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formerly from 	Lawrence Berkeley National Laboratory
		Network Research Group <>

This directory contains source code for libpcap, a system-independent
interface for user-level packet capture.  libpcap provides a portable
framework for low-level network monitoring.  Applications include
network statistics collection, security monitoring, network debugging,
etc.  Since almost every system vendor provides a different interface
for packet capture, and since we've developed several tools that
require this functionality, we've created this system-independent API
to ease in porting and to alleviate the need for several
system-dependent packet capture modules in each application.

For some platforms there are README.{system} files that discuss issues
with the OS's interface for packet capture on those platforms, such as
how to enable support for that interface in the OS, if it's not built in
by default.

The libpcap interface supports a filtering mechanism based on the
architecture in the BSD packet filter.  BPF is described in the 1993
Winter Usenix paper ``The BSD Packet Filter: A New Architecture for
User-level Packet Capture''.  A compressed PostScript version can be
found at


and a gzipped version can be found at

A PDF version can be found at

Although most packet capture interfaces support in-kernel filtering,
libpcap utilizes in-kernel filtering only for the BPF interface.
On systems that don't have BPF, all packets are read into user-space
and the BPF filters are evaluated in the libpcap library, incurring
added overhead (especially, for selective filters).  Ideally, libpcap
would translate BPF filters into a filter program that is compatible
with the underlying kernel subsystem, but this is not yet implemented.

BPF is standard in 4.4BSD, BSD/OS, NetBSD, FreeBSD, OpenBSD, DragonFly
BSD, and Mac OS X; an older, modified and undocumented version is
standard in AIX.  {DEC OSF/1, Digital UNIX, Tru64 UNIX} uses the
packetfilter interface but has been extended to accept BPF filters
(which libpcap utilizes).  Also, you can add BPF filter support to
Ultrix using the kernel source and/or object patches available in:

Linux, in the 2.2 kernel and later kernels, has a "Socket Filter"
mechanism that accepts BPF filters; see the README.linux file for
information on configuring that option.

Note to Linux distributions and *BSD systems that include libpcap:

There's now a rule to make a shared library, which should work on Linux
and *BSD, among other platforms.

It sets the soname of the library to ""; this is what it
should be, *NOT* or or something such as

We've been maintaining binary compatibility between libpcap releases for
quite a while; there's no reason to tie a binary linked with libpcap to
a particular release of libpcap.

Problems, bugs, questions, desirable enhancements, etc. should be sent
to the address "".  Bugs, support
requests, and feature requests may also be submitted on the GitHub issue
tracker for libpcap at

Source code contributions, etc. should be sent to the email address
above or submitted by forking the branch on GitHub at

and issuing a pull request.

Current versions can be found at

 - The TCPdump team