IP Filter Configuration File Examples

The following examples illustrate packet filtering rules that apply to a single host, a server, and a router.

Configuration files follow standard UNIX syntax rules:

• The pound sign (#) indicates a line containing comments.

• Rules and comments can coexist on the same line.

• Extraneous white space is allowed to keep rules easy to read.

• Rules can be more than one line long. Use the backslash (\) at the end of a line to indicate that the rule continues on the next line.

For more detailed syntax information, see Configuring Packet Filtering Rules.

Example 5-20 IP Filter Host Configuration

This example shows a configuration on a host machine with a net0 network interface.

# pass and log everything by default
pass in log on net0 all
pass out log on net0 all

# block, but don't log, incoming packets from other reserved addresses
block in quick on net0 from 10.0.0.0/8 to any
block in quick on net0 from 172.16.0.0/12 to any

# block and log untrusted internal IPs. 0/32 is notation that replaces 
# address of the machine running IP Filter.
block in log quick from 192.168.1.15 to <thishost>
block in log quick from 192.168.1.43 to <thishost>

# block and log X11 (port 6000) and remote procedure call 
# and portmapper (port 111) attempts
block in log quick on net0 proto tcp from any to net0/32 port = 6000 keep state
block in log quick on net0 proto tcp/udp from any to net0/32 port = 111 keep state

This rule set begins with two unrestricted rules that allow everything to pass into and out of the net0 interface. The second set of rules blocks any incoming packets from the private address spaces 10.0.0.0 and 172.16.0.0 from entering the firewall. The next set of rules blocks specific internal addresses from the host machine. Finally, the last set of rules blocks packets coming in on port 6000 and port 111.

Example 5-21 IP Filter Server Configuration

This example shows a configuration for a host machine acting as a web server. This machine has an net0 network interface.

# web server with an net0 interface
# block and log everything by default; 
# then allow specific services
# group 100 - inbound rules
# group 200 - outbound rules
# (0/32) resolves to our IP address)
*** FTP proxy ***

# block short packets which are packets 
# fragmented too short to be real.
block in log quick all with short

# block and log inbound and outbound by default, 
# group by destination
block in log on net0 from any to any head 100
block out log on net0 from any to any head 200


# web rules that get hit most often
pass in quick on net0 proto tcp from any \
to net0/32 port = http flags S keep state group 100
pass in quick on net0 proto tcp from any \
to net0/32 port = https flags S keep state group 100


# inbound traffic - ssh, auth
pass in quick on net0 proto tcp from any \
to net0/32 port = 22 flags S keep state group 100
pass in log quick on net0 proto tcp from any \
to net0/32 port = 113 flags S keep state group 100
pass in log quick on net0 proto tcp from any port = 113 \
to net0/32 flags S keep state group 100

# outbound traffic - DNS, auth, NTP, ssh, WWW, smtp
pass out quick on net0 proto tcp/udp from net0/32 \
to any port = domain flags S keep state group 200
pass in quick on net0 proto udp from any \
port = domain to net0/32 group 100

pass out quick on net0 proto tcp from net0/32 \
to any port = 113 flags S keep state group 200
pass out quick on net0 proto tcp from net0/32 port = 113 \
to any flags S keep state group 200

pass out quick on net0 proto udp from net0/32 to any \
port = ntp group 200
pass in quick on net0 proto udp from any \
port = ntp to net0/32 port = ntp group 100

pass out quick on net0 proto tcp from net0/32 \
to any port = ssh flags S keep state group 200

pass out quick on net0 proto tcp from net0/32 \
to any port = http flags S keep state group 200
pass out quick on net0 proto tcp from net0/32 \
to any port = https flags S keep state group 200

pass out quick on net0 proto tcp from net0/32 \
to any port = smtp flags S keep state group 200


# pass icmp packets in and out
pass in quick on net0 proto icmp from any to net0/32  keep state group 100
pass out quick on net0 proto icmp from net0/32 to any keep state group 200

# block and ignore NETBIOS packets
block in quick on net0 proto tcp from any \
to any port = 135 flags S keep state group 100

block in quick on net0 proto tcp from any port = 137 \
to any flags S keep state group 100
block in quick on net0 proto udp from any to any port = 137 group 100
block in quick on net0 proto udp from any port = 137 to any group 100

block in quick on net0 proto tcp from any port = 138 \
to any flags S keep state group 100
block in quick on net0 proto udp from any port = 138 to any group 100

block in quick on net0 proto tcp from any port = 139 to any flags S keep state
group 100
block in quick on net0 proto udp from any port = 139 to any group 100

Example 5-22 IP Filter Router Configuration

This example shows a configuration for a router that has an internal interface, net0, and an external interface, net1.

# internal interface is net0 at 192.168.1.1
# external interface is net1 IP obtained via DHCP
# block all packets and allow specific services
*** NAT ***
*** POOLS ***

# Short packets which are fragmented too short to be real.
block in log quick all with short

# By default, block and log everything.
block in log on net0 all
block in log on net1 all
block out log on net0 all
block out log on net1 all


# Packets going in/out of network interfaces that aren't on the loopback
# interface should not exist.
block in log quick on net0 from 127.0.0.0/8 to any
block in log quick on net0 from any to 127.0.0.0/8
block in log quick on net1 from 127.0.0.0/8 to any
block in log quick on net1 from any to 127.0.0.0/8


# Deny reserved addresses.
block in quick on net1 from 10.0.0.0/8 to any
block in quick on net1 from 172.16.0.0/12 to any
block in log quick on net1 from 192.168.1.0/24 to any
block in quick on net1 from 192.168.0.0/16 to any


# Allow internal traffic
pass in quick on net0 from 192.168.1.0/24 to 192.168.1.0/24
pass out quick on net0 from 192.168.1.0/24 to 192.168.1.0/24


# Allow outgoing DNS requests from our servers on .1, .2, and .3
pass out quick on net1 proto tcp/udp from net1/32 to any port = domain keep state
pass in quick on net0 proto tcp/udp from 192.168.1.2 to any port = domain keep state
pass in quick on net0 proto tcp/udp from 192.168.1.3 to any port = domain keep state


# Allow NTP from any internal hosts to any external NTP server.
pass in quick on net0 proto udp from 192.168.1.0/24 to any port = 123 keep state
pass out quick on net1 proto udp from any to any port = 123 keep state


# Allow incoming mail
pass in quick on net1 proto tcp from any to net1/32 port = smtp keep state
pass in quick on net1 proto tcp from any to net1/32 port = smtp keep state
pass out quick on net1 proto tcp from 192.168.1.0/24 to any port = smtp keep state


# Allow outgoing connections: SSH, WWW, NNTP, mail, whois
pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = 22 keep state
pass out quick on net1 proto tcp from 192.168.1.0/24 to any port = 22 keep state

pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = 80 keep state
pass out quick on net1 proto tcp from 192.168.1.0/24 to any port = 80 keep state
pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = 443 keep state
pass out quick on net1 proto tcp from 192.168.1.0/24 to any port = 443 keep state

pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = nntp keep state
block in quick on net1 proto tcp from any to any port = nntp keep state
pass out quick on net1 proto tcp from 192.168.1.0/24 to any port = nntp keep state

pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = smtp keep state

pass in quick on net0 proto tcp from 192.168.1.0/24 to any port = whois keep state
pass out quick on net1 proto tcp from any to any port = whois keep state


# Allow ssh from offsite
pass in quick on net1 proto tcp from any to net1/32 port = 22 keep state


# Allow ping out
pass in quick on net0 proto icmp all keep state
pass out quick on net1 proto icmp all keep state


# allow auth out
pass out quick on net1 proto tcp from net1/32 to any port = 113 keep state
pass out quick on net1 proto tcp from net1/32 port = 113 to any keep state


# return rst for incoming auth
block return-rst in quick on net1 proto tcp from any to any port = 113 flags S/SA


# log and return reset for any TCP packets with S/SA
block return-rst in log on net1 proto tcp from any to any flags S/SA

# return ICMP error packets for invalid UDP packets
block return-icmp(net-unr) in proto udp all