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libnetfilter_conntrack - userspace library for the connection tracking system

(C) 2005-2011 Pablo Neira Ayuso <[email protected]>

Connection Tracking System

The connection tracking system is a in-kernel subsystem that stores information
about the state of a connection in a memory structure that contains the source
and destination IP addresses, port number pairs, protocol types, state, and
timeout. With this extra information, we can define more intelligent filtering

Moreover, there are some application protocols, such as FTP, TFTP, IRC, PPTP
that have aspects that are hard to track for a firewall that follows the
traditional static filtering approach. The connection tracking system defines
a mechanism to track such aspects.

The connection tracking system does not alter the packets themselves; the
default behavior always lets the packets continue their travel through the
network stack, although there are a couple of very specific exceptions where
packets can be dropped (e.g., under memory exhaustion). So keep in mind that
the connection tracking system just tracks packets; it does not filter.

For further information on the connection tracking system, please see the
reference section at the bottom of this document.

What is libnetfilter_conntrack?

libnetfilter_conntrack is an userspace library that provides an interface to
the in-kernel connection tracking system.


libnetfilter_conntrack is released under GPLv2 or any later at your option.

Prerequirements for libnetfilter_conntrack

Linux kernel version >= 2.6.18 ( and enable support for:

  • connection tracking system (quite obvious ;)
  • nfnetlink
  • ctnetlink (ip_conntrack_netlink)
  • connection tracking event notification API


You can generate the doxygen-based documentation by invoking:
$ doxygen doxygen.cfg


You can find a set of handy examples on the use of libnetfilter_conntrack
under the directory utils/ distributed with this library. You can compile them
by invoking:
$ make check

Heads Up

libnetfilter_conntrack used to provided two different APIs: The old one had
several limitations, for that reason, it was deprecated time ago. The existing
library only provides the new API that solves former deficiencies. Thus, make
sure you use recent versions of libnetfilter_conntrack and, in case that
you are using the old API, consider porting your application to the new one.

Since libnetfilter_conntrack >= 0.9.1, you can use the same handler obtained
via nfct_open() to register conntrack and expectation callbacks (before this
version, this was not possible).


[1] Pablo Neira Ayuso. Netfilter's Connection Tracking System:

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