 Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. X.25, Internets, Intranets, and Extranets Chapter 7 Implementing Extranets In order to provide some sort of consistency, some definitions are required. These are not universal (and are violated frequently in the trade press) but help with the following discussions. Organizations are connected to the Internet and specifically permit access to the internal network (or portions of our network) by its customers. Now the organization is at risk from all sorts of hackers, crackers, and freaks. The goal then is to keep the outsiders (hackers and crackers) out, or at least slow them down and make them work for it, while giving friends and customers free, unfettered access. The first step then is to install a firewall. A firewall is simply a machine whose job it is to examine each packet arriv- ing and verify whether it is permitted based on a set of rules. This set of rules is referred to as the rule base. Firewalls essentially operate at the IP and TCP levels of the protocol stack. This means that the firewall examines each packet to see if it is coming from and going to the proper IP address and that it contains the proper or permitted TCP port number. These fire- walls can be set up as a one-way valve. Employees can surf the Net at will and send out any kind of packet, but the firewall will not permit any unso- licited packets. (This means that mail won't work; that is, the firewall won't let in any packets including mail.) The other problem with this approach is that it will permit internal employees to hack on the Net. If they are caught, the corporation may be liable SO think about this issue. This is not to say that firewalls are not useful but to point out that simplistic solutions will not achieve the organization's objectives. Performing basic packet filtering on a source and destination IP address is a useful feat