The
bpf
node type allows Berkeley Packet Filter (see
bpf(4))
filters to be applied to data travelling through a Netgraph network.
Each node allows an arbitrary number of connections to arbitrarily
named hooks.
With each hook is associated a
bpf(4)
filter program which is applied to incoming data only, a destination hook
for matching packets, a destination hook for non-matching packets,
and various statistics counters.
A
bpf(4)
program returns an unsigned integer, which is normally interpreted as
the length of the prefix of the packet to return.
In the context of this
node type, returning zero is considered a non-match, in which case the
entire packet is delivered out the non-match destination hook.
Returning a value greater than zero causes the packet to be truncated
to that length and delivered out the match destination hook.
Either or both destination hooks may be the empty string, or may
not exist, in which case the packet is dropped.
New hooks are initially configured to drop all packets.
A new filter program may be installed using the
NGM_BPF_SET_PROGRAM
control message.
HOOKS
This node type supports any number of hooks having arbitrary names.
CONTROL MESSAGES
This node type supports the generic control messages, plus the following:
NGM_BPF_SET_PROGRAM
This command sets the filter program that will be applied to incoming
data on a hook.
The following structure must be supplied as an argument:
struct ng_bpf_hookprog {
char thisHook[NG_HOOKSIZ]; /* name of hook */
char ifMatch[NG_HOOKSIZ]; /* match dest hook */
char ifNotMatch[NG_HOOKSIZ]; /* !match dest hook */
int32_t bpf_prog_len; /* #isns in program */
struct bpf_insn bpf_prog[]; /* bpf program */
};
The hook to be updated is specified in
thisHook
The BPF program is the sequence of instructions in the
bpf_prog
array; there must be
bpf_prog_len
of them.
Matching and non-matching incoming packets are delivered out the hooks named
ifMatch
and
ifNotMatch
respectively.
The program must be a valid
bpf(4)
program or else
Er EINVAL
is returned.
NGM_BPF_GET_PROGRAM
This command takes an
ASCII
string argument, the hook name, and returns the
corresponding
struct ng_bpf_hookprog
as shown above.
NGM_BPF_GET_STATS
This command takes an
ASCII
string argument, the hook name, and returns the
statistics associated with the hook as a
struct ng_bpf_hookstat
NGM_BPF_CLR_STATS
This command takes an
ASCII
string argument, the hook name, and clears the
statistics associated with the hook.
NGM_BPF_GETCLR_STATS
This command is identical to
NGM_BPF_GET_STATS
except that the statistics are also atomically cleared.
SHUTDOWN
This node shuts down upon receipt of a
NGM_SHUTDOWN
control message, or when all hooks have been disconnected.
EXAMPLES
It is possible to configure a node from the command line, using
tcpdump(1)
to generate raw BPF instructions which are then fed into an
awk(1)
script to create the ASCII form of a
NGM_BPF_SET_PROGRAM
control message, as demonstrated here:
When built as a loadable kernel module, this module includes the file
net/bpf_filter.c
Although loading the module should fail if
net/bpf_filter.c
already exists in the kernel, currently it does not, and the duplicate
copies of the file do not interfere.
However, this may change in the future.
