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
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.
SEE ALSO
bpf(4)
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.