Access Control Lists
Access Control Lists (ACLs) in the FortiOS firmware could be considered a granular or more specifically targeted blacklist. These ACLs drop IPv4 or IPv6 packets at the physical network interface before the packets are analyzed by the CPU. On a busy appliance this can really help the performance.
The ACL feature is available on FortiGates with NP6-accelerated interfaces. ACL checking is one of the first things that happens to the packet and checking is done by the NP6 processor. The result is very efficient protection that does not use CPU or memory resources.
The configuration of the Access Control List allow you to specify which in interface theACL will be applied to. There is a hardware limitation that needs to be taken into account. The ACL is a Layer 2 function and is offloaded to the ISF hardware, therefore no CPU resources are used in the processing of the ACL. It is handled by the inside switch chip which can do hardware acceleration, increasing the performance of the FortiGate. The drawback is that the ACL function is only supported on switch fabric driven interfaces. It also cannot be applied to hardware switch interfaces or their members. Ports such as WAN1 or WAN2 that are found on some models that use network cards that connect to the CPU through a PCIe bus will not support ACL.
Because the address portion of an entry is based on a FortiGate address object, id can be any of the address types used by the FortiGate, including address ranges. There is further granularity by specifying both the source and destination addresses. The traffic is blocked not on an either or basis of these addresses but the combination of the two, so that they both have to be correct for the traffic to be denied. Of course, If you want to block all of the traffic from a specific address all you have to do is make the destination address "all".
Because the blocking takes place at the interface based on the information in the packet header and before any processing such as NAT can take place, a slightly different approach may be required. For instance, if you are trying to protect a VIP which has an external address of x.x.x.x and is forwarded to an internal address of y.y.y.y, the destination address that should be used is x.x.x.x, because that is the address that will be in the packet's header when it hits the incoming interface.
Further granulation of the filter by which the traffic will be denied is done by specifying which service the traffic will use.