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Processing Classify Callouts Asynchronously

Last Updated: 9/2/2016

A WFP callout driver can authorize or deny a network operation, or admit or discard a network packet, by returning the action types FWP_ACTION_PERMIT, FWP_ACTION_CONTINUE, or FWP_ACTION_BLOCK from the classifyFn callout function. Frequently a callout driver cannot return an inspection decision from its classifyFn function until the indicated information, such as classifiable fields, metadata, or packets, can be forwarded for processing to another component, such as a user-mode application. In these cases a decision may have to be made asynchronously at some later time.

General Rules for Asynchronous Processing

WFP supports asynchronous processing of the classifyFn callout function. However, the mechanism for doing this differs according to the different layers.

Asynchronous ALE Classify
A callout driver must call the FwpsPendOperation0 function from classifyFn. The asynchronous operation must be completed with a call to the FwpsCompleteOperation0 function.

Asynchronous Packet Classify
A callout driver should return FWP_ACTION_BLOCK from the classifyFn function, with the FWPS_CLASSIFY_OUT_FLAG_ABSORB flag set. Network packets must be referenced or cloned. The asynchronous operation is completed by either reinjecting the cloned or modified packet or by silently discarding the packet.

Asynchronous ALE Classify That Includes Packets
A combination of the previous two procedures is used: the classify operation is pended and the packet is referenced or cloned, and at some time later the call to classifyFn is completed and the cloned packet is reinjected or discarded.

Special Cases and Considerations

ALE Connect vs. Receive/Accept Layers
When FwpsCompleteOperation0 is called to complete a pended classify operation at an ALE connect layer (FWPS_LAYER_ALE_AUTH_CONNECT_V4 or FWPS_LAYER_ALE_AUTH_CONNECT_V6), an ALE reauthorization classify operation is triggered at the respective ALE connect layer. The callout driver should return an inspection decision from this reauthorization classify operation. You can detect an ALE reauthorization classify operation by checking whether the FWP_CONDITION_FLAG_IS_REAUTHORIZE flag is set.

The callout driver must maintain a unique state for each pended ALE_AUTH_CONNECT classify operation in such a way that the inspection decision for each classify operation can be looked up during a FwpsCompleteOperation0-triggered reauthorization. If packets are referenced or cloned during a pended ALE_AUTH_CONNECT classify operation (for example, for non-TCP connections), they can be reinjected after reauthorization occurs.

When FwpsCompleteOperation0 is called during with a classify operation at an ALE receive/accept layer (FWPS_LAYER_ALE_AUTH_RECV_ACCEPT_V4 or FWPS_LAYER_ALE_AUTH_RECV_ACCEPT_V6), FwpsCompleteOperation0 does not trigger an ALE reauthorization. Instead a new call to classifyFn is made again when the cloned packet is reinjected incoming if the modification was not significant enough to bypass the filter. Permitting the self-injected clone from the ALE_RECV_ACCEPT layer effectively authorizes the incoming connection. If the incoming connection is not to be allowed, discard the incoming packet after it calls FwpsCompleteOperation0.

ALE Reauthorization
A callout driver can be reclassified at an ALE connect or receive/accept layer for events such as a policy change (for example, adding or removing a filter at the layer), detecting a new arrival interface, and re-keying a connection by using IPsec. Such a reauthorization cannot be pended by calling FwpsCompleteOperation0, and it is not necessary to do so. A callout driver should use the rules listed previously to process packets that are indicated during reauthorization.

Be aware that both incoming and outgoing packet can be reauthorized at ALE_AUTH_CONNECT or ALE_RECV_ACCEPT layers. For example, an incoming packet can be reauthorized at the ALE_AUTH_CONNECT layer. A callout driver must not assume that the direction of the packet is the same as the direction of the connection.

Asynchronous processing is not supported at these layers (FWPS_LAYER_ALE_FLOW_ESTABLISHED_V4 or FWPS_LAYER_ALE_FLOW_ESTABLISHED_V6).

A callout driver must not perform asynchronous processing of packets that require ALE classify processing at an incoming (inbound) transport layer (FWPS_LAYER_INBOUND_TRANSPORT_V4 or FWPS_LAYER_INBOUND_TRANSPORT_V6). Doing this can interfere with flow creation. When WFP calls the classifyFn callout function at an incoming transport layer, it sets the FWPS_METADATA_FIELD_ALE_CLASSIFY_REQUIRED flag for those packets that require ALE classify processing. A callout driver should permit such packets from an INBOUND_TRANSPORT layer and should defer processing them until they reach an ALE_RECV_ACCEPT layer.

At a stream layer (FWPS_LAYER_STREAM_V4 or FWPS_LAYER_STREAM_V6), TCP data segments are indicated instead of an IP or TCP header. The stream layer is also where a chain of net buffer lists can be indicated in one call to the classifyFn callout function. WFP makes available specialized clone and injection functions, FwpsCloneStreamData0 and FwpsStreamInjectAsync0, for stream layer callouts to use.

Because of the ordered delivery nature of stream layer data, a callout driver must continue to clone and absorb data as long any stream data is still pending. Mixing asynchronous and synchronous operations for a given stream flow can result in undefined behavior.

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