FortiOS 6 – Proxy options

Proxy options

Certain inspections defined in security profiles require that the traffic be held in proxy while the inspection is carried out. When a security profile requiring the use of a proxy is enabled in a policy, the Proxy Options field is displayed. The Proxy Options define the parameters of how the traffic will be processed and to what level the traffic will be processed. There can be multiple security profiles of a single type. There can also be a number of unique Proxy Option profiles. As the requirements for a policy differ from one policy to the next, a different Proxy Option profile for each individual policy can be configured or one profile can be repeatedly applied.

The Proxy Options refer to the handling of the following protocols:


The configuration for each of these protocols is handled separately.

The use of different proxy profiles and profile options

Just like other components of the FortiGate, different Proxy Option profiles can be configured to allow for granular control of the FortiGate. In the case of the Proxy Option profiles the thing that you will want to focus on is the matching up of the correct profile to a firewall policy that is using the appropriate protocols. If you are creating a Proxy Option profile that is designed for policies that control SMTP traffic into your network you only want to configure the settings that apply to SMTP. You do not need or want to configure the HTTP components.

Proxy Options profile components

Highlighted below are certain features available in the Proxy Options security profile.

Log Oversized Files

This setting enables logging of the occurrence of oversized files being processed. It does not change how they are processed. It only enables the FortiGate unit to log that they were either blocked or allowed through. A common practice is to allow larger files through without antivirus processing. This allows you to get an idea of how often this happens and decide on whether or not to alter the settings relating to the treatment of oversized files.

The setting of the threshold for oversized files and emails is found on theSecurity Profiles > Proxy Options page under Common Options.


FortiGate units with firmware version 5.4 and higher support RPC over HTTP. This protocol is used by the

Microsoft Exchange Server to perform virus scanning of Microsoft Exchange Server email that uses RPC over HTTP. To enable this feature, go to Security Profiles > Proxy Options and enable RPC over HTTP.

Protocol Port Mapping

To optimize the resources of the unit, the mapping and inspection of protocols can be enabled or disabled.

Each of the protocols listed in the GUI has a commonly used default TCP port, however, the port used by the protocols can be individually modified. It can also be set to inspect any port with flowing traffic for that particular protocol. The headers of the packets indicate which protocol generated the packet.

Comfort Clients

When proxy-based antivirus scanning is enabled, the FortiGate unit buffers files as they are downloaded. Once the entire file is captured, the FortiGate unit begins scanning the file. During the buffering and scanning procedure, the user must wait. After the scan is completed, if no infection is found, the file is sent to the next step in the process flow. If the file is a large one this part of the process can take some time. In some cases enough time that some users may get impatient and cancel the download.

The Comfort Clients feature mitigates this potential issue by feeding a trickle of data while waiting for the scan to complete. The user then knows that processing is taking place and that there hasn’t been a failure in the transmission. The slow transfer rate continues until the antivirus scan is complete. Once the file has been successfully scanned and found to be clean of any viruses, the transfer will proceed at full speed.

If there is evidence of an infection, the FortiGate unit caches the URL and drops the connection. The client does not receive any notification of what happened because the download to the client had already started. Instead, the download stops and the user is left with a partially downloaded file. If the user tries to download the same file again within a short period of time, the cached URL is matched and the download is blocked. A notification that the download has been blocked is displayed. The number of URLs in the cache is limited by the size of the cache.

Client comforting is available for HTTP and FTP traffic. If your FortiGate unit supports SSL content scanning and inspection, you can also configure client comforting for HTTPS and FTPS traffic.

Buffering the entire file allows the FortiGate unit to eliminate the danger of missing an infection due to fragmentation because the file is reassembled before examination. Client comforting can send unscanned and therefore potentially infected content to the client. You should only enable client comforting if you are prepared to accept this risk. Keeping the client comforting interval high and the amount low will reduce the amount of potentially infected data that is downloaded.

Block Oversized File/Email

This feature is related to antivirus scanning. The FortiGate unit has a finite amount of resources that can be used to buffer and scan a file. If a large file such as an ISO image or video file was to be downloaded this could overwhelm or exceed the memory of the FortiGate, especially if there were other large files being downloaded at the same time. For this reason, the treatment of large files needs to be addressed.

A threshold is assigned to identify an oversize file or email. This can be set at any size from 1 MB to 10 MB. Any file or email over this threshold will not be processed by policies applying the Antivirus security profile.

It should be noted that in terms of probability that malware is more likely to be found in smaller files than in larger files. A number of administrators take this into account when they lower the default threshold so as to lessen the impact on memory if they see the FortiGate unit going into conserve mode on a regular basis.

Chunked Bypass

The HTTP section allows the enabling of Chunked Bypass. This refers to the mechanism in version 1.1 of HTTP that allows a web server to start sending chunks of dynamically generated output in response to a request before actually knowing the actual size of the content. Where dynamically generated content is concerned, enabling this feature means that there is a faster initial response to HTTP requests. From a security stand point, enabling this feature means that the content will not be held in the proxy as an entire file before proceeding.

Allow Fragmented Messages

The specifications of RFC 2046 allow for the breaking up of emails and sending the fragments in parallel to be rebuilt and read at the other end by the mail server. It was originally designed to increase the performance over slower connections where larger email messages were involved. It will depend on your mail configuration if this is even possible for your network but outside of Microsoft Outlook and Outlook Express, not many email clients are set up to break up messages like this. The drawback of this feature is that if malware is broken up between multiple fragments of the message the risk is run that it will not be detected by some antivirus configurations because the code may not all be present at the same time to identify.

Append Email Signature

The Append Email Signature feature ensures that all of the emails going out of a particular network has the appropriate signature or corporate message, for example. These appended emails do not replace existing signatures.

Examples could include things like:

l Without prior approval the email should not be forwarded. l Please be environmentally friendly and don’t print out emails l For questions regarding the purchasing of our products please call…

It can be anything that the organization would like as long as it is in text format. The use of this feature usually works best in an environment where there is some standardization of what goes into the personal signatures of the senders so that there is no duplication or contradiction of information in the signatures.


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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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