#
#  This is an example of a fairly heavy firewall used to keep everyone
#  out of a particular network while still allowing people within that
#  network to get outside.
#
#  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, IP fragments so short the
#  filtering rules won't be able to examine the whole UDP/TCP header,
#  and anything with IP options.
#
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 in log quick on ppp0 from any to any with ipopts
#
#  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 all incoming UDP traffic except talk and DNS traffic.  NFS
#  and portmap are special-cased and logged.
#
block in on ppp0 proto udp from any to any
block in log on ppp0 proto udp from any to any port = sunrpc
block in log on ppp0 proto udp from any to any port = 2049
pass in on ppp0 proto udp from any to any port = domain
pass in on ppp0 proto udp from any to any port = talk
pass in on ppp0 proto udp from any to any port = ntalk
#
#  Block all incoming TCP traffic connections to known services,
#  returning a connection reset so things like ident don't take
#  forever timing out.  Don't log ident (auth port) as it's so common.
#
block return-rst in log on ppp0 proto tcp from any to any flags S/SA
block return-rst in on ppp0 proto tcp from any to any port = auth flags S/SA
#
#  Allow incoming TCP connections to ports between 1024 and 5000, as
#  these don't have daemons listening but are used by outgoing
#  services like ftp and talk.  For slightly more obscurity (though
#  not much more security), the second commented out rule can chosen
#  instead.
#
pass in on ppp0 proto tcp from any to any port 1024 >< 5000
#pass in on ppp0 proto tcp from any port = ftp-data to any port 1024 >< 5000
#
#  Now allow various incoming TCP connections to particular hosts, TCP
#  to the main nameserver so secondaries can do zone transfers, SMTP
#  to the mail host, www to the web server (which really should be
#  outside the firewall if you care about security), and ssh to a
#  hypothetical machine caled 'gatekeeper' that can be used to gain
#  access to the protected network from the outside world.
#
pass in on ppp0 proto tcp from any to ns1 port = domain
pass in on ppp0 proto tcp from any to mail port = smtp
pass in on ppp0 proto tcp from any to www port = www
pass in on ppp0 proto tcp from any to gatekeeper port = ssh