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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
     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 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.

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
     program.

     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
     Summer 1990.  Much of the design is due to Van Jacobson.  This manpage
     was written by Orla McGann.