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bpf — Berkeley Packet Filter
bpfattach(struct ifnet *ifp, u_int dlt, u_int hdrlen);
bpfattach2(struct ifnet *ifp, u_int dlt, u_int hdrlen,
struct bpf_if **driverp);
bpfdetach(struct ifnet *ifp);
bpf_tap(struct ifnet *ifp, u_char *pkt, u_int *pktlen);
bpf_mtap(struct ifnet *ifp, struct mbuf *m);
bpf_mtap2(struct bpf_if *bp, void *data, u_int dlen, struct mbuf *m);
bpf_filter(const struct bpf_insn *pc, u_char *pkt, u_int wirelen,
bpf_validate(const struct bpf_insn *fcode, int flen);
The Berkeley Packet Filter provides a raw interface, that is protocol
independent, to data link layers. It allows all packets on the network,
even those destined for other hosts, to be passed from a network
interface to user programs. Each program may specify a filter, in the
form of a bpf filter machine program. The bpf(4) manual page describes
the interface used by user programs. This manual page describes the
functions used by interfaces to pass packets to bpf and the functions for
testing and running bpf filter machine programs.
The bpfattach() function attaches a network interface to bpf. The ifp
argument is a pointer to the structure that defines the interface to be
attached to an interface. The dlt argument is the data link-layer type:
DLT_NULL (no link-layer encapsulation), DLT_EN10MB (Ethernet),
DLT_IEEE802_11 (802.11 wireless networks), etc. The rest of the link
layer types can be found in <net/bpf.h>. The hdrlen argument is the
fixed size of the link header; variable length headers are not yet
supported. The bpf system will hold a pointer to ifp->if_bpf. This
variable will set to a non-NULL value when bpf requires packets from this
interface to be tapped using the functions below.
The bpfattach2() function allows multiple bpf instances to be attached to
a single interface, by registering an explicit if_bpf rather than using
ifp->if_bpf. It is then possible to run tcpdump(1) on the interface for
any data link-layer types attached.
The bpfdetach() function detaches a bpf instance from an interface,
specified by ifp. The bpfdetach() function should be called once for
each bpf instance attached.
The bpf_tap() function is used by an interface to pass the packet to bpf.
The packet data (including link-header), pointed to by pkt, is of length
pktlen, which must be a contiguous buffer. The ifp argument is a pointer
to the structure that defines the interface to be tapped. The packet is
parsed by each processes filter, and if accepted, it is buffered for the
process to read.
The bpf_mtap() function is like bpf_tap() except that it is used to tap
packets that are in an mbuf chain, m. The ifp argument is a pointer to
the structure that defines the interface to be tapped. Like bpf_tap(),
bpf_mtap() requires a link-header for whatever data link layer type is
specified. Note that bpf only reads from the mbuf chain, it does not
free it or keep a pointer to it. This means that an mbuf containing the
link-header can be prepended to the chain if necessary. A cleaner
interface to achieve this is provided by bpf_mtap2().
The bpf_mtap2() function allows the user to pass a link-header data, of
length dlen, independent of the mbuf m, containing the packet. This
simplifies the passing of some link-headers.
The bpf_filter() function executes the filter program starting at pc on
the packet pkt. The wirelen argument is the length of the original
packet and buflen is the amount of data present. The buflen value of 0
is special; it indicates that the pkt is actually a pointer to an mbuf
chain (struct mbuf *).
The bpf_validate() function checks that the filter code fcode, of length
flen, is valid.
The bpf_filter() function returns -1 (cast to an unsigned integer) if
there is no filter. Otherwise, it returns the result of the filter
The bpf_validate() function returns 0 when the program is not a valid
The Enet packet filter was created in 1980 by Mike Accetta and Rick
Rashid at Carnegie-Mellon University. Jeffrey Mogul, at Stanford, ported
the code to BSD and continued its development from 1983 on. Since then,
it has evolved into the Ultrix Packet Filter at DEC, a STREAMS NIT module
under SunOS 4.1, and BPF.
Steven McCanne, of Lawrence Berkeley Laboratory, implemented BPF in
Summer 1990. Much of the design is due to Van Jacobson. This manpage
was written by Orla McGann.