A firewall is a network security device that monitors incoming and outgoing network traffic and decides whether to allow or block specific traffic based on a defined set of security rules.

firewalls are no cure-all solution to network security woes. A firewall is only as good as its rule set, and there are many ways an attacker can find common misconfigurations and errors in the rules. For example, if the firewall blocks all traffic except traffic originating from port 53 (DNS) so that everyone can resolve names, the attacker could then use this rule to his advantage. By changing the source port of his attack or scan to port 53, the firewall will allow all of his traffic through, because it assumes it is DNS traffic.

Bypassing firewalls is a whole study in itself and one which is very interesting (especially to those with a passion for networking), because it normally involves misusing the way TCP and IP are supposed to work. That said, firewalls today are becoming very sophisticated and a well-installed firewall can severely thwart a would-be attacker's plans.

It is important to remember that the firewall does not look into the data section of the packet. Thus, if you have a Web server that is vulnerable to a CGI exploit and