Home > Articles > Home & Office Computing > Microsoft Windows Desktop

Like this article? We recommend

Firewalls

In Internet communications, firewalls are normally established to buffer the intranet from outside attacks. Firewall functions are implemented on packet-forwarding devices, which usually—but not always—have at least two interfaces. They can take the form of a dual-homed host with two interface cards or a piece of network gear known as a router.

A typical firewall configuration consists of an external packet filtering firewall and a secondary firewall supporting the needed proxy services. The packet-filtering rules are configured to allow external Internet users access to HTTP and FTP services on the company's Web and FTP servers—or whatever ports you decide to let through, including SMTP or real audio. All other network and transport protocol packets not matching the packet filtering rules are dropped. Additional rule sets must be implemented to allow services, such as DNS and SMTP, to pass from the internal intranet through the packet filtering firewall to servers on the Internet. Servers located in the zone between the two firewalls, sometimes referred to as the Demilitarized Zone (DMZ), are given their own dedicated Ethernet segments. If one of them is compromised, a remotely installed network sniffer is prevented from monitoring traffic to the internal network. (This is really dependent on the firewall implementation.) The second firewall proxies Web and ftp services initiated by internal users to the Internet.

The firewall routes packets between interfaces performing packet filtering, stateful inspection, or proxy services. A review of each of these items is appropriate.

Packet Filtering

A packet filter is designed to restrict external connections to a limited set of services, protecting the internal network from the Internet. The firewall enforces rules for packet filtering. These rules typically filter on protocols, source ports, source addresses, destination ports and destination addresses, and allowed network interfaces a given address may use. They may also filter on whether the traffic is UDP or TCP. Packet rules define connection types that are accepted and those that are rejected. For simplicity and security, it is probably best to establish rules for packets that will be accepted. Everything else is dropped and not forwarded.

Packet filtering occurs mostly at the network and transport protocol levels. If the NetBIOS service ports are not specifically enabled, traffic targeting the NetBIOS services will be blocked from Internet attacks. Attacks using IP source routing can also be stopped using packet filtering.

Packet Filtering Example

The rule set configuration allows external clients to access the internal Web server through the packet filtering router. The rule set allows inbound requests and outbound responses with IP addresses and TCP port settings. It also allows internal intranet users to connect with external Web servers on the Internet. Packets with destination TCP port = 80 and return TCP port = random number above 1023 are considered inbound HTTP requests from Internet users (line A). In order for the internal Web server to respond to this client connection, the outbound destination TCP port = Random > 1023 and source TCP port = 80 (Line B) must be permitted. However, an Internet user attempting to connect to the intranet using an internal source IP address will not be permitted. Internal IP addresses are forwarded only from the internal to the external interface on the packet filter route.

The foregoing is a general rule and is by no means absolute. Additionally, a filter may be applied to the incoming or outgoing interface, and the ACK bits may be set. Because a cracker can make a request come from any port she wants, you must check the ACK bit in the header to make sure that incoming requests either are to a specific IP address or were already established.

A rule set similar to this example must be implemented for every network service that intends to communicate through the packet filter.

Allowing TFTP, SNMP, or any of the NetBIOS services in from the Internet is not recommended. This, of course, needs to be based on policy, but these services are relatively insecure.

Proxy Services

A proxy server is a popular way to give internal users access to the Internet without compromising external access. The proxy software runs on a host routing between the internal network and the Internet. The main idea is to allow internally initiated connections with the Internet, while preventing any that are externally initiated. For example, after installing and configuring Microsoft Proxy Server 2.0 for HTTP, the internal users designate the HTTP proxy server in their Web browser. All external Web site requests are directed to it. The proxy receives HTTP URL requests, and establishes its own connections with the destination Web sites. It then retrieves the data, and relays it back to the correct internal client. This translation between internal Web client and external Web server can be transparent to the user. A proxy firewall requires client proxy and server proxy software specifically designed for each proxy service implemented to provide this transparency.

Both Netscape and Internet Explorer provide client proxy software for HTTP. Noncaching proxies degrade network performance, but provide a secure firewall. Noncaching is included here because a caching server may actually respond much more quickly if the answer is in the cache. The Microsoft Proxy Server also has an added caching feature. Commonly visited sites are stored on the proxy's local hard disk, preventing the need for Internet retrieval. Clients receive the pages more quickly and reduce the required Internet bandwidth. The proxy can also retrieve and refresh popular Web sites automatically without client requests.

Stateful Inspection

Stateful inspection offers an alternate way to provide internal users with connectivity to Internet resources while preventing external users from initiating inbound connections. As a user makes an outbound request to an external server, the stateful inspection firewall records details about it. The information tracked includes the source address, destination address, protocol, port number, and so forth; and it is evaluated for a certain period of time. When the destination service responds to the internal user, the packet is permitted to return through the firewall.

The firewall performs intelligent packet filtering without requiring the server and client software to support each network service. However, although stateful inspection offers better performance than the proxy model, it reveals internal IP addresses to the Internet. Several stateful inspection products hide internal IP addresses through address translation.

In the final article of this series, we will examine the special security issues associated with the Internet and Web services.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.

Overview


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information


To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.

Surveys

Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.

Newsletters

If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information


Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.

Security


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.

Children


This site is not directed to children under the age of 13.

Marketing


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information


If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.

Choice/Opt-out


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information


Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents


California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure


Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.

Links


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact


Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice


We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020