FortiOS 6 – AntiVirus

Client comforting

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 scans it. If no infection is found, the file is sent along to the client. The client initiates the file transfer and nothing happens until the FortiGate finds the file clean, and releases it. Users can be impatient, and if the file is large or the download slow, they may cancel the download, not realizing that the transfer is in progress.

The client comforting feature solves this problem by allowing a trickle of data to flow to the client so they can see the file is being transferred. The default client comforting transfer rate sends one byte of data to the client every ten seconds. This slow transfer continues while the FortiGate unit buffers the file and scans it. If the file is infection-free, it is released and the client will receive the remainder of the transfer at full speed. If the file is infected, 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. The client receives the Infection cache message replacement message as a notification that the download has been blocked. The number of URLs in the cache is limited by the size of the cache.

Client comforting can send unscanned and 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.

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.

Enable and configure client comforting

  1. Go to Security Profiles > Proxy Options.
  2. Select a Proxy Options profile and choose Edit, or select Create New to make a new one.
  3. Scroll down to the Common Options section and enable the Comfort Clients This will set the option on all of the applicable protocols. The ability to set this feature on a protocol by protocol basis exists in the CLI.
  4. Select OK or Apply to save the changes.
  5. Apply this Proxy Options profile in any security policy for it to take effect on all traffic handled by the policy. The default values for Interval and Amount are 10 and 1, respectively. This means that when client comforting takes effect, 1 byte of the file is sent to the client every 10 seconds. You can change these values to vary the amount and frequency of the data transferred by client comforting.

Oversized files and emails

Downloaded files can range from a few Kilobytes to multiple Gigabytes. A FortiGate doesn’t have the memory to allow for a large number of people downloading large files. Imagine the memory required for a team of developers to all download the latest Linux OS distribution at once, in addition to the normal requirements of the firewall. Everything would come to a grinding halt if the FortiGate tried to store each of those Gigabyte+ files in memory. To give you some piece of mind, the chances of malware being in a large file like those is much smaller than in a smaller single Megabyte file, so the threat is somewhat limited, but you will probably want to use your computers antivirus software to scan those large files after they have been downloaded.

A threshold must be set to prevent the resources of the system from becoming overloaded. By default the threshold is 10 MB. Any files larger than the threshold will not be scanned for malware. With a maximum file size threshold in place, it must now be determined what is to be done with the files that are larger than threshold. There are only 2 choices; either the file is passed through without being scanned for malware or the file is blocked. The default action for oversized files is to pass them through.

If you wish to block the downloading of files over the threshold, this can be set within the Proxy Option profile found at Security Profiles > Proxy Options, under Common Options.

Enable Block Oversized File/Email.

This will reveal an additional option, Threshold (MB). The threshold of the files is set based upon the protocol being used to transfer the file. In the CLI and configuration file, the threshold variable is found in each of the protocol sections within the profile. Changing the value in this field will change the oversize-limit value for all of the protocols.

If you wish to change the oversize-limit value on the protocols covered in a Proxy Option profile you have two options.

  1. You can go into the CLI and change the value manually within each of the protocol sections.
  2. You can use the GUI to temporarily block oversized files, and when configuring it change the threshold to the new value that you want. Apply this setting. Then go back to the profile and turn off the block setting. If you now go into the CLI you will find that the configuration file has retained the new oversize-limit value. The settings can be found in the CLI by going to:

config firewall profile-protocol-options edit <profile_name> config <protocol> set oversize-limit <size_int>




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