#!/usr/bin/perl
#                     for debugging, use .../perl -d
##############################################################################
#
# TCP/IP server: 
#    - Listen to client
#    - Fork a child to perform communication
#    -    Child will execute whatever is received as a Unix command
#    - Go back and listen for more clients
#
# Usage:  TCPserver portNumber &      (always run server in the background) 
#
##############################################################################

use Socket;                                     # import Socket.pm

$serverPort = $ARGV[0] || 8467;                 # if none, use port 8467

&setConnect();                                  # setup connection

&clientListen();                                # listen to client(s)


##############################################################################
#  setConnect: Establish connection to listening port
##############################################################################
sub setConnect {

    ### Server information ###

    my $serverName = `hostname`;                       #cmd `hostname`
    chomp($serverName);

    my $serverAddrBin = gethostbyname($serverName);
    my @serverAddrNum = unpack("C4", $serverAddrBin);
    my $serverAddr    = join('.', @serverAddrNum);    

#   my $serverAddr    = inet_ntoa($serverAddrBin);     #same as prev 2 lines

    print "Server name: $serverName \n";
    print "IP address : $serverAddr \n";
    print "Port number: $serverPort \n\n";


    my $protocol      = getprotobyname("tcp");

#   my $serverSocket  = pack_sockaddr_in($serverPort, undef);
    my $serverSocket  = pack_sockaddr_in($serverPort, INADDR_ANY);


    ### create socket filehandle ###

    socket(LISTENSOCKET, PF_INET, SOCK_STREAM, $protocol)
                     || die "$0: Cannot create $protocol socket- $!";


    ### set socket options ###

    setsockopt(LISTENSOCKET, SOL_SOCKET, SO_REUSEADDR, pack("l", 1))
                     || die "$0: Cannot set socket options- $!";


    ### bind socket to local port number ###

    bind(LISTENSOCKET, $serverSocket)
                     || die "$0: Cannot bind socket to server- $!";


    ### Establish the length of the waiting queue ###

    listen(LISTENSOCKET, SOMAXCONN)
                     || die "$0: Cannot establish listening queue- $!";
}

##############################################################################
#  clientListen: listen to clients on generic socket,
#                when a new client arrives --> fork a child to serve him 
##############################################################################
sub clientListen {

    select LISTENSOCKET;
    $| = 1;                                 # set LISTENSOCKET to unbuffered
    select CLIENTSOCKET;
    $| = 1;                                 # set CLIENTSOCKET to unbuffered
    select STDOUT;
    $| = 1;                                 # set STDOUT to unbuffered

    for ($connect = 1; ; $connect++) {      # loop infinitely  

        print("Listening for connection $connect.....\n\n");


        ### accept socket connection from client ### 

        ($clientSocket = accept(CLIENTSOCKET, LISTENSOCKET) )
                     || die "$0: Cannot accept client connection- $!";

        $pid = fork();                   # fork a new process (dupl this pgm)

        defined $pid || die "$0: $!";    # pid == null? then die

        if ($pid == 0) {

            &child();                    # perform reading from the socket

        }
    }
}

##############################################################################
#  child: I'm the child, read from the socket, and write to STDOUT
#                        do whatever the child wants
#                        write results into the socket, and write to STDOUT
##############################################################################
sub child {

    my ($clientPort, $clientAddrBin) = unpack_sockaddr_in($clientSocket);

    my $clientName    = gethostbyaddr($clientAddrBin, AF_INET);

    my @clientAddrNum = unpack("C4", $clientAddrBin);
    my $clientAddr    = join('.', @clientAddrNum);    

#   my $clientAddr    = inet_ntoa($clientAddrBin);     #same as prev 2 lines

    print "Connect to : $clientName \n";
    print "at IP addr : $clientAddr \n"; 
    print "Port number: $clientPort \n\n";

    while ($data = <CLIENTSOCKET>) {        # read from client socket
        print STDOUT $data;                 # print what's received locally           
#       $data =~ tr/[a-z]/[A-Z]/;           # translate to upcase (or whatever)

        $result = `$data`;                  # execute whatever is requested

        print CLIENTSOCKET $result;         # write back into socket

        print STDOUT $result;               # print what's sent back locally
    }
	
    close(CLIENTSOCKET);
        
    exit (0);
}