freebsd-dev/share/man/man9/pfil.9

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.Dd September 8, 2003
.Dt PFIL 9
.Os
.Sh NAME
.Nm pfil ,
.Nm pfil_head_register ,
.Nm pfil_head_unregister ,
.Nm pfil_head_get ,
.Nm pfil_hook_get ,
.Nm pfil_add_hook ,
.Nm pfil_remove_hook ,
.Nm pfil_run_hooks
.Nd packet filter interface
.Sh SYNOPSIS
.In sys/param.h
.In sys/mbuf.h
.In net/if.h
.In net/pfil.h
.Ft int
.Fn pfil_head_register "struct pfil_head *head"
.Ft int
.Fn pfil_head_unregister "struct pfil_head *head"
.Ft struct pfil_head *
.Fn pfil_head_get "int af" "u_long dlt"
.Ft struct packet_filter_hook *
.Fn pfil_hook_get "int dir" "struct pfil_head *head"
.Ft void
.Fn pfil_add_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
.Ft void
.Fn pfil_remove_hook "int (*func)()" "void *arg" "int flags" "struct pfil_head *"
.Ft int
.Fn (*func) "void *arg" "struct mbuf **mp" "struct ifnet *" "int dir"
.Ft int
.Fn pfil_run_hooks "struct pfil_head *head" "struct mbuf **mp" "struct ifnet *" "int dir"
.Sh DESCRIPTION
The
.Nm
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.
.Pp
Packet filtering points are registered with
.Fn pfil_head_register .
Filtering points are identified by a key (void *) and a data link type
(int) in the
.Em 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
.Xr 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
.Fn pfil_head_unregister
function.
.Pp
Packet filters register/unregister themselves with a filtering point
with the
.Fn pfil_add_hook
and
.Fn pfil_remove_hook
functions, respectively.
The head is looked up using the
.Fn 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.
.Pp
When a filter is invoked, the packet appears just as if it
.Dq 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) 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.
.Pp
The
.Nm
interface is enabled in the kernel via the
.Sy PFIL_HOOKS
option.
.Sh RETURN VALUES
If successful,
.Fn pfil_head_get 
returns the pfil_head structure for the given key/dlt. 
.Fn pfil_add_hook 
and
.Fn pfil_remove_hook
return 0 if successful. If called with flag PFIL_WAITOK, 
.Fn pfil_remove_hook
is expected to always succeed.
.Pp
.Fn pfil_head_unregister
might sleep!
.Sh HISTORY
The
.Nm
interface first appeared in
.Nx 1.3 .
The
.Nm
input and output lists were originally implemented as
.Fd \*[Lt]sys/queue.h\*[Gt]
.Dv LIST
structures;
however this was changed in
.Nx 1.4
to
.Dv 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.
.Pp
The
.Nm
interface was changed in 1.4T to accept a 3rd parameter to both
.Fn pfil_add_hook
and
.Fn pfil_remove_hook ,
introducing the capability of per-protocol filtering.
This was done primarily in order to support filtering of IPv6.
.Pp
In 1.5K, the
.Nm
framework was changed to work with an arbitrary number of filtering points,
as well as be less IP-centric.
.Pp
Fine-grained locking was added in
.Fx 5.2 .
.Sh BUGS
.Fn pfil_hook_get
is only safe for internal use.
.Pp
FreeBSD implements only hooks for AF_INET and AF_INET6.
Packets diverted through these hooks have data in
host byte order contrary to the above statements.
.Pp
The
.Xr bridge 4
diverts inbound AF_INET traffic, but contrary to the above
statements, the data is provided in host byte order.
.Pp
When a pfil_head is being modified no traffic is diverted
(to avoid deadlock).
This means that unwanted traffic may flow for a short period
of time.
.Sh SEE ALSO
.Xr bpf 4 ,
.Xr bridge 4