Web filter – Fortinet FortiGate

Web filter

This section describes FortiGate web filtering for HTTP traffic. The three main parts of the web filtering function, the Web Content Filter, the URL Filter, and the FortiGuard Web Filtering Service interact with each other to provide maximum control over what the Internet user can view as well as protection to your network from many Internet content threats. Web Content Filter blocks web pages containing words or patterns that you specify. URL filtering uses URLs and URL patterns to block or exempt web pages from specific sources. FortiGuard Web Filtering provides many additional categories you can use to filter web traffic.

This section describes the Web Content Filter and URL Filter functions. For information on FortiGuard Web Filtering, see “FortiGuard W eb Filter” on page 133 The following topics are included in this section:

  • Web filter concepts
  • Inspections Modes
  • FortiGuard Web Filtering Service
  • Overriding FortiGuard website categorization
  • SafeSearch
  • YouTube Education Filter
  • Web Site Filter
  • Web content filter
  • Advanced web filter configurations
  • Working with the Interface
  • Web filtering example

Web filter concepts

Web filtering is a means of controlling the content that an Internet user is able to view. With the popularity of web applications, the need to monitor and control web access is becoming a key component of secure content management systems that employ antivirus, web filtering, and messaging security. Important reasons for controlling web content include:

  • lost productivity because employees are accessing the web for non-business reasons
  • network congestion — when valuable bandwidth is used for non-business purposes, legitimate business applications suffer
  • loss or exposure of confidential information through chat sites, non-approved email systems, instant messaging, and peer-to-peer file sharing
  • increased exposure to web-based threats as employees surf non-business-related web sites
  • legal liability when employees access/download inappropriate and offensive material
  • copyright infringement caused by employees downloading and/or distributing copyrighted material.

As the number and severity of threats increase on the World Wide Web, the risk potential increases within a company’s network as well. Casual non-business related web surfing has caused many businesses countless hours of legal litigation as hostile environments have been created by employees who download and view offensive content. Web-based attacks and

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threats are also becoming increasingly sophisticated. Threats and web-based applications that cause additional problems for corporations include:

  • spyware/grayware
  • phishing
  • pharming
  • instant messaging
  • peer-to-peer file sharing
  • streaming media
  • blended network attacks.

Spyware, also known as grayware, is a type of computer program that attaches itself to a user’s operating system. It does this without the user’s consent or knowledge. It usually ends up on a computer because of something the user does such as clicking on a button in a pop-up window. Spyware can track the user’s Internet usage, cause unwanted pop-up windows, and even direct the user to a host web site. For further information, visit the FortiGuard Center.

Some of the most common ways of grayware infection include:

  • downloading shareware, freeware, or other forms of file-sharing services
  • clicking on pop-up advertising
  • visiting legitimate web sites infected with grayware.

Phishing is the term used to describe attacks that use web technology to trick users into revealing personal or financial information. Phishing attacks use web sites and email that claim to be from legitimate financial institutions to trick the viewer into believing that they are legitimate. Although phishing is initiated by spam email, getting the user to access the attacker’s web site is always the next step.

Pharming is a next generation threat that is designed to identify and extract financial, and other key pieces of information for identity theft. Pharming is much more dangerous than phishing because it is designed to be completely hidden from the end user. Unlike phishing attacks that send out spam email requiring the user to click to a fraudulent URL, pharming attacks require no action from the user outside of their regular web surfing activities. Pharming attacks succeed by redirecting users from legitimate web sites to similar fraudulent web sites that have been created to look and feel like the authentic web site.

Instant messaging presents a number of problems. Instant messaging can be used to infect computers with spyware and viruses. Phishing attacks can be made using instant messaging. There is also a danger that employees may use instant messaging to release sensitive information to an outsider.

Peer-to-peer (P2P) networks are used for file sharing. Such files may contain viruses. Peer-to-peer applications take up valuable network resources and may lower employee productivity but also have legal implications with the downloading of copyrighted or sensitive company material.

Streaming media is a method of delivering multimedia, usually in the form of audio or video to Internet users. Viewing streaming media impacts legitimate business by using valuable bandwidth.

Blended network threats are rising and the sophistication of network threats is increasing with each new attack. Attackers learn from each previous successful attack and enhance and update attack code to become more dangerous and fast spreading. Blended attacks use a combination of methods to spread and cause damage. Using virus or network worm techniques combined with known system vulnerabilities, blended threats can quickly spread through email, web sites, and Trojan applications. Examples of blended threats include Nimda, Code Red, Slammer, and Blaster. Blended attacks can be designed to perform different types of attacks, which include disrupting network services, destroying or stealing information, and installing stealthy backdoor applications to grant remote access.

Different ways of controlling access

The methods available for monitoring and controlling Internet access range from manual and educational methods to fully automated systems designed to scan, inspect, rate and control web activity.

Common web access control mechanisms include:

  • establishing and implementing a well-written usage policy in the organization on proper Internet, email, and computer conduct
  • installing monitoring tools that record and report on Internet usage
  • implementing policy-based tools that capture, rate, and block URLs.

The final method is the focus of this topic. The following information shows how the filters interact and how to use them to your advantage.

Order of web filtering

The FortiGate unit applies web filters in a specific order:

  1. URL filter
  2. FortiGuard Web Filter
  3. web content filter
  4. web script filter
  5. antivirus scanning.

If you have blocked a FortiGuard Web Filter category but want certain users to have access to URLs within that pattern, you can use the Override within the FortiGuard Web Filter. This will allow you to specify which users have access to which blocked URLs and how long they have that access. For example, if you want a user to be able to access www.example.com for one hour, you can use the override to set up the exemption. Any user listed in an override must fill out an online authentication form that is presented when they try to access a blocked URL before the FortiGate unit will grant access to it. For more information, see “FortiGuard Web  Filter” on page 133.

Inspections Modes


Proxy-based inspection involves buffering the traffic and examining it as a whole before determining an action. The process of having the whole of the data to analyze allow this process to include more points of data to analyze than the flow-based or DNS methods.

The advantage of a proxy-based method is that the inspection can be more thorough than the other methods, resulting in fewer false positive or negative results in the analysis of the data.


The Flow-based inspection method examines the file as it passes through the FortiGate unit without any buffering. As each packet of the traffic arrives it is process and forwarded without waiting for the complete file or web page, etc.

The advantage of the flow-based method is that the user sees a faster response time for HTTP requests and there is less chance of a time-out error due to the server at the other end responding slowly.

The disadvantages of this method are that there is a higher probability of a false positive or negative in the analysis of the data and that a number of points of analysis that can be used in the proxy-based method are not available in the flow-based inspection method. There is also fewer actions available to choose from based on the categorization of the website by FortiGuard services.


The DNS inspection method uses the same categories as the FortiGuard Service. It is lightweight in terms of resource usage because it doesn’t involve any proxy-based or flow-based inspection.

A DNS request is typically the first part of any new session to a new website. This inspection method takes advantage of that and places the results of the categorization of websites right on the FortiGuard DNS servers. When the FortiGate resolves a URL, in addition to the IP address of the website it also receives a domain rating.

In the same way that the flow-based inspection method had fewer filters and points of analysis than the proxy-based inspection method, DNS has fewer settings still. All of its inspection is based on the IP address, the domain name and the rating provided by the FortiGuard DNS server.

FortiGuard Web Filtering Service

FortiGuard Web Filter is a managed web filtering solution available by subscription from Fortinet. FortiGuard Web Filter enhances the web filtering features supplied with your FortiGate unit by sorting billions of web pages into a wide range of categories users can allow or block. The FortiGate unit accesses the nearest FortiGuard Web Filter Service Point to determine the category of a requested web page, and then applies the security policy configured for that user or interface.

FortiGuard Web Filter includes over 45 million individual ratings of web sites that apply to more than two billion pages. Pages are sorted and rated into several dozen categories administrators can allow or block. Categories may be added or updated as the Internet evolves. To make configuration simpler, you can also choose to allow or block entire groups of categories. Blocked pages are replaced with a message indicating that the page is not accessible according to the Internet usage policy.

FortiGuard Web Filter ratings are performed by a combination of proprietary methods including text analysis, exploitation of the web structure, and human raters. Users can notify the FortiGuard Web Filter Service Points if they feel a web page is not categorized correctly, so that the service can update the categories in a timely fashion.

Before you begin to use the FortiGuard Web Filter options you should verify that you have a valid subscription to the service for your FortiGate firewall.

FortiGuard Web Filter and your FortiGate unit

When FortiGuard Web Filter is enabled in a web filter profile, the setting is applied to all firewall policies that use this profile. When a request for a web page appears in traffic controlled by one of these firewall policies, the URL is sent to the nearest FortiGuard server. The URL category is returned. If the category is blocked, the FortiGate unit provides a replacement message in place of the requested page. If the category is not blocked, the page request is sent to the requested URL as normal.

Figure 12:Webfiltering flowchart

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