1. Netfilter.
http://www.netfilter.org
2. Libpcap.
http://www.tcpdump.org
3. Snort.
http://www.snort.org
4. The Bro Network Security Monitor.
http://www.bro.org
5. Wehrle, K., Pählke, F., Ritter, H., Müller, D., Bechler, M.: The Linux Networking Architecture: Design and Implementation of Network Protocols in the Linux Kernel (2004)