http://netfilter.org/projects/libnfnetlink/index.html

What is nfnetlink?
nfnetlink is a netlink(7) based kernel/userspace transport layer.  It
provides a unified kernel/userspace interface for the various netfilter
subsystems, such as connection tracking, logging and queueing.

What is libnfnetlink?
libnfnetlink is a userspace library that provides some low-level
nfnetlink handling functions.  It is used as a foundation for other, netfilter
subsystem specific libraries such as libnfnetlink_conntrack, libnfnetlink_log
and libnfnetlink_queue.

Prerequirements for libnfnetlink
You will need kernel headers from a kernel that has nfnetlink support.  The
first official kernel release to include nfnetlink support is 2.6.14.
You can specify the location of your kernel sources using the "--with-kernel="
configure option.

Where can I find documentation?
At the moment, you will have to RTFS.  Sorry, but we have barely enough
manpower to keep development ongoing.  If you want to conribute documentation,
please contact us.

It has bugs.  Where do I report them?

Please report all libnfnetlink bugs to https://bugzilla.netfilter.org/, using
the "libnfnetlink" Product.

Where do I get support?

The netfilter@lists.netfilter.org and netfilter-devel@lists.netfilter.org
mailinglists server as support forum.

libnetfilter_queue is a userspace library providing an API to packets that have been queued by the kernel packet filter. It is is part of a system that deprecates the old ip_queue / libipq mechanism.

libnetfilter_queue has been previously known as libnfnetlink_queue.