Berkeley Packet Filter

The Berkeley Packet Filter (BPF) provides a raw interface to data link layers, permitting raw link-layer packets to be sent and received. It is available on most Unix-like operating systems. In addition, 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.

BPF supports filtering packets, allowing a userspace process to supply a filter program that specifies which packets it wants to receive. For example, a tcpdump process may only want to receive packets that initiate a TCP connection. BPF only returns 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.

BPF is sometimes used to refer just to 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.

Raw interface

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.

Filtering

BPF's filtering capabilities are implemented as an interpreter for a machine language for the BPF virtual machine. 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.

Traditional Unix-like BPF implementations can be used in userspace, despite being written for kernel-space. This is accomplished using preprocessor conditions.

Extensions and optimizations

Some projects use BPF instruction sets or execution techniques different from the originals.

Some platforms, including FreeBSD, NetBSD, and WinPcap, use a just-in-time (JIT) compiler to convert BPF instructions into native code in order to improve performance. Linux includes a BPF JIT compiler which is disabled by default.

Kernel-mode interpreters for that same virtual machine language are used in raw data link layer mechanisms in other operating systems, such as Tru64 Unix, and for socket filters in the Linux kernel and in the WinPcap packet capture mechanism. Since version 3.18, the Linux kernel includes an extended BPF virtual machine, termed extended BPF (eBPF). It can be used for non-networking purposes, such as for attaching eBPF programs to various tracepoints. Since kernel version 3.19, eBPF filters can be attached to sockets, and, since kernel version 4.1, to traffic control classifiers.

A user-mode interpreter for BPF is provided with the libpcap/WinPcap implementation of the pcap API, so that, when capturing packets on systems without kernel-mode support for that filtering mechanism, packets can be filtered in user mode; code using the pcap API will work on both types of systems, although, on systems where the filtering is done in user mode, all packets, including those that will be filtered out, are copied from the kernel to user space. That interpreter can also be used when reading a file containing packets captured using pcap.

History

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

In August 2003, SCO Group publicly claimed that the Linux kernel was infringing Unix code which they owned. Programmers quickly discovered the code in question was the Berkeley Packet Filter. Clearly, SCO never owned the BPF code; however, SCO has not explained or acknowledged the mistake, although the ongoing legal action may eventually force an answer.

See also

• Data link layer

External links

• bpfc, a Berkeley Packet Filter compiler, Linux BPF JIT disassembler (part of netsniff-ng)