libnetfilter_cthelper is the userspace library that provides a programming interface (API) to user-space connection tracking helpers. Basically, you can find here a set of helper functions that you can use together with libmnl.
libnetfilter_cthelper homepage is: http://netfilter.org/projects/libnetfilter_cthelper/
libnetfilter_cthelper requires libmnl >= 1.0.0 and the Linux kernel that includes the nfnetlink_cthelper subsystem (i.e. 3.3.x or any later).
The current development version of libnetfilter_cthelper can be accessed at https://git.netfilter.org/cgi-bin/gitweb.cgi?p=libnetfilter_cthelper.git
To write your own program using libnetfilter_cthelper, you should start by reading the documentation for libmnl to understand basic operation with Netlink sockets. Moreover, you may want to check the examples available under examples/ in the libnetfilter_cthelper source code. You can compile these examples by invoking `make check'.
libnetfilter_cthelper has been written by Pablo Neira Ayuso.