now we will intentionally make a mistake to see what happens when
the name `8888' (the so-called port) is already used by another service.
Start the server program in both windows. The first one works, but the
second one complains that it could not open the connection. Each port
on a single machine can only be used by one server program at a time.
Now terminate the server program and change the name `8888' to `echo'.
After restarting it, the server program does not run any more and you
know why: there already is an `echo' service running on your machine.
But even if this isn't true, you would not get your own `echo' server
running on a Unix machine, because the ports with numbers smaller than
1024 (`echo' is at port 7) are reserved for `root'.  On machines
running some flavor of Microsoft Windows, there is no restriction that
reserves ports 1 to 1024 for a privileged user; hence you can start an
`echo' server there.

   Turning this short server program into something really useful is
simple.  Imagine a server that first reads a file name from the client
through the network connection, then does something with the file and
sends a result back to the client. The server-side processing could be:

     BEGIN {
       NetService = "/inet/tcp/8888/0/0"
       NetService |& getline
       CatPipe    = ("cat " $1)    # sets $0 and the fields
       while ((CatPipe | getline) > 0)
         print $0 |& NetService
       close(NetService)
     }

and we would have a remote copying facility. Such a server reads the
name of a file from any client that connects to it and transmits the
contents of the named file across the net. The server-side processing
could also be the execution of a command that is transmitted across the
network. From this example, you can see how simple it is to open up a
security hole on your machine. If you allow clients to connect to your
machine and execute arbitrary commands, anyone would be free to do `rm
-rf *'.