Protocol options
Firewall policies contain a Protocol Options field that defines the parameters for handling protocol-specific traffic. Multiple protocol options profiles can be configured in FortiOS since the requirements may differ between policies. A single protocol options profile is applied per policy, but the profile can be used in multiple policies.
To create a protocol options profile, go to Policy & Objects > Protocol Options. The following settings can be configured.
Log oversized files
Enable this option to log the occurrence of oversized files being processed. This does not change how they are processed. It only allows the FortiGate to log that they were either blocked or allowed through.
It is common practice to allow larger files through without antivirus processing. Monitor the logs for the frequency of oversized file processing to determine whether or not to alter the settings for treating oversized files. The threshold setting for oversized files and emails is located in the Common Options section.
RPC over HTTP
This protocol is used by Microsoft Exchange Servers to perform virus scanning on emails that use RPC over HTTP.
Protocol port mapping
To optimize the FortiGate’s resources, the mapping and inspection of the following protocols can be enabled or disabled:
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Each protocol has a default TCP port. The ports can be modified to inspect any port with flowing traffic. The packet headers indicate which protocol generated the packet.
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Protocol port mapping only works with proxy-based inspection. Flow-based inspection inspects all ports regardless of the protocol port mapping configuration. |
Common options
The Comfort Clients and Block Oversized File/Email options apply to multiple protocols.
Comfort clients
When proxy-based antivirus scanning is enabled, the FortiGate buffers files as they are downloaded. Once the entire file is captured, the FortiGate begins scanning the file. The user must wait during the buffering and scanning procedure. After the scan is completed and if no infection is found, the file is sent to the next step in the process flow. If the file is large, 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 option mitigates this potential issue by feeding a trickle of data while waiting for the scan to complete. The user is aware that processing is taking place, and that there has not been a failure in the transmission. The slow transfer rate continues until the antivirus scan is complete. The transfer will proceed at full speed once the file is scanned successfully and does not contain any viruses.
If there is evidence of an infection, the FortiGate caches the URL and drops the connection. The client does not receive any notification of what happened because the download to the client has 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 is displayed that the download was blocked. 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 the FortiGate supports SSL content scanning and inspection, client comforting can be configured for HTTPS and FTPS traffic.
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Buffering the entire file allows the FortiGate to eliminate the danger of missing an infection due to fragmentation because the file is reassembled before examination. This buffering is performed whenever the Comfort Clients option is disabled. Client comforting can send unscanned and potentially infected content to the client, so only enable this option 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 files and emails
This option is related to antivirus scanning. The FortiGate has a finite amount of resources to buffer and scan a file. If a large file (such as an ISO image or video file) is downloaded, this could overwhelm or exceed the FortiGate’s memory, especially if other large files are being downloaded at the same time.
A threshold is assigned to identify an oversize file or email. The default is 10 MB. The range varies per model, and the minimum is 1 MB. Any file or email over this threshold will not be processed by policies applying the antivirus security profile.
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If the FortiGate enters conserve mode on a regular basis, lowering the threshold can lessen the impact of processing the files on memory. This can increase risk, even though malware is more likely to be in smaller files. |
Web options
The Chunked Bypass option applies to traffic containing web protocols.
Chunked bypass
Chunked bypass is a mechanism in HTTP 1.1 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. For dynamically generated content, enabling chunked bypass speeds up the initial response to HTTP requests, but the content is not held in the proxy as an entire file before proceeding.
Email options
The Allow Fragmented Messages and Append Signature (SMTP) options apply to email protocols.
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. Feasibility of using this function depends on the mail configuration. Outside of Microsoft Outlook, 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, there is a risk that it will not be detected by some antivirus configurations because all the code may not be present at the same time to identify the malware.
Append signature
This option adds a plain text email signature to SMTP email messages as they pass through the FortiGate. The message maximum is 1023 characters.
This feature works best in an environment where there is some standardization of what goes into the senders' personal signatures so that there is no duplication or contradiction of information. For example:
- This email should not be forwarded without prior approval.
- Please consider the environment before printing this email.
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