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gawkinet: Setting Up

 
 2.5 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 (Establishing a TCP Connection TCP
 Connecting.) 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 is already 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 *'.
 

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