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
.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
.Vt 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
.Vt mbuf chain, it does not free it or keep a pointer to it. This means that an
.Vt 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
.Vt 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 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 program.
The bpf_validate function returns 0 when the program is not a valid filter program.