Firewall

What is a firewall?

A firewall is a set of related programs, located at a network gateway server, that protects the resources of a private network from users from other networks.

Basically, a firewall is a barrier to keep destructive forces away from your property. A firewall is simply a program or hardware device that filters the information coming through the Internet connection into your private network or computer system. If an incoming packet of information is flagged by the filters, it is not allowed through.

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 analyzed 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 traveling from inside the firewall to the outside is monitored for specific defining characteristics, then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.

Firewall Configuration

Firewalls are customizable. This means that you can add or remove filters based on several conditions. Some of these are:

• IP addresses - Each machine on the Internet is assigned a unique address called an IP address. IP addresses are 32-bit numbers, normally expressed as four "octets" in a "dotted decimal number." A typical IP address looks like this: 216.27.61.137. For example, if a certain IP address outside the company is reading too many files from a server, the firewall can block all traffic to or from that IP address.
• Domain names - Because it is hard to remember the string of numbers that make up an IP address, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. For example, it is easier for most of us to remember www.howstuffworks.com than it is to remember 216.27.61.137. A company might block all access to certain domain names, or allow access only to specific domain names.
• Protocols - The protocol is the pre-defined way that someone who wants to use a service talks with that service. The "someone" could be a person, but more often it is a computer program like a Web browser. Protocols are often text, and simply describe how the client and server will have their conversation. The http in the Web's protocol. Some common protocols that you can set firewall filters for include:
  
  
  
  
  
  • IP (Internet Protocol) - the main delivery system for information over the Internet
  • TCP (Transmission Control Protocol) - used to break apart and rebuild information that travels over the Internet
  • HTTP (Hyper Text Transfer Protocol) - used for Web pages
  • FTP (File Transfer Protocol) - used to download and upload files
  • UDP (User Datagram Protocol) - used for information that requires no response, such as streaming audio and video
  • ICMP (Internet Control Message Protocol) - used by a router to exchange the information with other routers
  • SMTP (Simple Mail Transport Protocol) - used to send text-based information (e-mail)
  • SNMP (Simple Network Management Protocol) - used to collect system information from a remote computer
  • Telnet - used to perform commands on a remote computer
  A company might set up only one or two machines to handle a specific protocol and ban that protocol on all other machines.
  
• Ports - Any server machine makes its services available to the Internet using numbered ports, one for each service that is available on the server (see How Web Servers Work for details). For example, if a server machine is running a Web (HTTP) server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. A company might block port 21 access on all machines but one inside the company.
• Specific words and phrases - This can be anything. The firewall will sniff (search through) each packet of information for an exact match of the text listed in the filter. For example, you could instruct the firewall to block any packet with the word "X-rated" in it. The key here is that it has to be an exact match. The "X-rated" filter would not catch "X rated" (no hyphen). But you can include as many words, phrases and variations of them as you need.