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BPF(9)		       FreeBSD Kernel Developer's Manual		BPF(9)

NAME
     bpf -- Berkeley Packet Filter

SYNOPSIS
     #include <net/bpf.h>

     void
     bpfattach(struct ifnet *ifp, u_int	dlt, u_int hdrlen);

     void
     bpfattach2(struct ifnet *ifp, u_int dlt, u_int hdrlen,
	 struct	bpf_if **driverp);

     void
     bpfdetach(struct ifnet *ifp);

     void
     bpf_tap(struct ifnet *ifp,	u_char *pkt, u_int *pktlen);

     void
     bpf_mtap(struct ifnet *ifp, struct	mbuf *m);

     void
     bpf_mtap2(struct bpf_if *bp, void *data, u_int dlen, struct mbuf *m);

     u_int
     bpf_filter(const struct bpf_insn *pc, u_char *pkt,	u_int wirelen,
	 u_int buflen);

     int
     bpf_validate(const	struct bpf_insn	*fcode,	int flen);

DESCRIPTION
     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 inter-
     face 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 func-
     tions 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	sup-
     ported.  The bpf system will hold a pointer to ifp-_if_bpf.  This vari-
     able 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, spec-
     ified 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	inter-
     face 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 sim-
     plifies 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.

RETURN VALUES
     The bpf_filter() function returns -1 (cast	to an unsigned integer)	if
     there is no filter.  Otherwise, it	returns	the result of the filter pro-
     gram.

     The bpf_validate()	function returns 0 when	the program is not a valid
     filter program.

SEE ALSO
     tcpdump(1), bpf(4)

HISTORY
     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.

AUTHORS
     Steven McCanne, of	Lawrence Berkeley Laboratory, implemented BPF in Sum-
     mer 1990.	Much of	the design is due to Van Jacobson.  This manpage was
     written by	Orla McGann.

FreeBSD	10.1		       December	13, 2006		  FreeBSD 10.1

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | SEE ALSO | HISTORY | AUTHORS

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