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bpf (Berkeley Packet Filter)
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Overview IOCTLS BPF header Filter machine Authors Bugs Files History
#include <sys/types.h>
#include <sys/time.h>
#include <sys/ioctl.h>
#include <net/bpf.h>

int 
bpf_open(char * name, int flags, int mode);

Overview

The Berkeley Packet Filter provides a raw interface to data link layers in a protocol independent fashion. All packets on the network, even those destined for other hosts, are accessible through this mechanism.

The packet filter appears as a character special device, /dev/bpf0, /dev/bpf1, etc. After opening the device, the file descriptor must be bound to a specific network interface with the BIOCSETIF ioctl. A given interface can be shared by multiple listeners, and the filter underlying each descriptor will see an identical packet stream.

A separate device file is required for each minor device. If a file is in use, the bpf_open will fail and errno will be set to EBUSY.

Associated with each open instance of a bpf file is a user-settable packet filter. Whenever a packet is received by an interface, all file descriptors listening on that interface apply their filter. Each descriptor that accepts the packet receives its own copy.

Reads from these files return the next group of packets that have matched the filter. To improve performance, the buffer passed to read must be the same size as the buffers used internally by bpf. This size is returned by the BIOCGBLEN ioctl (see below), and can be set with BIOCSBLEN. Note that an individual packet larger than this size is necessarily truncated.

The packet filter will support any link level protocol that has fixed length headers. Currently, only Ethernet, SLIP, and PPP drivers have been modified to interact with bpf.

Since packet data is in network byte order, applications should use the byteorder(3) macros to extract multi-byte values.

A packet can be sent out on the network by writing to a bpf file descriptor. The writes are unbuffered, meaning only one packet can be processed per write. Currently, only writes to Ethernets and SLIP links are supported.

The bpf_open() mode and flags are as defined for open()

IOCTLS

The ioctl(2) command codes below are defined in <net/bpf.h>. All commands require these includes:

     #include <sys/types.h>
     #include <sys/time.h>
     #include <sys/ioctl.h>
     #include <net/bpf.h>


Additionally, BIOCGETIF and BIOCSETIF require <sys/socket.h> and <net/if.h>.

In addition to FIONREAD and SIOCGIFADDR, the following commands may be applied to any open bpf file. The (third) argument to ioctl(2) should be a pointer to the type indicated.

BPF header

The following structure is prepended to each packet returned by read(2):

struct bpf_hdr {
             struct timeval bh_tstamp;     /* time stamp */
             u_long bh_caplen;             /* length of captured portion */
             u_long bh_datalen;            /* original length of packet */
             u_short bh_hdrlen;            /* length of bpf header (this struct
                                              plus alignment padding */
     };

The fields, whose values are stored in host order, and are:

The bh_hdrlen field exists to account for padding between the header and the link level protocol. The purpose here is to guarantee proper alignment of the packet data structures, which is required on alignment sensitive architectures and improves performance on many other architectures. The packet filter insures that the bpf_hdr and the network layer header will be word aligned. Suitable precautions must be taken when accessing the link layer protocol fields on alignment restricted machines. (This is not a problem on an Ethernet, since the type field is a short falling on an even offset, and the addresses are probably accessed in a bytewise fashion).

Additionally, individual packets are padded so that each starts on a word boundary. This requires that an application has some knowledge of how to get from packet to packet. The macro BPF_WORDALIGN is defined in <net/bpf.h> to facilitate this process. It rounds up its argument to the nearest word aligned value (where a word is BPF_ALIGNMENT bytes wide).

For example, if `p' points to the start of a packet, this expression will advance it to the next packet:

p = (char *)p + BPF_WORDALIGN(p->bh_hdrlen + p->bh_caplen)

For the alignment mechanisms to work properly, the buffer passed to read(2) must itself be word aligned. The malloc(3) function will always return an aligned buffer.

Filter machine

A filter program is an array of instructions, with all branches forwardly directed, terminated by a return instruction. Each instruction performs some action on the pseudo-machine state, which consists of an accumulator, index register, scratch memory store, and implicit program counter.

The following structure defines the instruction format:

struct bpf_insn {
         u_short code;
         u_char  jt;
         u_char  jf;
         u_long k;
     };

The k field is used in different ways by different instructions, and the jt and jf fields are used as offsets by the branch instructions. The opcodes are encoded in a semi-hierarchical fashion. There are eight classes of instructions: BPF_LD, BPF_LDX, BPF_ST, BPF_STX, BPF_ALU, BPF_JMP, BPF_RET, and BPF_MISC. Various other mode and operator bits are or'd into the class to give the actual instructions. The classes and modes are defined in <net/bpf.h>.

Below are the semantics for each defined bpf instruction. We use the convention that A is the accumulator, X is the index register, P[] packet data, and M[] scratch memory store. P[i:n] gives the data at byte offset 'i' in the packet, interpreted as a word (n=4), unsigned halfword (n=2), or unsigned byte (n=1). M[i] gives the i'th word in the scratch memory store, which is only addressed in word units. The memory store is indexed from 0 to BPF_MEMWORDS - 1. k, jt, and jf are the corresponding fields in the instruction definition. 'len' refers to the length of the packet.

The bpf interface provides the following macros to facilitate array initializers: BPF_STMT(opcode, operand) and BPF_JUMP(opcode, operand, true_offset, false_offset).

Authors

Steven McCanne, of Lawrence Berkeley Laboratory, implemented BPF in Summer 1990. Much of the design is due to Van Jacobson.

Bugs

The read buffer must be of a fixed size (returned by the BIOCGBLEN ioctl).

A file that does not request promiscuous mode may receive promiscuously received packets as a side effect of another file requesting this mode on the same hardware interface. This could be fixed in the kernel with additional processing overhead. However, we favor the model where all files must assume that the interface is promiscuous, and if so desired, must utilize a filter to reject foreign packets.

Data link protocols with variable length headers are not currently supported.

The SEESENT, DIRECTION, and FEEDBACK settings have been observed to work incorrectly on some interface types, including those with hardware loop- back rather than software loopback, and point-to-point interfaces. They appear to function correctly on a broad range of Ethernet-style interfaces.

Files

/dev/bpfn the packet filter device

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.

Requirements

Versions Link to
INtime 4.0 clib.lib
netlib.lib
See Also
pcap (Packet Capture library)
tcpdump

 

 

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