netCD.gif

networking curriculum
support CD for senior
secondary studies

Topics
. Home
. Networking History
. Why network?
. OSI model
. Connecting up
. Protocols
. Technology
. Network types
. Topologies
. Issues
. Design factors
. Economic effects
. www links
. Site map
Print Formattednetworking > issues > firewalls

Firewalls

Connecting to the Internet can immediately expose you to malicious activity by others. When it connects to the Internet, your computer's operating system opens up many communication ports. For example, when your browser requests and receives HTTP (web page) data that traffic goes in and out port 80. FTP (File Transfer Protocol) traffic usually uses port 21. IRC (Internet Relay Chat) uses a different port. There are many, many other ports that are usually idle, but can be used by outsiders to gain access to your computer.

Other people can do "port probing" or "port sniffing" using automated software that scans the web looking for connected computers. When a computer is found, the software interrogates the computer's Internet ports to see if they are protected against intrusion. A simple command sent to an unprotected port makes the operating system automatically respond, indicating the port is open. If open ports are found, the person can sometimes gain access to your computer's data, or even control your computer.

A firewall is a hardware or software component that blocks unauthorised Internet traffic from entering or leaving a computer through any of its 65,536 internet communication ports. Unauthorised incoming traffic is often a sign that someone is trying to do a port probe, looking for weaknesses in the computer's security. When a weakness is found, the outsider could take control of the computer. Unauthorised outgoing traffic can be a sign that a Trojan horse is active and trying to illicitly communicate with the outside world.

A Trojan Horse is software that appears to be harmless, but in fact hides in a user's computer and performs undesirable activities by stealth, such as logging the user's typing (e.g. passwords, credit card information) or participating in Distributed Denial of Service attacks under the control of a remote hacker.

A Distributed Denial of Service (DDOS) attack is where a person remotely sends commands to many Trojan-infected computers to bombard a web server with millions of bogus requests for information. The sheer amount of incoming traffic can swamp the targeted server and slow down or stop its normal operations.

A free and effective firewall, Zone Alarm can be downloaded from www.zonelabs.com. It can be customised by the operator to give permission only to legitimate programs to access the internet. Any unauthorised or unrecognised program that attempts to transmit to the internet raises a warning to the operator who can allow or disallow the communication.

Firewalls use one or more of three methods to control traffic flowing in and out of the network:

  • Packet filtering - Packets (small chunks of data) are analysed against a set of filters. Packets that make it through the filters are sent to the requesting system and all others are discarded.
  • Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.
  • Stateful inspection - A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information travelling from inside the firewall to the outside is monitored for specific defining characteristics, and then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.

worksheets
. history
. why network
. osi model
. connecting
. protocols
. technology
. topologies (tla)
. issues
. network types
. design
. economics
. economics (cs)
   - solutions -

quizzes
. why network
. osi model
. connecting
. protocols
. technology
. topology
. issues
. network types
. design
. economics

| Home | History | Why Network? | OSI Model | Connecting up | Protocols | Technology | Networks Types | Topologies | Issues | Design Factors | Economic effects |
:: copyright | credits | sitemap ::

© 2003 Victorian Information Technology Teachers Association
http://www.vitta.org.au/