problem with dmz firewall script - cant connect to inet via plan

From: bob (bob@kr8.co.uk)
Date: 12/10/02


From: bob@kr8.co.uk (bob)
Date: 9 Dec 2002 17:40:00 -0800

Hi,
can anyone tell me why my firewall script is not working correctly.
Everything is fine but I can not access the internet from my private
lan. I have a dmz and seperate trusted private lan multihomed on the
firewall box connected to the internet via a dsl connection. The
external interface card is aliased with real ip addresses and dnat'ed
through to the dmz. The box runs linux redhat 7.2 and I am using
iptables (userspace) as my packet filter. I have looked at this for a
while but am now at a loss for further testing. The strange thing is
that windows clients on the private lan have internet access ok but
when I try to connect a linux client to the internet via the private
lan it does not work. Does the static route table have to be set up as
normal on the private lan box ? i.e. add host, networks and default
gateway - if so should I be able to ping the dsl router via the
firewall ?

 
I have included my firewall script :

#!/bin/sh
# rc.DMZ.firewall - DMZ IP Firewall script for Linux 2.4.x and
iptables
# 1. Configuration options.

######### Internet Configuration.
INET_IP="81.2.94.210"
HTTP_IP="81.2.94.211"
DNS_IP="81.2.94.212"
INET_IFACE="eth0"

######## Local Area Network configuration.
LAN_IP="192.168.0.2"
LAN_IP_RANGE="192.168.0.0/16"
LAN_BROADCAST_ADDRESS="192.168.0.255"
LAN_IFACE="eth1"

######## Display status information
echo "---------------------------------------------------------"
echo "HTTP IP: $HTTP_IP"
echo "DNS IP: $DNS_IP"
echo "External Network Device: $INET_IP"
echo "External IP: $INET_IP"
echo "Local Network Address: $LAN_IP"
echo "---------------------------------------------------------"

ALLOW_ICMP="0 3 4 5 8 11 12"

######## DMZ Configuration.
DMZ_HTTP_IP="192.168.1.3"
DMZ_DNS_IP="192.168.1.4"
DMZ_IP="192.168.1.2"
DMZ_IP_RANGE="192.168.1.0/16"
DMZ_IFACE="eth2"

######## Localhost Configuration.
LO_IFACE="lo"
LO_IP="127.0.0.1"

# IPTables Configuration.
IPTABLES="/sbin/iptables"

# Other Configuration.

# Module loading.
# Needed to initially load modules
/sbin/depmod -a
# 2.1 Required modules
/sbin/modprobe ip_tables
/sbin/modprobe ip_conntrack
/sbin/modprobe iptable_filter
/sbin/modprobe iptable_mangle
/sbin/modprobe iptable_nat
/sbin/modprobe ipt_LOG
/sbin/modprobe ipt_limit
/sbin/modprobe ipt_state

# 2.2 Non-Required modules
#/sbin/modprobe ipt_owner
#/sbin/modprobe ipt_REJECT
#/sbin/modprobe ipt_MASQUERADE
#/sbin/modprobe ip_conntrack_ftp
#/sbin/modprobe ip_conntrack_irc
#/sbin/modprobe ip_nat_ftp
#/sbin/modprobe ip_nat_irc

# 3. /proc set up.
# 3.1 Required proc configuration

echo "1" > /proc/sys/net/ipv4/ip_forward

# 3.2 Non-Required proc configuration
#echo "1" > /proc/sys/net/ipv4/conf/all/rp_filter
#echo "1" > /proc/sys/net/ipv4/conf/all/proxy_arp
#echo "1" > /proc/sys/net/ipv4/ip_dynaddr

# Rules set up.
###### FILTER TABLE ############

# Set policies
$IPTABLES -P INPUT DROP
$IPTABLES -P OUTPUT DROP
$IPTABLES -P FORWARD DROP

# Create userspecified chains

# Create chain for bad tcp packets
$IPTABLES -N bad_tcp_packets

# Create separate chains for ICMP, TCP and UDP to traverse
$IPTABLES -N allowed
$IPTABLES -N icmp_packets
$IPTABLES -N tcp_packets
$IPTABLES -N udpincoming_packets

# Create content in userspecified chains

# bad_tcp_packets chain
$IPTABLES -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j
LOG \
--log-prefix "New not syn:"
$IPTABLES -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j
DROP

# allowed chain
$IPTABLES -A allowed -p TCP --syn -j ACCEPT
$IPTABLES -A allowed -p TCP -m state --state ESTABLISHED,RELATED -j
ACCEPT
$IPTABLES -A allowed -p TCP -j DROP

# ICMP rules
# Changed rules totally
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 0 -j ACCEPT
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j ACCEPT
$IPTABLES -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT

######## INPUT chain ############

# Bad TCP packets we don't want
$IPTABLES -A INPUT -p tcp -j bad_tcp_packets

# Packets from the Internet to this box
$IPTABLES -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets

# Packets from LAN, DMZ or LOCALHOST

# From DMZ Interface to DMZ firewall IP
$IPTABLES -A INPUT -p ALL -i $DMZ_IFACE -d $DMZ_IP -j ACCEPT

# From LAN Interface to LAN firewall IP
$IPTABLES -A INPUT -p ALL -i $LAN_IFACE -d $LAN_IP_RANGE -j ACCEPT
$IPTABLES -A INPUT -p ALL -i $LAN_IFACE -d $LAN_BROADCAST_ADDRESS -j
ACCEPT

# From Localhost interface to Localhost IP's
$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $LO_IP -j ACCEPT
$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $LAN_IP_RANGE -j ACCEPT
$IPTABLES -A INPUT -p ALL -i $LO_IFACE -s $INET_IP -j ACCEPT

# Special rule for DHCP requests from LAN, which are not caught
properly
# otherwise.
$IPTABLES -A INPUT -p UDP -i $LAN_IFACE --dport 67 --sport 68 -j
ACCEPT

# All established and related packets incoming from the internet to
the
# firewall
#

$IPTABLES -A INPUT -p ALL -d $INET_IP -m state --state
ESTABLISHED,RELATED \
-j ACCEPT

# Log weird packets that don't match the above.
$IPTABLES -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
--log-level DEBUG --log-prefix "IPT INPUT packet died: "

######## FORWARD chain ###########

# Bad TCP packets we don't want
$IPTABLES -A FORWARD -p tcp -j bad_tcp_packets

## DMZ section
## General rules
$IPTABLES -A FORWARD -i $DMZ_IFACE -o $INET_IFACE -j ACCEPT
$IPTABLES -A FORWARD -i $INET_IFACE -o $DMZ_IFACE -m state \
--state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A FORWARD -i $LAN_IFACE -o $DMZ_IFACE -j ACCEPT
$IPTABLES -A FORWARD -i $DMZ_IFACE -o $LAN_IFACE -j ACCEPT
#set up allow traffic from lan to inet and back again
$IPTABLES -A FORWARD -i $LAN_IFACE -o $INET_IFACE -j ACCEPT
$IPTABLES -A FORWARD -i $INET_IFACE -o $LAN_IFACE -m state \
--state ESTABLISHED,RELATED -j ACCEPT

## HTTP server
$IPTABLES -A FORWARD -p TCP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_HTTP_IP \
--dport 80 -j allowed
$IPTABLES -A FORWARD -p ICMP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_HTTP_IP \
-j icmp_packets
$IPTABLES -A FORWARD -p TCP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_HTTP_IP \
-s 0/0 --dport 22 -j allowed

## DNS server
$IPTABLES -A FORWARD -p TCP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_DNS_IP \
--dport 53 -j allowed
$IPTABLES -A FORWARD -p UDP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_DNS_IP \
--dport 53 -j ACCEPT
$IPTABLES -A FORWARD -p ICMP -i $INET_IFACE -o $DMZ_IFACE -d
$DMZ_DNS_IP \
-j icmp_packets

## LAN section
$IPTABLES -A FORWARD -p TCP -i $LAN_IFACE -j ACCEPT
$IPTABLES -A FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT

## Log weird packets that don't match the above.
$IPTABLES -A FORWARD -m limit --limit 3/minute --limit-burst 3 -j LOG
\
--log-level DEBUG --log-prefix "IPT FORWARD packet died: "

######## OUTPUT chain ############

# Bad TCP packets we don't want.
$IPTABLES -A OUTPUT -p tcp -j bad_tcp_packets

# Special OUTPUT rules to decide which IP's to allow.
$IPTABLES -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
$IPTABLES -A OUTPUT -p ALL -s $LAN_IP_RANGE -j ACCEPT
$IPTABLES -A OUTPUT -p ALL -s $INET_IP -j ACCEPT

# Log weird packets that don't match the above.
$IPTABLES -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
--log-level DEBUG --log-prefix "IPT OUTPUT packet died: "

### nat table

### Set policies

### Create user specified chains

### Create content in user specified chains

######## PREROUTING chain ##############
# send packets from internet to web server
$IPTABLES -t nat -A PREROUTING -p TCP -i $INET_IFACE -d $HTTP_IP
--dport 80 -j DNAT --to-destination $DMZ_HTTP_IP
$IPTABLES -t nat -A PREROUTING -p TCP -i $INET_IFACE -d $DNS_IP
--dport 53 -j DNAT --to-destination $DMZ_DNS_IP
$IPTABLES -t nat -A PREROUTING -p UDP -i $INET_IFACE -d $DNS_IP
--dport 53 -j DNAT --to-destination $DMZ_DNS_IP
$IPTABLES -t nat -A PREROUTING -p TCP -i $INET_IFACE -d $HTTP_IP
--dport 22 -j DNAT --to-destination $DMZ_HTTP_IP
$IPTABLES -t nat -A PREROUTING -p TCP -i $LAN_IFACE -d $INET_IP
--dport 80 -j DNAT --to-destination $INET_IP

###### POSTROUTING chain ##############
# Enable simple IP Forwarding and Network Address Translation
$IPTABLES -t nat -A POSTROUTING -o $INET_IFACE -j SNAT --to-source
$INET_IP

## OUTPUT chain
# mangle table
# Set policies
# Create user specified chains
# Create content in user specified chains

# PREROUTING chain
# INPUT chain
# FORWARD chain
# OUTPUT chain
# POSTROUTING chain

many thanks,

james fox



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