jftpgw is a proxy server for the FTP protocol. It is highly configurable, allowing you to use an XML-like configuration file to define almost any combination of the client's IP, the server's IP, the user name, the destination port, and so on. jftpgw supports inetd/daemon mode, transparent proxying, forwards, caching, logging, data IP and port specification (port ranges), limiting of the number of simultaeous sessions, and several proxy login styles for clients that support FTP proxies.
knock is a server/client set that implements the idea known as port-knocking. Port-knocking is a method of accessing a backdoor to your firewall through a special sequence of port hits. This can be useful for opening up temporary holes in a restrictive firewall for SSH access or similar.
Libconnect is a simple library wrapper written in assembly language that intercepts the calls applications make to establish TCP connections and transparently proxies them as necessary. Existing applications can use this library without any recompilation or modification. If you provide multiple proxies they will be cycled connection after connection.
libnetfilter_cttimeout is an interface to Netfilter Connection Tracking timeouts. It provides a userspace library which provides a programming interface to the fine-grained connection tracking timeout infrastructure. With this library, you can create, update, and delete timeout policies which can be attached to traffic flows.