This publication addresses three network security related topics based on open source. Network flow analysis addresses the problem of finding suspicious packet flows on a big network, and also gives an introduction to how a proposal for an unfriendly firewall should look. Redundant firewalling gives an overview about redundant firewall technologies in routing and bridging mode with state synchronization for IPv6 and IPv4 networks. IPsec VPN deals with the implementation of a reliable open source IPsec VPN server that is secure and supports all standard clients with their standard connection tools. This is a very technical book inlcuding a lot of configuration examples and helpful hints. The whole implementation is based on open source tools. You may find additional information on homepage of the author: http://www.wogri.com