#!/sbin/ipf -f -
#
# SAMPLE: PERMISSIVE FILTER RULES
#
# THIS EXAMPLE IS WRITTEN FOR IP FILTER 3.3
#
# ppp0 - (external) PPP connection to ISP, address a.b.c.d/32
#
# ed0 - (internal) network interface, address w.x.y.z/32
#
# This file contains the basic rules needed to construct a firewall for the
# above situation.
#
#-------------------------------------------------------
# *Nasty* packets we don't want to allow near us at all!
# short packets which are packets fragmented too short to be real.
block in log quick all with short
#-------------------------------------------------------
# Group setup.
# ============
# By default, block and log everything.  This maybe too much logging
# (especially for ed0) and needs to be further refined.
#
block in log on ppp0 all head 100
block out log on ppp0 all head 150
block in log on ed0 from w.x.y.z/24 to any head 200
block out log on ed0 all head 250
#-------------------------------------------------------
# Invalid Internet packets.
# =========================
#
# Deny reserved addresses.
#
block in log quick from 10.0.0.0/8 to any group 100
block in log quick from 192.168.0.0/16 to any group 100
block in log quick from 172.16.0.0/12 to any group 100
#
# Prevent IP spoofing.
#
block in log quick from a.b.c.d/24 to any group 100
#
#-------------------------------------------------------
# Localhost packets.
# ==================
# packets going in/out of network interfaces that aren't on the loopback
# interface should *NOT* exist.
block in log quick from 127.0.0.0/8 to any group 100
block in log quick from any to 127.0.0.0/8 group 100
block in log quick from 127.0.0.0/8 to any group 200
block in log quick from any to 127.0.0.0/8 group 200
# And of course, make sure the loopback allows packets to traverse it.
pass in quick on lo0 all
pass out quick on lo0 all
#-------------------------------------------------------
# Allow any communication between the inside network and the outside only.
#
# Allow all outgoing connections (SSH, TELNET, FTP, WWW, gopher, etc)
#
pass in log quick proto tcp all flags S/SA keep state group 200 
#
# Support all UDP `connections' initiated from inside.
#
# Allow ping out
#
pass in log quick proto icmp all keep state group 200
#-------------------------------------------------------
# Log these:
# ==========
# * return RST packets for invalid SYN packets to help the other end close
block return-rst in log proto tcp from any to any flags S/SA group 100
# * return ICMP error packets for invalid UDP packets
block return-icmp(net-unr) in proto udp all group 100