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l7-filter is a software package which provides a classifier for Linux's Netfilter subsystem which can categorize Internet Protocol packets based on their application layer data. The major goal of this tool is to make possible the identification of peer-to-peer programs, which use unpredictable port numbers. There are two versions for this software. The first is implemented as a kernel module for Linux 2.4 and 2.6. The second experimental version was released in December 2006 which runs as a user-space program and relies on netfilter's user-space libraries for the classification process.

Both versions of l7-filter use regular expressions (though the user-space and kernel modules use different regular expression libraries) to identify the network protocol. This technique, used in conjunction with Linux's QoS system, allows application-specific yet port-independent traffic shaping.

All versions of l7-filter have been released under the GNU General Public License.

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