Linux Iptables Example Firewall Script
SYSCTL="/sbin/sysctl -w"
IPT="/sbin/iptables"
IPTS="/sbin/iptables-save"
IPTR="/sbin/iptables-restore"
# Interface Connected to Internet
INET_IFACE="eth0"
INET_ADDRESS=" 209.223.237 .66"
# Localhost Interface
LO_IFACE="lo"
LO_IP="127.0.0.1"
# Save and Restore arguments handled here
if [ "$1" = "save" ]
then
echo -n "Saving firewall to /etc/sysconfig/iptables ... "
$IPTS > /etc/sysconfig/iptables
echo "done"
exit 0
elif [ "$1" = "restore" ]
then
echo -n "Restoring firewall from /etc/sysconfig/iptables ... "
$IPTR < /etc/sysconfig/iptables
echo "done"
exit 0
fi
###############################################################################
#
# Load Modules
#
echo "Loading kernel modules ..."
# /sbin/depmod -a
# core netfilter module
/sbin/modprobe ip_tables
# the stateful connection tracking module
/sbin/modprobe ip_conntrack
# filter table module
# /sbin/modprobe iptable_filter
# mangle table module
# /sbin/modprobe iptable_mangle
# nat table module
# /sbin/modprobe iptable_nat
# LOG target module
# /sbin/modprobe ipt_LOG
# This is used to limit the number of packets per sec/min/hr
# /sbin/modprobe ipt_limit
# masquerade target module
# /sbin/modprobe ipt_MASQUERADE
# filter using owner as part of the match
# /sbin/modprobe ipt_owner
# REJECT target drops the packet and returns an ICMP response.
# The response is configurable. By default, connection refused.
# /sbin/modprobe ipt_REJECT
# This target allows packets to be marked in the mangle table
# /sbin/modprobe ipt_mark
# This target affects the TCP MSS
# /sbin/modprobe ipt_tcpmss
# This match allows multiple ports instead of a single port or range
# /sbin/modprobe multiport
# This match checks against the TCP flags
# /sbin/modprobe ipt_state
# This match catches packets with invalid flags
# /sbin/modprobe ipt_unclean
# The ftp nat module is required for non-PASV ftp support
/sbin/modprobe ip_nat_ftp
# the module for full ftp connection tracking
/sbin/modprobe ip_conntrack_ftp
# the module for full irc connection tracking
/sbin/modprobe ip_conntrack_irc
# SYN flood protection.
if [ "$SYSCTL" = "" ]
then
echo "1" > /proc/sys/net/ipv4/tcp_syncookies
else
$SYSCTL net.ipv4.tcp_syncookies="1"
fi
# Source validation by reversed path according to RFC1812.
if [ "$SYSCTL" = "" ]
then
echo "1" > /proc/sys/net/ipv4/conf/all/rp_filter
else
$SYSCTL net.ipv4.conf.all.rp_filter="1"
fi
# Ignore all ICMP echo requests sent to the broadcast address.
if [ "$SYSCTL" = "" ]
then
echo "1" > /proc/sys/net/ipv4/icmp_echo_ignore_broadcasts
else
$SYSCTL net.ipv4.icmp_echo_ignore_broadcasts="1"
fi
# Accept or refuse source routed packets.
if [ "$SYSCTL" = "" ]
then
echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route
else
$SYSCTL net.ipv4.conf.all.accept_source_route="0"
fi
# This option accepts only from gateways in the default gateways list.
if [ "$SYSCTL" = "" ]
then
echo "1" > /proc/sys/net/ipv4/conf/all/secure_redirects
else
$SYSCTL net.ipv4.conf.all.secure_redirects="1"
fi
# This option logs packets from impossible addresses.
if [ "$SYSCTL" = "" ]
then
echo "1" > /proc/sys/net/ipv4/conf/all/log_martians
else
$SYSCTL net.ipv4.conf.all.log_martians="1"
fi
###############################################################################
#
# Flush Any Existing Rules or Chains
#
echo "Flushing Tables ..."
# Reset Default Policies
$IPT -P INPUT ACCEPT
$IPT -P FORWARD ACCEPT
$IPT -P OUTPUT ACCEPT
$IPT -t nat -P PREROUTING ACCEPT
$IPT -t nat -P POSTROUTING ACCEPT
$IPT -t nat -P OUTPUT ACCEPT
$IPT -t mangle -P PREROUTING ACCEPT
$IPT -t mangle -P OUTPUT ACCEPT
# Flush all rules
$IPT -F
$IPT -t nat -F
$IPT -t mangle -F
# Erase all non-default chains
$IPT -X
$IPT -t nat -X
$IPT -t mangle -X
if [ "$1" = "stop" ]
then
echo "Firewall completely flushed! Now running with no firewall."
exit 0
fi
###############################################################################
#
# Rules Configuration
#
####################################################### ########################
#
# Filter Table
#
###############################################################################
# Set Policies
$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP
###############################################################################
#
# User-Specified Chains
#
# Create user chains to reduce the number of rules each packet
# must traverse.
echo "Create and populate custom rule chains ..."
# Create a chain to filter INVALID packets
$IPT -N bad_packets
# Create another chain to filter bad tcp packets
$IPT -N bad_tcp_packets
# Create separate chains for icmp, tcp (incoming and outgoing),
# and incoming udp packets.
$IPT -N icmp_packets
# Used for UDP packets inbound from the Internet
$IPT -N udp_inbound
# Used to block outbound UDP services from internal network
# Default to allow all
$IPT -N udp_outbound
# Used to allow inbound services if desired
# Default fail except for established sessions
$IPT -N tcp_inbound
# Used to block outbound services from internal network
# Default to allow all
$IPT -N tcp_outbound
###############################################################################
#
# Populate User Chains
#
# bad_packets chain
#
# Drop INVALID packets immediately
$IPT -A bad_packets -p ALL -m state --state INVALID -j LOG \
--log-prefix "Invalid packet: "
$IPT -A bad_packets -p ALL -m state --state INVALID -j DROP
# Then check the tcp packets for additional problems
$IPT -A bad_packets -p tcp -j bad_tcp_packets
# All good, so return
$IPT -A bad_packets -p ALL -j RETURN
# Stop port scannings.
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j LOG \
--log-prefix "New not syn: "
$IPT -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL NONE -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL ALL -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j LOG \
--log-prefix "Stealth scan: "
$IPT -A bad_tcp_packets -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP
# All good, so return
$IPT -A bad_tcp_packets -p tcp -j RETURN
# icmp_packets chain
$IPT -A icmp_packets --fragment -p ICMP -j LOG \
--log-prefix "ICMP Fragment: "
$IPT -A icmp_packets --fragment -p ICMP -j DROP
# Echo
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 8 -j DROP
# Time Exceeded
$IPT -A icmp_packets -p ICMP -s 0/0 --icmp-type 11 -j ACCEPT
# Not matched, so return so it will be logged
$IPT -A icmp_packets -p ICMP -j RETURN
# TCP & UDP
# udp_inbound chain
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 137 -j DROP
$IPT -A udp_inbound -p UDP -s 0/0 --destination-port 138 -j DROP
# Not matched, so return for logging
$IPT -A udp_inbound -p UDP -j RETURN
# udp_outbound chain
# No match, so ACCEPT
$IPT -A udp_outbound -p UDP -s 0/0 -j ACCEPT
# tcp_inbound chain
# Web Server
# HTTP
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 80 -j ACCEPT
# HTTPS (Secure Web Server)
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 443 -j ACCEPT
# FTP Server (Control)
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 21 -j ACCEPT
# FTP Client (Data Port for non-PASV transfers)
$IPT -A tcp_inbound -p TCP -s 0/0 --source-port 20 -j ACCEPT
# Passive FTP
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 62000:64000 -j ACCEPT
# sshd
$IPT -A tcp_inbound -p TCP -s 0/0 --destination-port 2 2 -j ACCEPT
# Not matched, so return so it will be logged
$IPT -A tcp_inbound -p TCP -j RETURN
# tcp_outbound chain
# No match, so ACCEPT
$IPT -A tcp_outbound -p TCP -s 0/0 -j ACCEPT
###############################################################################
#
# INPUT Chain
#
echo "Process INPUT chain ..."
# Allow all on localhost interface
$IPT -A INPUT -p ALL -i $LO_IFACE -j ACCEPT
# Drop bad packets
$IPT -A INPUT -p ALL -j bad_packets
$IPT -A INPUT -p ALL -d 224.0.0.1 -j DROP
# Inbound Internet Packet Rules
# Accept Established Connections
$IPT -A INPUT -p ALL -i $INET_IFACE -m state --state ESTABLISHED,RELATED \
-j ACCEPT
# Route the rest to the appropriate user chain
$IPT -A INPUT -p TCP -i $INET_IFACE -j tcp_inbound
$IPT -A INPUT -p UDP -i $INET_IFACE -j udp_inbound
$IPT -A INPUT -p ICMP -i $INET_IFACE -j icmp_packets
# Drop without logging broadcasts that get this far.
$IPT -A INPUT -m pkttype --pkt-type broadcast -j DROP
# Log packets that still don't match
$IPT -A INPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
--log-prefix "INPUT packet died: "
###############################################################################
#
# FORWARD Chain
#
echo "Process FORWARD chain ..."
# Used if forwarding for a private network
###############################################################################
#
# OUTPUT Chain
#
echo "Process OUTPUT chain ..."
# Generally trust the firewall on output
$IPT -A OUTPUT -m state -p icmp --state INVALID -j DROP
# Localhost
$IPT -A OUTPUT -p ALL -s $LO_IP -j ACCEPT
$IPT -A OUTPUT -p ALL -o $LO_IFACE -j ACCEPT
# To internet
$IPT -A OUTPUT -p ALL -o $INET_IFACE -j ACCEPT
# Log packets that still don't match
$IPT -A OUTPUT -m limit --limit 3/minute --limit-burst 3 -j LOG \
--log-prefix "OUTPUT packet died: "
###############################################################################
#
# nat table
#
###############################################################################
echo "Load rules for nat table ..."
###############################################################################
#
# PREROUTING chain
#
###############################################################################
#
# POSTROUTING chain
#
###############################################################################
#
# mangle table
#
###############################################################################
echo "Load rules for mangle table ..."
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