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, Cloud Access Security Inspection (CASI), Web Filtering, 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 and CASI, and flow-based Web Filtering happen together. CASI signatures are applied as part of application control. Flow-based antivirus 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.