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IP spoofing
packet filtering
proxy server

A system designed to prevent unauthorized access to or from a private network. Firewalls can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

There are several types of firewall techniques:

  • Packet filter: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules. Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
  • Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose a performance degradation.
  • Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, packets can flow between the hosts without further checking.
  • Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.
  • In practice, many firewalls use two or more of these techniques in concert.

    A firewall is considered a first line of defense in protecting private information. For greater security, data can be encrypted.

    More Information

    Outstanding Page Firewall lab test - article
    Data Communications article (March 21, 1997) on the performance results of 20 firewall products. Reveals trends, product features and downfalls. Updated on Aug 5, 1998

    Outstanding Page ICSA 3rd Annual Firewall Buyer's Guide
    A free resource for end users who need to learn more about firewall technology, or who want some guidance in selecting a commercial firewall. The guide includes 90 pages of educational material, and thorough descriptions of 50 leading commercial firewall products.

    Outstanding Page
    Home page of ICSA, a leading membership organization for information security, including firewalls, reliability, ethics, and privacy issues, plus defense against virus, Trojan horse and other malicious code attacks.

    Outstanding Page Information Security Standards
    Provides information on standards and specifications that can be used to ensure the security of data interchanged between two open systems. Updated on May 9, 1998

    Outstanding Page Paper on Internet firewalls and security
    This paper, from 3Com, focuses on many of the issues relating to the construction of Internet firewalls, including their benefits, limitations, building blocks, and examples of firewall system topologies. Updated on Apr 25, 1998

    Outstanding Page Rotherwick firewall resources
    Contains over 900 firewall and Internet security links as well as introductory information on firewalls and network security. Updated on Mar 28, 1998

      Considerations for LAN and Internet Security
    Article by Gary C. Kessler and Carol A. Monaghan (February, 1997) that provides an excellent overview of LAN security issues, including the weaknesses of TCP/IP and the use of firewalls. Updated on Feb 12, 1998

      Firewall product overview
    Contains a directory of commercial firewall products. Updated on Aug 5, 1998

      Firewalls mailing list
    This is the sign-up page for an Internet firewall mailing list. Updated on Apr 17, 1998

      H.323 and Firewalls
    White paper from Intel that describes the problems and pitfalls of getting H.323 safely through firewalls. Updated on Apr 10, 1998

      Internet Firewalls FAQ
    Answers general questions about firewalls and includes many links to sites with firewall information.

      NetGuard home page
    Home page of NetGuard, Inc., a software company specializing in security solutions for corporate networks attached to the Internet. This page highlights their Guardian firewall and Guidepost Bandwidth Control System products.

      Network Security Buyer's Guide
    Provides information about network security, utilities and virus protection. Offers a searchable database of products, links to vendor sites, and a library of white papers, press releases, and product presentations. Updated on Aug 4, 1998

      Why You Need a Firewall
    Describes security issues when connecting to the Internet, with information on how to protect your network. This page is hosted by Cisco Systems Inc.

      Yahoo!'s firewall companies page
    Yahoo!'s directory of firewall companies. Updated on Aug 1, 1998

      Yahoo!'s firewalls page
    Yahoo!'s directory of firewalls. Updated on Aug 4, 1998


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