All intranets are vulnerable to attack. Their underlying TCP/IP architecture is identical to that of the Internet. Since the Internet was built for maximum openness and communication, there are countless techniques that can be used to attack intranets. Attacks can involve the theft of vital company information and even cash. Attacks can destroy or deny a company's computing resources and services. Attackers can break in or pose as a company employee to use the company's intranet resources.
Firewalls are hardware and software combinations that block intruders from access to an intranet while still allowing people on the intranet to access the resources of the Internet. Depending on how secure a site needs to be, and on how much time, money, and resources can be spent on a firewall, there are many kinds that can be built. Most of them, though, are built using only a few elements. Servers and routers are the primary components of firewalls.
Most firewalls use some kind of packet filtering. In packet filtering, a screening router or filtering router looks at every packet of data traveling between an intranet and the Internet. See Chapter 13 for more information on filtering.
Proxy servers on an intranet are used when someone from the intranet wants to access a server on the Internet. A request from the user's computer is sent to the proxy server instead of directly to the Internet. The proxy server contacts the server on the Internet, receives the information from the Internet, and then sends the information to the requester on the intranet. By acting as a go-between like this, proxy servers can filter traffic and maintain security as well as log all traffic between the Internet and the network.
Bastion hosts are heavily fortified servers that handle all incoming requests from the Internet, such as FTP requests. A single bastion host handling incoming requests makes it easier to maintain security and track attacks. In the event of a break in, only that single host has been compromised, instead of the entire network. In some firewalls, multiple bastion hosts can be used, one for each different kind of intranet service request.
Firewalls protect intranets from any attacks launched against them from the Internet. They are designed to protect an intranet from unauthorized access to corporate information, and damaging or denying computer resources and services. They are also designed to stop people on the intranet from accessing Internet services that can be dangerous, such as FTP.