FortiOS 6 – Inspection Modes

Inspection modes

You can select one of two inspection modes from the System > Settings page to control the security profile inspection mode for your FortiGate or VDOM.

  • Proxy-based inspection, that reconstructs content passing through the FortiGate unit and inspects the content for security threats, or
  • Flow-based inspection, that takes a snapshot of content packets and uses pattern matching to identify security threats in the content.

Each inspection component plays a role in the processing of traffic en route to its destination. Having control over flow and proxy mode is helpful if you want to be sure that only flow inspection mode is used (and that proxy inspection mode is not used). In most cases proxy mode is preferred because more security profile features are available and more configuration options for these individual features are available. Yet, some implementations may require all security profile scanning to only use flow mode. In this case, you can set your FortiGate to flow mode knowing that proxy mode inspection will not be used. While both modes offer significant security, proxybased provides more features and flow-based is designed to optimize performance.

This section addresses the following topics:

Proxy-based inspection

Flow-based inspection

Changing between proxy and flow mode

Comparison of inspection types

Proxy-based inspection

If a FortiGate or VDOM is configured for proxy-based inspection, then a mixture of flow-based and proxy-based inspection occurs. Traffic initially encounters the IPS engine, which applies single-pass IPS, Application Control, and CASI, if configured in the firewall policy accepting the traffic.

The traffic is then sent for proxy-based inspection. Proxy-based inspection extracts and caches content, such as files and web pages, from a content session and inspects the cached content for threats. Content inspection takes place in the following order: VoIP inspection, DLP, AntiSpam, Web Filtering, AntiVirus, and ICAP.

If no threat is found, the proxy relays the content to its destination. If a threat is found, the proxy can block the threat and send a replacement message in its stead. The proxy can also block VoIP traffic that contains threats.

Transparent web proxy mode

In proxy mode, FortiOS 5.6 functions just like FortiOS 5.4 with the addition of the new Transparent Web Proxy mode. See New Operating mode for Transparent web proxy in What’s New in FortiOS 5.6.

Flow-based inspection

Flow-based inspection

Flow-based inspection identifies and blocks security threats in real time as they are identified using single-pass Direct Filter Approach (DFA) pattern matching to identify possible attacks or threats.

If a FortiGate or a VDOM is configured for flow-based inspection, depending on the options selected in the firewall policy that accepted the session, flow-based inspection can apply IPS, Application Control, Web Filtering, DLP, and AntiVirus. Flow-based inspection is all done by the IPS engine and, as you would expect, no proxying is involved.

All of the applicable flow-based security modules are applied simultaneously in one single pass, and pattern matching is offloaded and accelerated by CP8 or CP9 processors. IPS, Application Control, flow-based Web Filtering, and flow-based DLP filtering happen together. Flow-based AntiVirus scanning caches files during protocol decoding and submits cached files for virus scanning while the other matching is carried out.

Flow-based inspection typically requires fewer processing resources than proxy-based inspection and does not change packets, unless a threat is found and packets are blocked. Flow-based inspection cannot apply as many features as proxy inspection. For example, flow-based inspection does not support client comforting and some aspects of replacement messages.

In FortiOS 5.6, flow-based inspection requires the new NGFW mode.

Changing between proxy and flow mode

You can see which inspection mode your FortiGate is using by looking at the System Information widget on your Dashboard.

To change inspection modes, go to System > Settings and scroll down to Inspection Mode. You can select Flow-based to operate in Flow mode or Proxy to operate in Proxy mode.

When you select Flow-based, all proxy mode profiles are converted to flow mode, removing any proxy settings. As well proxy mode only features (for example, Web Application Profile) are removed from the GUI.

In addition, selecting Flow-based inspection will cause the Explicit Web Proxy and Explicit FTP Proxy features to be removed from the GUI and the CLI. This includes Explicit Proxy firewall policies.

W hen you select Flow-based you can only configure Virtual Servers (under Policy & Objects > Virtual Servers) with Type set to HTTP, TCP, UDP, or IP.

If required, you can change back to proxy mode through the System > Settings page.

If your FortiGate has multiple VDOMs, you can set the inspection mode independently for each VDOM. Use the top left drop-down menu to go to Global > System > VDOM. Click Editfor the VDOM you wish to change and select the Inspection Mode.

From the GUI, you can only configure antivirus and web filter security profiles in proxy mode. From the CLI you can configure flow-based antivirus profiles, web filter profiles and DLP profiles and they will appear on the GUI and include their inspection mode setting. Also, flow-based profiles created when in flow mode are still available when you switch to proxy mode.

NGFW profile-based and NGFW policy-based modes

When you select Flow-based as the Inspection Mode, you have the option in FortiOS 5.6 to select an NGFW Mode. NGFW Profile-based mode works the same as flow-based mode did in FortiOS 5.4

When selecting NGFW policy-based mode you can also select the SSL/SSH Inspection mode that is applied to all policies.

In the new NGFW Policy-based mode, you add applications and web filtering profiles directly to a policy without having to first create and configure Application Control or Web Filtering profiles. See NGFW Policy Mode on page


When you change to flow-based inspection, all proxy mode profiles are converted to flow mode, removing any proxy settings. And proxy-mode only features (for example, Web Application Profile) are removed from the GUI.

If your FortiGate has multiple VDOMs, you can set the inspection mode independently for each VDOM. Go to System > VDOM. Click Edit for the VDOM you wish to change and select the Inspection Mode. CLI syntax

The following CLI commands can be used to configure inspection and NGFW (called “policy” in the CLI) modes:

config system settings set inspection-mode {proxy | flow} set policy-mode {standard | ngfw}


Comparison of inspection types

The tables in this section show how different security features map to different inspection types and present the strengths and weaknesses of proxy- vs. flow-based inspection.

Security profile features mapped to inspection mode

The table below lists FortiOS security profile features and shows whether they are available in flow-based or proxy-based inspection modes.

Security Profile Feature Flow-based inspection Proxy-based inspection
AntiVirus x x
Web Filter x x


Security Profile Feature Flow-based inspection Proxy-based inspection
DNS Filter x x
Application Control x x
Intrusion Protection x x
Anti-Spam   x
Data Leak Protection   x
VoIP   x
ICAP   x
Web Application Firewall   x
FortiClient Profiles x x
Proxy Options x x
SSL Inspection x x
SSH Inspection   x
Web Rating Overrides x x
Web Profile Overrides   x

Individual security profile considerations

In flow mode, AntiVirus and Web Filter profiles only include flow-mode features. Web filtering and virus scanning are still done with the same engines and to the same accuracy, but some inspection options are limited or not available in flow mode. Application control, intrusion protection, and FortiClient profiles are not affected when switching between flow and proxy mode.

Application control uses flow-based inspection; if you apply an additional security profile to your traffic that is proxy-based, the connection will simply timeout rather than display the warning, or replacement, message. However, Application Control will still function.

Even though VoIP profiles are not available from the GUI in flow mode, the FortiGate can process VoIP traffic. In this case the appropriate session helper is used (for example, the SIP session helper).

Setting flow or proxy mode doesn’t change the settings available from the CLI. However, when in flow mode you can’t save security profiles that are set to proxy mode.

You can also add proxy-only security profiles to firewall policies from the CLI. So, for example, you can add a VoIP profile to a security policy that accepts VoIP traffic. This practice isn’t recommended because the setting will not be visible from the GUI.

If you set flow-based to use external servers for FortiWeb and FortiMail you must use the CLI to set a Web Application Firewall profile or Anti-Spam profile to external mode and add the Web Application Firewall profile or AntiSpam profile to a firewall policy.

Proxy mode and flow mode antivirus and web filter profile options

The following tables list the antivirus and web filter profile options available in proxy and flow modes.

Antivirus features in proxy and flow mode

Feature Proxy Flow
Scan Mode (Quick or Full) no yes
Detect viruses (Block or Monitor) yes yes
Inspected protocols yes no (all relevant protocols are inspected)
Inspection Options yes yes (not available for quick scan mode)
Treat Windows Executables in Email Attachments as Viruses yes yes
Send Files to FortiSandbox Appliance for Inspection yes yes
Use FortiSandbox Database yes yes
Include Mobile Malware Protection yes yes

Web filter features in proxy and flow mode

Feature Proxy Flow
FortiGuard category based filter yes yes (show, allow, monitor, block)
Category Usage Quota yes no
Allow users to override blocked categories (on some models) yes no
Search Engines yes no


Feature   Proxy Flow
  Enforce ‘Safe Search’ on Google, Yahoo!, Bing, Yandex yes no
Restrict YouTube Access yes no
Log all search keywords yes no
Static URL Filter   yes yes
  Block invalid URLs yes no
URL Filter yes yes
Block malicious URLs discovered by FortiSandbox yes yes
Web Content Filter yes yes
Rating Options   yes yes
  Allow websites when a rating error occurs yes yes
Rate URLs by domain and IP Address yes yes
Block HTTP redirects by rating yes no
Rate images by URL yes no
Proxy Options   yes no
  Restrict Google account usage to specific domains yes no
Provide details for blocked HTTP 4xx and 5xx errors yes no
HTTP POST Action yes no
Remove Java Applets yes no
Remove ActiveX yes no
Remove Cookies yes no
Filter Per-User Black/White List yes no

AntiVirus scanning differences between versions of FortiOS 5.x

In FortiOS 5.0, 5.2, 5.4, 5.6 and 6.0, there are several AntiVirus (AV) scanning inspection modes available. FortiOS 5.0 includes proxy and flow-based virus scanning. FortiOS 5.2 also uses proxy-based and flowbased scanning, but the flow-based mode in FortiOS 5.2 uses a new approach to flow-based scanning (that is sometimes called deepflow or deep flow scanning). FortiOS 5.4 and onward offer another flow-based mode, quick mode, to inspect traffic efficiently.

The databases used for AV scanning does not change from proxy to flow mode unless quick mode is enabled. In flow-based quick mode, a compact antivirus database is used.

AntiVirus scanning examines files in HTTP, HTTPS, email, and FTP traffic for threats as they pass through your FortiGate. If the traffic contains compressed files, they are also examined. Go to the SysAdmin Note on the Fortinet Cookbook site for detailed information on supported compression formats in antivirus scanning.

If the AV scanner finds a threat such as a virus or some other malware, FortiOS protects your network by blocking the file.

FortiOS includes a number of AntiVirus features that make virus scanning more user-friendly. One of these features, called replacement messages, sends a customizable message to anyone whose file is blocked by AV scanning, to explain what happened and why. Other features make communication between the client and the server more seamless. The availability of these changes depending on the inspection mode.

Proxy-based AV scanning

Proxy-based AV scanning is the most feature-rich AV scanning mode. This mode uses a proxy to manage the communication between client and server. The proxy extracts content packets from the data stream as they arrive and buffers the content until the complete file is assembled. Once the file is whole, the AV scanner examines the file for threats. If no threats are found, the file is sent to its destination. If a threat is found, the file is blocked.

Because proxy-based scanning is applied to complete files, including compressed files, it provides very effective threat detection. Proxy-based scanning also supports a full range of features, including replacement messages and client comforting, making proxy-based scanning the most user friendly inspection mode. In addition the proxy manages the communication between the client and the server, improving the user experience. For example, in flow mode if a virus is found, the last part of the file is not downloaded and the connection just times out and the user cannot tell what is going on. In proxy mode, the users gets a message about the file being blocked.

Proxy-based scanning inspects all files under the oversized threshold. Since the FortiGate unit has a limited amount of memory, files larger than a certain size do not fit within the memory buffer. The default buffer size is 10 MB. You can use the uncompsizelimitCLI command to adjust the size of this memory buffer. Files larger than the threshold are passed to the destination without scanning. You can use the Oversized File/Email setting in Security Profiles > Proxy Options to block files larger than the antivirus buffer if allowing files that are too large to be scanned is an unacceptable security risk.

During the buffering and scanning procedure, the client must wait. With a default configuration, the file is released to the client only after it is scanned. You can enable client comforting in the Proxy Options security profile to feed the client a trickle of data to prevent them from possibly thinking the transfer is stalled and consequently canceling the download.

Flow-based AV scanning

Although the name “flow-based scanning” is used in FortiOS 5.0, 5.2, 5.4, and 5.6, the different versions handle this mode in very different ways.

Flow AV in FortiOS 5.4 and 5.6

In FortiOS 5.4 and 5.6, there are two modes available for flow-based virus scanning: Quick and Full scan mode. Full mode is the same as flow-based scanning in FortiOS 5.2 (see below). Quick mode uses a compact antivirus database and advanced techniques to improve performance. You can designate quick or full scan mode when configuring the antivirus profile in the GUI. Alternatively, use the following CLI command to enable quick or full mode:

config antivirus profile edit <profile> set scan-mode {quick | full}


Flow AV in FortiOS 5.2 (deepflow or deep flow)

FortiOS 5.2 introduced a new type of flow-based AV scanning, that is sometimes called deepflow or deep flow, and that takes a hybrid approach where content packets are buffered while simultaneously being sent to their destination. When all of the files packets have been collected and buffered, but before the final packet is delivered, the buffered file is scanned. If a threat is found, the last packet is blocked and the client application has to deal with not getting the completed file. If no threat is found the final packet is sent and the user gets their file.

Deepflow AV scanning is as good as proxy-based AV scanning at detecting threats. There may be a small performance advantage over proxy-based AV as files get larger based on the difference between sending the whole file after analysis and just sending the last packet. Deepflow’s most notable limitation is that, just like the flow-based AV in 5.0, it does not support many of the user-friendly features provided by proxy-based AV.

Flow AV in FortiOS 5.0

In FortiOS 5.0, flow-based AV scanning examines the content of individual data packets as they pass through the FortiGate. There is no proxy involved so packets are not changed by the proxy and files are not buffered for analysis. Potentially less memory and CPU resources are used, resulting in a potential performance increase compared to using proxy-based mode. FortiOS 5.0 flow-based AV scanning is also not limited by file size.

Flow AV uses the IPS engine and the AV database and is effective at many kinds of threat detection; however, because it can only analyze what is in an individual packet rather than a complete file, flow-based scanning cannot detect some types of malware, including polymorphic code. Malware in documents, compressed files, and some archives are also less likely to be detected.

Flow AV does not actually block files, it stops delivering a file’s packets once a threat has been detected. This means that parts of the file may already have been delivered when the threat has been detected and the recipient application is responsible for dealing with the partially complete content.

In addition flow AV can be less user friendly. Replacement messages are not supported and clients may have to wait for sessions to time out without knowing why content has been blocked.

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About Mike

Michael Pruett, CISSP has a wide range of cyber-security and network engineering expertise. The plethora of vendors that resell hardware but have zero engineering knowledge resulting in the wrong hardware or configuration being deployed is a major pet peeve of Michael's. This site was started in an effort to spread information while providing the option of quality consulting services at a much lower price than Fortinet Professional Services. Owns PacketLlama.Com (Fortinet Hardware Sales) and Office Of The CISO, LLC (Cybersecurity consulting firm).

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