SRFirewall is an easy to use, but still quite capable firewall for Netfilter/iptables. It is a complete rewrite of Firewall/SOSDG, designed to make it easy for users to configure and protect their Linux servers, routers, and gateways.
Covert_qos uses the Quality of service fields in IPv4 and IPv6 to set up a unidirectional covert channel. In IPv4, this is a single byte per packet; in IPv6 it is a maximum of 3 bytes per packet. Files can be sent over the channel. It uses Linux netfilter queues as a basis.