The
pfil framework allows for a specified function to be invoked for every incoming or outgoing packet for a particular network I/O stream. These hooks may be used to implement a firewall or perform packet transformations.
Packet filtering points are registered with
pfil_head_register(). Filtering points are identified by a key (void *) and a data link type (int) in the
pfil_head structure. Packet filters use the key and data link type to look up the filtering point with which they register themselves. The key is unique to the filtering point. The data link type is a
bpf(4) DLT constant indicating what kind of header is present on the packet at the filtering point. Filtering points may be unregistered with the
pfil_head_unregister() function.
Packet filters register/unregister themselves with a filtering point with the
pfil_add_hook() and
pfil_remove_hook() functions, respectively. The head is looked up using the
pfil_head_get() function, which takes the key and data link type that the packet filter expects. Filters may provide an argument to be passed to the filter when invoked on a packet.
When a filter is invoked, the packet appears just as if it “came off the wire”. That is, all protocol fields are in network byte order. The filter is called with its specified argument, the pointer to the pointer to the mbuf containing the packet, the pointer to the network interface that the packet is traversing, and the direction (
PFIL_IN or
PFIL_OUT, see also below) that the packet is traveling. The filter may change which mbuf the mbuf ** argument references. The filter returns an errno if the packet processing is to stop, or 0 if the processing is to continue. If the packet processing is to stop, it is the responsibility of the filter to free the packet.
The
flags parameter, used in the
pfil_add_hook() and
pfil_remove_hook() functions, indicates when the filter should be called. The flags are:
PFIL_IN
call me on incoming packets
PFIL_OUT
call me on outgoing packets
PFIL_ALL
call me on all of the above
PFIL_IFADDR
call me on interface reconfig (mbuf ** is ioctl #)
PFIL_IFNET
call me on interface attach/detach (mbuf ** is either PFIL_IFNET_ATTACH or PFIL_IFNET_DETACH)
PFIL_WAITOK
OK to call malloc with M_WAITOK.
The
pfil interface is enabled in the kernel via the
PFIL_HOOKS option.