fwknop implements an authorization scheme called Single Packet Authorization (SPA). This method of authorization is based around a default-drop packet filter (fwknop supports both iptables on Linux systems and ipfw on FreeBSD and Mac OS X systems) and libpcap. SPA is essentially next generation port knocking.
SPA requires only a single encrypted packet in order to communicate various pieces of information including desired access through a firewall policy and/or complete commands to execute on the target system.
By using a firewall to maintain a “default drop” stance, the main application of fwknop is to protect services such as OpenSSH with an additional layer of security in order to make the exploitation of vulnerabilities (both 0-day and unpatched code) much more difficult. With fwknop deployed, anyone using nmap to look for sshd can’t even tell that it is listening; it makes no difference if they have a 0-day exploit or not.
The authorization server passively sniffs authorization packets via libcap and hence there is no “server” to which to connect in the traditional sense. Access to a protected service is only granted after a valid encrypted and non-replayed packet is monitored from an fwknop client.
fwknop 2.0 is the first production release of the fully re-written C version of fwknop, and is the culmination of an effort to provide Single Packet Authorization to multiple open source firewalls, embedded systems, mobile devices, and more.
On the “server” side, supported firewalls now include iptables on Linux, ipfw on FreeBSD and Mac OS X, and pf on OpenBSD. The fwknop client is known to run on all of these platforms, and also functions on Windows systems running under Cygwin. There is also an Android client, and a good start on a iPhone client as well.