Berkeley Packet Filter

The Berkeley Packet Filter (BPF; also BSD Packet Filter, classic BPF or cBPF) is a network tap and packet filter which permits computer network packets to be captured and filtered at the operating system level. It provides a raw interface to data link layers, permitting raw link-layer packets to be sent and received, and allows a userspace process to supply a filter program that specifies which packets it wants to receive. For example, a tcpdump process may want to receive only packets that initiate a TCP connection. BPF returns only packets that pass the filter that the process supplies. This avoids copying unwanted packets from the operating system kernel to the process, greatly improving performance. The filter program is in the form of instructions for a virtual machine, which are interpreted, or compiled into machine code by a just-in-time (JIT) mechanism and executed, in the kernel.

BPF is used by programs that need to, among other things, analyze network traffic. If the driver for the network interface supports promiscuous mode, it allows the interface to be put into that mode so that all packets on the network can be received, even those destined to other hosts.

The BPF filtering mechanism is available on most Unix-like operating systems. BPF is sometimes used to refer to just the filtering mechanism, rather than to the entire interface. Some systems, such as Linux and Tru64 UNIX, provide a raw interface to the data link layer other than the BPF raw interface but use the BPF filtering mechanisms for that raw interface.

The Linux kernel provides an extended version of the BPF filtering mechanism, called eBPF, which uses a JIT mechanism, and which is used for packet filtering, as well as for other purposes in the kernel. eBPF is also available for Microsoft Windows.

The original paper was written by Steven McCanne and Van Jacobson in 1992 while at Lawrence Berkeley Laboratory.

BPF provides pseudo-devices that can be bound to a network interface; reads from the device will read buffers full of packets received on the network interface, and writes to the device will inject packets on the network interface.

In 2007, Robert Watson and Christian Peron added zero-copy buffer extensions to the BPF implementation in the FreeBSD operating system, allowing kernel packet capture in the device driver interrupt handler to write directly to user process memory in order to avoid the requirement for two copies for all packet data received via the BPF device. While one copy remains in the receipt path for user processes, this preserves the independence of different BPF device consumers, as well as allowing the packing of headers into the BPF buffer rather than copying complete packet data.

BPF's filtering capabilities are implemented as an interpreter for a machine language for the BPF virtual machine, a 32-bit machine with fixed-length instructions, one accumulator, and one index register. Programs in that language can fetch data from the packet, perform arithmetic operations on data from the packet, and compare the results against constants or against data in the packet or test bits in the results, accepting or rejecting the packet based on the results of those tests.