Killer Game Programming in Java

Firewalls

Firewalls are unfortunately a part of today's networking experience. Most companies, government institutions, universities, and so on, utilize firewalls to block access to the wider Internet: socket creation on all nonstandard ports, and most standard ones, are typically prohibited, and web pages must be retrieved through a proxy that filters (limits) the traffic.

Tip

The firewall-related examples in this section can be found in the NetBasics/Firewall/ directory.

This situation means that the code described so far may not work because it requires the creation of sockets. The DayPing example given below is a Java application that attempts to contact a "time-of-day" server. The example below uses the server at the National Institute of Standards and Technology in Boulder, Colorado:

    >java DayPing time-A.timefreq.bldrdoc.gov
    time-A.timefreq.bldrdoc.gov is alive at

    53394 05-01-24 09:16:12 00 0 0 525.7 UTC(NIST) *

DayPing opens a socket to the host at port 13, where the standard time-of-day service is listening. The response is printed out using println() after layering a BufferedReader stream on top of the network link:

 public class DayPing { public static void main(String args[]) throws IOException { if (args.length != 1) { System.out.println("usage: java DayPing <host> "); System.exit(0); } Socket sock = new Socket(args[0],13); // host and port BufferedReader br = new BufferedReader( new InputStreamReader( sock.getInputStream() ) ); System.out.println( args[0] + " is alive at ...