This is a significant commit that allows iptables firewalls to be used
as an "SPA gateway" for all ports/protocols upon providing a valid SPA
packet. Additional commits will be made to extend this capability, but
this commit adds two new access.conf keywords: FORWARD_ALL and
DISABLE_DNAT. These are used in conjunction to add ACCEPT rules for all
ports/protocols in the FORWARD chain, and also disable DNAT rules at the
same time. Then, by buildling the SNAT chain to provide translation for
an internal network (where an SPA cliet is located), but DROP all
forwarded traffic by default at the same time, SPA can be used to gain
access to the internet. So, this would allow, say, an RFC 1918 internal
network to have IP's assigned via DHCP but they wouldn't be able to
access the internet before sending a SPA packet to the gateway. This
scenario was suggested by spartan1833 to the fwknop list and tracked via
github issue 131.
Additional commits will be made to fully support this feature.
6b7a3bbdae