Handy Linux iptables script

Here is a script I put together some years ago to create a simple Iptables firewall on my linux webserver. Some of the code was borrowed from a linux security book but I do not remember which one. Anyway it is a pretty handy script to give you some control and protection.

First create three files in /usr/local/etc:

ipblack.lst – this file contains a list of ip addresses you want to blacklist. One ip or subnet per line.

Example:

94.178.222.17
87.0.0.0/8

ipwhite.lst – this file contains a list of ip addresses that you allow unrestricted access (Be careful with this). One ip or subnet per line. Make sure you add localhost to this file.

Example:

localhost
10.10.1.1 #Home IP Address

ports.lst – this file contains a list of ports you allow.

Example:

22 #SSH
25 #SMTP
53 #DNS/Domain
80 #HTTPD
443 #HTTPS

Add this iptables.sh script to /usr/local/sbin

#!/bin/sh

#Iptables for webserver

IPTABLES=/sbin/iptables
WHITELIST=/usr/local/etc/ipwhite.lst
BLACKLIST=/usr/local/etc/ipblack.lst
PORTSLIST=/usr/local/etc/ports.lst

#—-Flood Variables—–#

# Overall Limit for TCP-SYN-Flood detection
TCPSYNLIMIT=”5/s”
# Burst Limit for TCP-SYN-Flood detection
TCPSYNLIMITBURST=”10″

# Overall Limit for Loggging in Logging-Chains
LOGLIMIT=”2/s”
# Burst Limit for Logging in Logging-Chains
LOGLIMITBURST=”10″

# Overall Limit for Ping-Flood-Detection
PINGLIMIT=”5/s”

# Burst Limit for Ping-Flood-Detection
PINGLIMITBURST=”10″

#Clear any current filters
$IPTABLES -F

#Process Whitelist
for x in `grep -v ^# $WHITELIST | awk ‘{print $1}’`; do
echo “Permitting $x…”
$IPTABLES -A INPUT -t filter -s $x -j ACCEPT
done

#Process Blacklist
for x in `grep -v ^# $BLACKLIST | awk ‘{print $1}’`; do
echo “Blocking $x…”
#$IPTABLES -A INPUT -t filter -s $x -j LOG
$IPTABLES -A INPUT -t filter -s $x -j DROP
done

#Allow Ports list
for port in `grep -v ^# $PORTSLIST | awk ‘{print $1}’`; do
echo “Accepting port $port…”
$IPTABLES -A INPUT -t filter -p tcp –dport $port -j ACCEPT
done

$IPTABLES -A INPUT -t filter -p tcp –syn -j DROP

#ICMP TIMESTAMP REQUEST AND REPLY
$IPTABLES -A INPUT -p icmp –icmp-type timestamp-request -j DROP
$IPTABLES -A FORWARD -p icmp –icmp-type timestamp-request -j DROP

#Logging of possible TCP-SYN-Floods
$IPTABLES -N LSYNFLOOD
$IPTABLES -A LSYNFLOOD -m limit –limit $LOGLIMIT –limit-burst $LOGLIMITBURST -j LOG –log-prefix “fp=SYNFLOOD:1 a=DROP ”
$IPTABLES -A LSYNFLOOD -j DROP

#INVALID SYN packets
$IPTABLES -A INPUT -i eth0 -p tcp –tcp-flags ALL ACK,RST,SYN,FIN -j DROP
$IPTABLES -A INPUT -i eth0 -p tcp –tcp-flags SYN,FIN SYN,FIN -j DROP
$IPTABLES -A INPUT -i eth0 -p tcp –tcp-flags SYN,RST SYN,RST -j DROP

#Logging of possible Ping-Floods
$IPTABLES -N LPINGFLOOD
$IPTABLES -A LPINGFLOOD -m limit –limit $LOGLIMIT –limit-burst $LOGLIMITBURST -j LOG –log-prefix “fp=PINGFLOOD:1 a=DROP ”
$IPTABLES -A LPINGFLOOD -j DROP

Add /usr/local/sbin/iptables.sh to rc.local so that it runs when the machine starts up.

Anytime you make changes to the ipblack.lst, ipwhite.lst, or ports.lst files rerun the iptables.sh script to apply the rules.

The script also applies iptable rules to help protect against ping floods, SYN flood, and invalid SYN packets.