Iptables Tutorial 1.1.19

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# 1. Configuration options.

#

#

# 1.1 Internet Configuration.

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INET_IFACE="eth0"

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# 1.1.1 DHCP

#

#

# Information pertaining to DHCP over the Internet, if needed.

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# Set DHCP variable to no if you don't get IP from DHCP. If you get DHCP

# over the Internet set this variable to yes, and set up the proper IP

# address for the DHCP server in the DHCP_SERVER variable.

#

DHCP="no"

DHCP_SERVER="195.22.90.65"

#

# 1.1.2 PPPoE

#

# Configuration options pertaining to PPPoE.

#

# If you have problem with your PPPoE connection, such as large mails not

# getting through while small mail get through properly etc, you may set

# this option to «yes» which may fix the problem. This option will set a

# rule in the PREROUTING chain of the mangle table which will clamp

# (resize) all routed packets to PMTU (Path Maximum Transmit Unit).

#

# Note that it is better to set this up in the PPPoE package itself, since

# the PPPoE configuration option will give less overhead.

#

PPPOE_PMTU="no"

#

# 1.2 Local Area Network configuration.

#

# your LAN's IP range and localhost IP. /24 means to only use the first 24

# bits of the 32 bit IP address. the same as netmask 255.255.255.0

#

LAN_IP="192.168.0.2"

LAN_IP_RANGE="192.168.0.0/16"

LAN_IFACE="eth1"

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# 1.3 DMZ Configuration.

#

#

# 1.4 Localhost Configuration.

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LO_IFACE="lo"

LO_IP="127.0.0.1"

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# 1.5 IPTables Configuration.

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IPTABLES="/usr/sbin/iptables"

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# 1.6 Other Configuration.

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###########################################################################

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# 2. Module loading.

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#

# Needed to initially load modules

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/sbin/depmod -a

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# 2.1 Required modules

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/sbin/modprobe ip_conntrack

/sbin/modprobe ip_tables

/sbin/modprobe iptable_filter

/sbin/modprobe iptable_mangle

/sbin/modprobe iptable_nat

/sbin/modprobe ipt_LOG

/sbin/modprobe ipt_limit

/sbin/modprobe ipt_MASQUERADE

#

# 2.2 Non-Required modules

#

#/sbin/modprobe ipt_owner

#/sbin/modprobe ipt_REJECT

#/sbin/modprobe ip_conntrack_ftp

#/sbin/modprobe ip_conntrack_irc

#/sbin/modprobe ip_nat_ftp

#/sbin/modprobe ip_nat_irc

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# 3. /proc set up.

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# 3.1 Required proc configuration

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

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# 3.2 Non-Required proc configuration

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#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

###########################################################################

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# 4. rules set up.

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######

# 4.1 Filter table

#

#

# 4.1.1 Set policies

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$IPTABLES -P INPUT DROP

$IPTABLES -P OUTPUT DROP

$IPTABLES -P FORWARD DROP

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# 4.1.2 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 tcp_packets

$IPTABLES -N udp_packets

$IPTABLES -N icmp_packets

#

# 4.1.3 Create content in userspecified chains

#

#

# bad_tcp_packets chain

#

$IPTABLES -A bad_tcp_packets -p tcp –tcp-flags SYN,ACK SYN,ACK \

–m state –state NEW -j REJECT –reject-with tcp-reset

$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

#

# TCP rules

#

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 21 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 22 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 80 -j allowed

$IPTABLES -A tcp_packets -p TCP -s 0/0 –dport 113 -j allowed

#

# UDP ports

#

$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 53 -j ACCEPT

if [ $DHCP == «yes» ] ; then

$IPTABLES -A udp_packets -p UDP -s $DHCP_SERVER –sport 67 \

–dport 68 -j ACCEPT

fi

#$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 53 -j ACCEPT

#$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 123 -j ACCEPT

$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 2074 -j ACCEPT

$IPTABLES -A udp_packets -p UDP -s 0/0 –source-port 4000 -j ACCEPT

#

# In Microsoft Networks you will be swamped by broadcasts. These lines

# will prevent them from showing up in the logs.

#

#$IPTABLES -A udp_packets -p UDP -i $INET_IFACE \

#–destination-port 135:139 -j DROP

#

# If we get DHCP requests from the Outside of our network, our logs will

# be swamped as well. This rule will block them from getting logged.

#

#$IPTABLES -A udp_packets -p UDP -i $INET_IFACE -d 255.255.255.255 \

#–destination-port 67:68 -j DROP

#

# ICMP rules

#

$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

#

# 4.1.4 INPUT chain

#

#

# Bad TCP packets we don't want.

#

$IPTABLES -A INPUT -p tcp -j bad_tcp_packets

#

# Rules for special networks not part of the Internet

#

$IPTABLES -A INPUT -p ALL -i $LAN_IFACE -s $LAN_IP_RANGE -j ACCEPT

$IPTABLES -A INPUT -p ALL -i $LO_IFACE -j ACCEPT

#

# Special rule for DHCP requests from LAN, which are not caught properly