freebsd/share/man/man9/bpf.9

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.Dd May 19, 2004
.Dt BPF 9
.Os
.\"
.Sh NAME
.Nm bpf
.Nd "Berkeley Packet Filter"
.\"
.Sh SYNOPSIS
.In net/bpf.h
.\"
.Ft void
.Fn bpfattach "struct ifnet *ifp" "u_int dlt" "u_int hdrlen"
.Ft void
.Fn bpfattach2 "struct ifnet *ifp" "u_int dlt" "u_int hdrlen" "struct bpf_if **driverp"
.Ft void
.Fn bpfdetach "struct ifnet *ifp"
.Ft void
.Fn bpf_tap "struct ifnet *ifp" "u_char *pkt" "u_int *pktlen"
.Ft void
.Fn bpf_mtap "struct ifnet *ifp" "struct mbuf *m"
.Ft void
.Fn bpf_mtap2 "struct ifnet *bp" "void *data" "u_int dlen" "struct mbuf *m"
.Ft u_int
.Fn bpf_filter "const struct bpf_insn *pc " "u_char *pkt" "u_int *wirelen" "u_int *buflen"
.Ft int
.Fn bpf_validate "const struct bpf_insn *fcode" "int flen"
.\"
.Sh 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.
.Xr bpf 4
describes the interface used by user programs.
This man page describes the functions used by interfaces to pass packets to
.Nm 
and the functions for testing and running
.Nm
filter machine programs.
.Pp
.Fn bpfattach
attaches a network interface to
.Nm .
.Em ifp
is a pointer to the structure that defines the interface to be
attached to an interface.
.Em dlt
is the data link-layer type:
DLT_NULL
.Po no link-layer encapsulation
.Pc ,
DLT_EN10MB
.Po Ethernet
.Pc ,
DLT_IEEE802_11
.Po 802.11 wireless networks
.Pc ,
etc.
The rest of the link layer types can be found in
.Pa /usr/src/sys/net/bpf.h .
.Em hdrlen
is the fixed size of the link header;
variable length headers are not yet supported.
The
.Nm
system will hold a pointer to
.Em ifp->if_bpf .
This variable will set to a non-NULL value when
.Nm
requires packets from this interface to be tapped using the functions below.
.Pp
.Fn bpfattach2
allows multiple bpf instances to be attached to a single interface,
by registering an explicit
.Em if_bpf
rather than using 
.Em ifp->if_bpf .
It is then possible to run
.Xr tcpdump 1
on the interface for any data link-layer types attached.
.Pp
.Fn bpfdetach
detaches a
.Nm
instance from an interface,
specified by
.Em ifp .
.Fn bpfdetach
should be called once for each
.Nm bpf
instance attached.
.Pp
.Fn bpf_tap
is used by an interface to pass the packet to
.Nm .
The packet data (including link-header),
pointed to by
.Em pkt ,
is of length
.Em pktlen ,
which must be a contiguous buffer.
.Em ifp
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.
.Pp
.Fn bpf_mtap
is
like
.Fn bpf_tap
except that it is used to tap packets that are in an mbuf chain,
.Em m .
.Em ifp
is a pointer to the structure that defines the interface to be tapped.
Like
.Fn bpf_tap ,
.Fn bpf_mtap
requires a link-header for whatever data link layer type is specified.
Note that
.Nm
only reads from the mbuf chain,
it does not free it or keep a pointer to it.
This means that a mbuf containing the link-header
can be prepended to the chain if necessary.
A cleaner interface to achieve this is provided by
.Fn bpf_mtap2 .
.Pp
.Fn bpf_mtap2
allows the user to pass a link-header
.Em data ,
of length
.Em dlen ,
independent of the mbuf
.Em m ,
containing the packet.
This simplifies the passing of some link-headers.
.Pp
.Fn bpf_filter
executes the filter program starting at
.Em pc
on the packet
.Em pkt .
.Em wirelen
is the length of the original packet and
.Em buflen
is the amount of data present.
.Pp
.Fn bpf_validate
checks that the filter code
.Em fcode ,
of length
.Em flen ,
is valid.
.\"
.Sh RETURN VALUES
.Fn bpf_filter
returns -1
.Po cast to an unsigned integer
.Pc
if there is no filter.
Otherwise, it returns the result of the filter program.
.Pp
.Fn bpf_validate
returns 0 when the program is not a valid filter program.
.\"
.Sh SEE ALSO
.Xr tcpdump 1 ,
.Xr bpf 4 .
.\"
.Sh 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
.Bx
and continued its development from 1983 on.
Since then,
it has evolved into the Ultrix Packet Filter at
.Tn DEC ,
a
.Tn STREAMS
.Tn NIT
module under
.Tn SunOS 4.1 ,
and
.Tn BPF .
.\"
.Sh AUTHORS
.An -nosplit
.An Steven McCanne ,
of Lawrence Berkeley Laboratory, implemented BPF in Summer 1990.
Much of the design is due to
.An Van Jacobson .
This manpage by was written by
.An Orla McGann .