Flow mode inspection (default mode)
When a firewall policy’s inspection mode is set to flow, traffic flowing through the policy will not be buffered by the FortiGate. Unlike proxy mode, the content payload passing through the policy will be inspected on a packet by packet basis with the very last packet held by the FortiGate until the scan returns a verdict. If a violation is detected in the traffic, a reset packet is issued to the receiver, which terminates the connection, and prevents the payload from being sent successfully.
Because of this method, flow mode inspection cannot be as thorough as proxy mode inspection and will have some feature limitations. For example, flow mode inspection determines a file’s size by identifying the file size information in the protocol exchange. If a file’s size is not present in the protocol exchange, the file’s size cannot be identified. The flow-based policy will automatically block or pass the file (based on the configuration) despite the file meeting the file size requirements.
The objective of flow-based policy is to optimize performance and increase throughput. Although it is not as thorough as a proxy-based policy, flow mode inspection is still very reliable.