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Old 01-21-2005, 11:59 PM   #1
fotoguy
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Registered: Mar 2003
Location: Brisbane Queensland Australia
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Can someone check my Iptables script


Just starting to write an iptables script and would like someone to take a look at it and see if i'm on the right track. Maybe make some suggestions on how I could improve or add to the script.

The script is written for a static IP, and to allow all internal traffic to the internet. I have written some of it myself, and grab some snippets from samples on the internet.

#!/bin/bash
#
#==============================================================
# SECTION 1

# Set the ethernet cards to the network and internet sides

INET_IP="35.35.35.35"
INET_IFACE="eth0"

LAN_IP="192.168.0.1"
LAN_IFACE="eth1"
INTERNAL_ADDRESS_RANGE="192.168.0.0/24"
LAN_BCAST_ADDRESS="192.168.0.255"


LO_IP="127.0.0.1"
LO_IFACE="lo"

ifdown $INET_IFACE
ifdown $LAN_IFACE
ifconfig $INET_IFACE 0.0.0.0
ifconfig $LAN_IFACE 0.0.0.0

#===============================================================================
# SECTION 2

# Load the netfilter 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

# Load 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

#=================================================================================
# SECTION 3

# Set iptables variable path

iptables=/sbin/iptables

# Allow internal machines to forward packets to the internet

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

# Prevent SYN floods from cosuming to much resources

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

# Disable response to ping

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

# Disable response to broadcasts

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

# Don't accept source routed packets

echo "0" > /proc/sys/net/ipv4/conf/all/accept_source_route

# Disable ICMP redirect acceptance

for interface in /proc/sys/net/ipv4/conf/*/accept_redirects; do
/bin/echo "0" > ${interface}
done

# Turn on reverse path filtering. This helps make sure that packets use
# legitimate source addresses

for interface in /proc/sys/net/ipv4/conf/*/rp_filter; do
/bin/echo "1" > ${interface}
done

# Log spoofed packets, source routed packets, redirect packets.

/bin/echo "1" > /proc/sys/net/ipv4/conf/all/log_martians

# Enable bad error message protection.

/bin/echo "1" > /proc/sys/net/ipv4/icmp_ignore_bogus_error_responses

#================================================================================
# SECTION 4

# Flush all existing rule

$IPTABLES -F INPUT
$IPTABLES -F OUTPUT
$IPTABLES -F FORWARD
$IPTABLES -F -t nat
$IPTABLES -X

# Set the default policy for FORWARD, INPUT and OUTPUT rules to drop all packets

$IPTABLES -P FORWARD DROP
$IPTABLES -P INPUT DROP
$IPTABLES -P OUTPUT DROP

# Create separate chains for ICMP, TCP and UDP to traverse
# Sourced from internet sample script

$IPTABLES -N allowed
$IPTABLES -N tcp_packets
$IPTABLES -N udp_packets
$IPTABLES -N icmp_packets

# Create a chain for bad TCP packets

$IPTABLES -N bad_tcp_packets

#=========================================================================================
# SECTION 5

# First line to accept packets on the Internal network interface first ($LAN_IFACE)
# Second then forwards all packets from $LAN_IFACE (Internal Network) to $INET_IFACE (Internet)
# Third line then allows the ($INET_IFACE) to accept the packets to pass out into the internet

$IPTABLES -A INPUT -i $LAN_IFACE -s 0/0 -d 0/0 -j ACCEPT
$IPTABLES -A FORWARD -i $LAN_IFACE -o $INET_IFACE -j ACCEPT
$IPTABLES -A OUTPUT -o $INET_IFACE -s 0/0 -d 0/0 -j ACCEPT

# These 2 lines are to accept packets on the Internal network ($LAN_IFACE loopback)
# And also NAT loopback as well, may not be needed but wont hurt to have it

$IPTABLES -A INPUT -i lo -s 0/0 -d 0/0 -j ACCEPT
$IPTABLES -A OUTPUT -o lo -s 0/0 -d 0/0 -j ACCEPT
$IPTABLES -t nat -A PREROUTING -i lo -j ACCEPT
$IPTABLES -t nat -A POSTROUTING -o lo -j ACCEPT

# First we need to allow the firewall to accept existing and related connections from the internet ($INET_IFACE)
# Then forward those connections to the Internal Network ($LAN_IFACE), so ($LAN_IFACE) can then allow packets to travel into
# the network to their destination

$IPTABLES -A INPUT -i $INET_IFACE -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A FORWARD -i $INET_IFACE -o $LAN_IFACE -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPTABLES -A OUTPUT -o $LAN_IFACE -j ACCEPT

# Enable SNAT on the Internet Interface ($INET_IFACE) this is used for Accounts that have a static IP
# All data from the Internal network ID will be give the static IP address as it passes through
# to the internet. 35.35.35.35 replace with your static IP address assigned by your ISP

$IPTABLES -A POSTROUTING -t nat -s 192.168.0.0/24 -o $INET_IFACE -j SNAT --to -source 35.35.35.35

# If you have a dynamic IP address (DHCP) assigned to you by your ISP, you will need to use
# Masquerade, just comment out the above SNAT line, an uncomment the MASQUERADE line
# below

# $IPTABLES -A POSTROUTING -t nat -s 192.168.0.0/24 -o $INET_IFACE -j MASQUERADE

#==================================================================================
# SECTION 6

# Need to block a specific internal IP address from accessing the internet

#$IPTABLES -A OUTPUT -o $INET_IFACE -s 192.168.1.10 -j DROP

# Need to block a specific port on an internal IP address

#$IPTABLES -A OUTPUT -o $INET_IFACE -s 192.168.1.10 --sport 80 --dport 80 -j DROP

# Need to block a specific IP address and log any attemps from accessing the firewall from the internet

#$IPTABLES -A INPUT -i $INET_IFACE -s 210.210.210.210 -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "BLOCKED IP ALERT: "
#$IPTABLES -A INPUT -i $INET_IFACE -s 210.210.210.210 -j DROP

# To block a machine by its physical address (MAC) from accessing the internet

#$IPTABLES -A OUTPUT -o $INET_IFACE --mac-source 00:0BB:45:56:42 -j DROP

# To allow only specified (MAC) addresses to pass through the firewall
# To use this feature, you will need to comment-out the default FORWARD rule (SECTION 5) allowing all internal network
# packets to pass from ($LAN_IFACE) to ($INET_IFACE), an then un-comment this line.

#$IPTABLES -A FORWARD -i $LAN_IFACE -o $INET_IFACE --mac-source 00:0BB:45:56:42 -j ACCEPT

# Need to FORWARD packets to a specified IP address behind the firewall on the internal network

#$IPTABLES -A INPUT -p TCP -i $INET_IFACE -s 0/0 -d 192.168.0.10 -sport 80 -dport 80 -j ACCEPT
#$IPTABLES -A FORWARD -p TCP -i $INET_IFACE -o $LAN_IFACE -s 0/0 -d 192.168.0.10 -sport 80 -dport 80 -j ACCEPT

#===================================================================================
# SECTION 7

# All below the hash line come from internet sample scripts


# Rules for incoming packets from anywhere.

$IPTABLES -A INPUT -p TCP -j tcp_packets
$IPTABLES -A INPUT -p UDP -j udp_packets
$IPTABLES -A INPUT -p ICMP -j icmp_packets

# 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

# 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 -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "New not syn:"
$IPTABLES -A bad_tcp_packets -p tcp ! --syn -m state --state NEW -j DROP

# Do some checks for obviously spoofed IP's

$IPTABLES -A bad_tcp_packets -i $INET_IFACE -s 192.168.0.0/24 -j DROP
$IPTABLES -A bad_tcp_packets -i $INET_IFACE -s 10.0.0.0/8 -j DROP
$IPTABLES -A bad_tcp_packets -i $INET_IFACE -s 172.16.0.0/12 -j DROP


# Take care of bad TCP packets that we don't want.

$IPTABLES -A INPUT -p tcp -j bad_tcp_packets
$IPTABLES -A OUTPUT -p tcp -j bad_tcp_packets
$IPTABLES -A FORWARD -p tcp -j bad_tcp_packets

#===================================================================================
# SECTION 8

# This section came from sample scripts on the internet

# Log weird packets that don't match any rules above.

$IPTABLES -A INPUT -m limit --limit 6/h --limit-burst 5 -j LOG --log-level DEBUG --log-prefix "IPT INPUT packet died: "

# Any udp not already allowed is logged and then dropped.

$IPTABLES -A INPUT -i $INET_IFACE -p udp -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "IPTABLES UDP-IN: "
$IPTABLES -A INPUT -i $INET_IFACE -p udp -j DROP

# Any icmp not already allowed is logged and then dropped.

$IPTABLES -A INPUT -i $INET_IFACE -p icmp -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "IPTABLES ICMP-IN: "
$IPTABLES -A INPUT -i $INET_IFACE -p icmp -j DROP

# Any tcp not already allowed is logged and then dropped.

$IPTABLES -A INPUT -i $INET_IFACE -p tcp -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "IPTABLES TCP-IN: "
$IPTABLES -A INPUT -i $INET_IFACE -p tcp -j DROP

# Anything else not already allowed is logged and then dropped.
# It will be dropped by the default policy anyway ........ but let's be paranoid.

$IPTABLES -A INPUT -i $INET_IFACE -m limit --limit 6/h --limit-burst 5 -j LOG --log-prefix "IPTABLES PROTOCOL-X-IN: "
$IPTABLES -A INPUT -i $INET_IFACE -j DROP


###################################################################################
 
Old 01-23-2005, 02:10 PM   #2
peter_robb
Senior Member
 
Registered: Feb 2002
Location: Szczecin, Poland
Distribution: Gentoo, Debian
Posts: 2,458

Rep: Reputation: 47
It looks like you want to be a firewll for pc's on a LAN ..
But you appear to have confused the use of the INPUT and OUTPUT chains.
These only apply to packets the firewall itself creates or is the final point for..

I suggest having a read of this tutorial and maybe using one of the scripts at the end..
 
Old 01-25-2005, 12:32 AM   #3
fotoguy
Senior Member
 
Registered: Mar 2003
Location: Brisbane Queensland Australia
Distribution: KirraMail Live Email Server
Posts: 1,276

Original Poster
Rep: Reputation: 61
Thanks for that, will check out the tutorial
 
  


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