Single Packet Authorization (SPA).
fwknop stands for the "FireWall KNock OPerator", and
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 requires only a single encrypted packet in order to
communicate various pieces of information including desired
access through an iptables policy and/or complete commands
to execute on the target system. By using iptables to
maintain a "default drop" stance, the main application of
this program 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 monitors
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 (see the following network diagram; the SSH
session can only take place after the SPA packet is monitored):
PR: ports/118229
Submitted by: Sean Greven <sean.greven@gmail.com>
