Returns REQ_PROCEED if the specified filter was inserted successfully, or REQ_NOACTION if the specified filter was not inserted because it was not required. Any other return value indicates an error.
SYS_NETFD sd is NULL (reserved for future use).
pblock *pb is a set of parameters to pass to the specified filter’s init method.
Session *sn identifies the Session structure.
Request *rq identifies the Request structure.
void *data is filter-defined private data.
const Filter *filter is the filter to insert.