It is well known that the Web is populated by mean and unscrupulous people who want to mess up your site. Many conservative citizens think that a firewall is the way to stop them. The purpose of a firewall is to prevent the Internet from connecting to arbitrary machines or services on your own LAN/WAN. Another purpose, depending on your environment, may be to stop users on your LAN from roaming freely around the Internet.
The term firewall does not mean anything standard. There are lots of ways to achieve the objectives just stated. Two extremes are presented in this section, and there are lots of possibilities in between. This is a big subject: here we are only trying to alert the webmaster to the problems that exist and to sketch some of the ways to solve them. For more information on this subject, see Building Internet Firewalls, by D. Brent Chapman and Elizabeth D. Zwicky (O'Reilly & Associates).
This technique is the simplest firewall. In essence, you restrict packets that come in from the Internet to safe ports. Packet-filter firewalls are usually implemented using the filtering built into your Internet router. This means that no access is given to ports below 1024 except for certain specified ones connecting to safe services, such as SMTP, NNTP, DNS, FTP, and HTTP. The benefit is that access is denied to potentially dangerous services, such as the following:
Gives a list of logged-in users, and in the process tells ...