NAME
pfil,
pfil_head_register,
pfil_head_unregister,
pfil_head_get,
pfil_hook_get,
pfil_add_hook,
pfil_remove_hook,
pfil_run_hooks —
packet filter interface
SYNOPSIS
#include
<sys/param.h>
#include <sys/mbuf.h>
#include <net/if.h>
#include <net/pfil.h>
int
pfil_head_register(struct
pfil_head *ph);
int
pfil_head_unregister(struct
pfil_head *ph);
struct pfil_head *
pfil_head_get(int
af, u_long
dlt);
struct packet_filter_hook *
pfil_hook_get(int
dir, struct pfil_head
*ph);
int
pfil_add_hook(int
(*func)(), void
*arg, int flags,
struct pfil_head
*ph);
int
pfil_remove_hook(int
(*func)(), void
*arg, int flags,
struct pfil_head
*ph);
int
(*func)(void
*arg, struct mbuf
**mp, struct ifnet
*, int dir);
int
pfil_run_hooks(struct
pfil_head *ph, struct
mbuf **mp, struct ifnet
*ifp, int dir);
DESCRIPTION
Thepfil 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_ATTACHorPFIL_IFNET_DETACH) - PFIL_WAITOK
- OK to call malloc with M_WAITOK.
SEE ALSO
HISTORY
The pfil interface first appeared in
NetBSD 1.3. The pfil input
and output lists were originally implemented as
<sys/queue.h>
LIST structures; however this was changed in
NetBSD 1.4 to TAILQ
structures. This change was to allow the input and output filters to be
processed in reverse order, to allow the same path to be taken, in or out of
the kernel.
The pfil interface was changed in 1.4T to
accept a 3rd parameter to both pfil_add_hook() and
pfil_remove_hook(), introducing the capability of
per-protocol filtering. This was done primarily in order to support
filtering of IPv6.
In 1.5K, the pfil framework was changed to
work with an arbitrary number of filtering points, as well as be less
IP-centric.
AUTHORS
The pfil interface was designed and
implemented by Matthew R. Green, with help from
Darren Reed, Jason R.
Thorpe, and Charles M. Hannum.
Darren Reed added support for IPv6 in addition to
IPv4. Jason R. Thorpe added support for multiple
hooks and other clean up.
BUGS
The current pfil implementation will need
changes to suit a threaded kernel model.