NG_BPF(4) FreeBSD Kernel Interfaces Manual NG_BPF(4)
NAME
ng_bpf - Berkeley packet filter netgraph node type
SYNOPSIS
#include <sys/types.h>
#include <net/bpf.h>
#include <netgraph.h>
#include <netgraph/ng_bpf.h>
DESCRIPTION
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 (setprogram)
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; /* #insns 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
EINVAL is returned.
NGM_BPF_GET_PROGRAM (getprogram)
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 (getstats)
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 (clrstats)
This command takes an ASCII string argument, the hook name, and
clears the statistics associated with the hook.
NGM_BPF_GETCLR_STATS (getclrstats)
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 transformed
into the ASCII form of a NGM_BPF_SET_PROGRAM control message, as
demonstrated here:
#!/bin/sh
PATTERN="tcp dst port 80"
NODEPATH="my_node:"
INHOOK="hook1"
MATCHHOOK="hook2"
NOTMATCHHOOK="hook3"
BPFPROG=$( tcpdump -s 8192 -p -ddd ${PATTERN} | \
( read len ; \
echo -n "bpf_prog_len=$len " ; \
echo -n "bpf_prog=[" ; \
while read code jt jf k ; do \
echo -n " { code=$code jt=$jt jf=$jf k=$k }" ; \
done ; \
echo " ]" ) )
ngctl msg ${NODEPATH} setprogram { thisHook=\"${INHOOK}\" \
ifMatch=\"${MATCHHOOK}\" \
ifNotMatch=\"${NOTMATCHHOOK}\" \
${BPFPROG} }
Based on the previous example, it is possible to prevent a jail (or a VM)
from spoofing by allowing only traffic that has the expected ethernet and
IP addresses:
#!/bin/sh
NODEPATH="my_node:"
JAIL_MAC="0a:00:de:ad:be:ef"
JAIL_IP="128.66.1.42"
JAIL_HOOK="jail"
HOST_HOOK="host"
DEBUG_HOOK="nomatch"
bpf_prog() {
local PATTERN=$1
tcpdump -s 8192 -p -ddd ${PATTERN} | (
read len
echo -n "bpf_prog_len=$len "
echo -n "bpf_prog=["
while read code jt jf k ; do
echo -n " { code=$code jt=$jt jf=$jf k=$k }"
done
echo " ]"
)
}
# Prevent jail from spoofing (filter packets coming from jail)
ngctl msg ${NODEPATH} setprogram { \
thisHook=\"${JAIL_HOOK}\" \
ifMatch=\"${HOST_HOOK}\" \
ifNotMatch=\"${DEBUG_HOOK}\" \
$(bpf_prog "ether src ${JAIL_MAC} && src ${JAIL_IP}") \
}
# Prevent jail from receiving spoofed packets (filter packets
# coming from host)
ngctl msg ${NODEPATH} setprogram { \
thisHook=\"${HOST_HOOK}\" \
ifMatch=\"${JAIL_HOOK}\" \
ifNotMatch=\"${DEBUG_HOOK}\" \
$(bpf_prog "ether dst ${JAIL_MAC} && dst ${JAIL_IP}") \
}
SEE ALSO
bpf(4), netgraph(4), ngctl(8)
HISTORY
The ng_bpf node type was implemented in FreeBSD 4.0.
AUTHORS
Archie Cobbs <archie@FreeBSD.org>
BUGS
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.
FreeBSD 13.1-RELEASE-p6 September 20, 2020 FreeBSD 13.1-RELEASE-p6
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