$user = <$client_socket>;
 chomp($user); #remove the \n
 print "$prefix USER = $user\n";
	unless (open (USERNAME, "username")) 
	 {
       print "ERROR:Cannot open username file.\n";
       exit(1); # ERROR
      }
	my $found;
	while(my $newusername = <USERNAME>)
		{
				chomp($newusername);
				if($newusername eq $user) 
				{
				$found = 1;
				last;
				}
				
				unless ($found) {
				print $client_socket "ERR\n";
				print "$prefix ERROR: user $user not allowed  Exiting.\n";
				close(USERNAME);
				exit(1);
	 			}
		
	}
     #else, continue
     print $client_socket "ACK\n";  # approve the user.  
	 #print "user $user is allowed.";
	 close(USERNAME);