#
#  This is an example of a very light firewall used to guard against
#  some of the most easily exploited common security holes.
#
#  The example assumes it is running on a gateway with interface ppp0
#  attached to the outside world, and interface ed0 attached to
#  network 192.168.4.0 which needs to be protected.
#
#
#  Pass any packets not explicitly mentioned by subsequent rules
#
pass out from any to any
pass in from any to any
#
#  Block any inherently bad packets coming in from the outside world.
#  These include ICMP redirect packets and IP fragments so short the
#  filtering rules won't be able to examine the whole UDP/TCP header.
#
block in log quick on ppp0 proto icmp from any to any icmp-type redir
block in log quick on ppp0 proto tcp/udp all with short
#
#  Block any IP spoofing atempts.  (Packets "from" our network
#  shouldn't be coming in from outside).
#
block in log quick on ppp0 from 198.168.4.0/24 to any
block in log quick on ppp0 from localhost to any
#
#  Block any incoming traffic to NFS ports, to the RPC portmapper, and
#  to X servers.
#
block in log on ppp0 proto tcp/udp from any to any port = sunrpc
block in log on ppp0 proto tcp/udp from any to any port = 2049
block in log on ppp0 proto tcp from any to any port = 6000