#!/sbin/ipf -f -
#
# SAMPLE: RESTRICTIVE 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 in log proto tcp all flags S/SA head 101 group 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 in log proto tcp all flags S/SA head 201 group 200
block in log proto udp all head 202 group 200
block out log on ed0 all head 250
#-------------------------------------------------------
# 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
#-------------------------------------------------------
# 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
#
#-------------------------------------------------------
# Allow outgoing DNS requests (no named on firewall)
#
pass in quick proto udp from any to any port = 53 keep state group 202
#
# If we were running named on the firewall and all internal hosts talked to
# it, we'd use the following:
#
#pass in quick proto udp from any to w.x.y.z/32 port = 53 keep state group 202
#pass out quick on ppp0 proto udp from a.b.c.d/32 to any port = 53 keep state
#
# Allow outgoing FTP from any internal host to any external FTP server.
#
pass in quick proto tcp from any to any port = ftp keep state group 201
pass in quick proto tcp from any to any port = ftp-data keep state group 201
pass in quick proto tcp from any port = ftp-data to any port > 1023 keep state group 101
#
# Allow NTP from any internal host to any external NTP server.
#
pass in quick proto udp from any to any port = ntp keep state group 202
#
# Allow outgoing connections: SSH, TELNET, WWW
#
pass in quick proto tcp from any to any port = 22 keep state group 201
pass in quick proto tcp from any to any port = telnet keep state group 201
pass in quick proto tcp from any to any port = www keep state group 201
#
#-------------------------------------------------------
block in log proto tcp from any to a.b.c.d/32 flags S/SA head 110 group 100
#
# Allow incoming to the external firewall interface: mail, WWW, DNS
#
pass in log quick proto tcp from any to any port = smtp keep state group 110
pass in log quick proto tcp from any to any port = www keep state group 110
pass in log quick proto tcp from any to any port = 53 keep state group 110
pass in log quick proto udp from any to any port = 53 keep state group 100
#-------------------------------------------------------
# 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