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#
# 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
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