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What is a firewall?

A firewall is a system or network that acts as a barrier between two or more networks. This barrier can control what passes between networks. A firewall is designed to permit authorized communications and block unauthorized traffic like viruses. Firewalls can be in the form of software like a program or hardware like a router.

All information entering or leaving a network goes through the firewall. The firewall will then scan the information and either allow or deny access based on the security settings. Firewalls are also used to prevent intruders from accessing sensitive or personal information.

Firewall control methods

There are several types of firewall methods that can be used to control incoming and outgoing traffic.

  • A packet filter looks at each packet (small group) of information and decides if the packet can be allowed through based on user defined restrictions.

  • A application gateway restricts and grants access on specific applications such as FTP (file transfer Protocol) and Telnet (teletype network) servers. This gateway allows communication from outside users to the network while hiding specific computers behind the firewall. This method is very effective but requires a large amount of memory (webopedia).

  • A Circuit-level gateway secures TCP (Transmission Control Protocol) UDP (User Datagram Protocol) when they are established. Once this connection has been allowed, information passes through the gateway without further checking.

  • A Proxy server effectively protects users by acting as a go between for clients requesting resources from a server. The actual server address is hidden behind the proxy server. A proxy server also caches responses for quick loading on frequently requested information. Unlike other firewall methods where traffic flows through the wall directly to each network after passing the filters, proxy servers create a new network connection for each request, eliminating the need for direct connections between networks.

  • A Stateful inspection, also known as dynamic packet filtering, works by examining the contents of a packet rather then just the heading of the packet which is what a packet filter does. This allows the firewall to filter information based on previously allowed packets as well as the user defined rules (webopedia).

Firewall Filters

Firewalls can be set up with several different filters which look for specific attributes from the information passing through the firewall software. Some common filters block access from:

  • IP addresses which are unique identifiers for each device connected to the Internet. This address is in the form of 32-bit numbers “normally expressed as four 'octets' in a 'dotted decimal number.'” A firewall can filter out and deny access to all information coming from a certain IP address.

  • Domain names are smiliar to IP addresses because they are also unique but are expressed in words. Some firewalls will filter packets out by domain name because IP address can be changed but servers can be identifed by the domain name.

  • Protocols are a established method of communication between something like a web browser to a server. Web browsers use the protocol “http.” Other common protocols are

                - 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

                 - Telnet - used to perform commands on a remote computer

  • Ports allow server machines to access the Internet through numbered ports. Filters can customized to only allow one machine in a company to access a specific port.

  • Key words: Filters can also be programmed to search through packets of information to find key words. These key words must be an exact match other wise the filter will not recognize the word(s).

Firewalls in home networks are also known as gateways because they provide the only access point between the home network and the Internet. When a router is used to connect computers on a network, the router is considered the gateway (howstuffworks.com).