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IP Tables

What is it all about?

Just a short collection of how to use iptables in your Linux environment. The example over here is mainly intended for a typical web server running CentOS 6.

Figure out the current iptables

iptables -L

Save iptables and run them on startup

/etc/init.d/iptables save
chkconfig --level 53 iptables on

And here a script installing all the typical stuff you need with some rate limiting as well. Use the script on your own risk. By the way, this is for a small web server – so adapt it if you are going big ;)

#!/bin/bash
# A sample firewall shell script 
IPT="/sbin/iptables"
SPAMLIST="blockedip"
SPAMDROPMSG="BLOCKED IP DROP"
SYSCTL="/sbin/sysctl"
BLOCKEDIPS="/root/scripts/blocked.ips.txt"

# logging rsyslog 
# /etc/rsyslog.d/iptables.conf with the following contents:
# :msg, startswith, "iptables: " -/var/log/iptables.log
# & ~

# Max connection in seconds range
SECONDS=10
# Max connections per IP
BLOCKCOUNT=20

# Stop certain attacks
echo "Setting sysctl IPv4 settings..."
$SYSCTL net.ipv4.ip_forward=0
$SYSCTL net.ipv4.conf.all.send_redirects=0
$SYSCTL net.ipv4.conf.default.send_redirects=0
$SYSCTL net.ipv4.conf.all.accept_source_route=0
$SYSCTL net.ipv4.conf.all.accept_redirects=0
$SYSCTL net.ipv4.conf.all.secure_redirects=0
$SYSCTL net.ipv4.conf.all.log_martians=1
$SYSCTL net.ipv4.conf.default.accept_source_route=0
$SYSCTL net.ipv4.conf.default.accept_redirects=0
$SYSCTL net.ipv4.conf.default.secure_redirects=0
$SYSCTL net.ipv4.icmp_echo_ignore_broadcasts=1
#$SYSCTL net.ipv4.icmp_ignore_bogus_error_messages=1
$SYSCTL net.ipv4.tcp_syncookies=1
$SYSCTL net.ipv4.conf.all.rp_filter=1
$SYSCTL net.ipv4.conf.default.rp_filter=1
$SYSCTL kernel.exec-shield=1
$SYSCTL kernel.randomize_va_space=1
 
echo "Starting IPv4 Firewall..."
$IPT -F
$IPT -X
$IPT -t nat -F
$IPT -t nat -X
$IPT -t mangle -F
$IPT -t mangle -X
 
# load modules
modprobe ip_conntrack
 
[ -f "$BLOCKEDIPS" ] && BADIPS=$(egrep -v -E "^#|^$" "${BLOCKEDIPS}")
 
# interface connected to the Internet 
PUB_IF="eth0"
 
#Unlimited traffic for loopback
$IPT -A INPUT -i lo -j ACCEPT
$IPT -A OUTPUT -o lo -j ACCEPT
 
# DROP all incomming traffic
$IPT -P INPUT DROP
$IPT -P OUTPUT DROP
$IPT -P FORWARD DROP
 
if [ -f "${BLOCKEDIPS}" ];
then
# create a new iptables list
$IPT -N $SPAMLIST
 
for ipblock in $BADIPS
do
   $IPT -A $SPAMLIST -s $ipblock -j LOG --log-prefix "ipt: $SPAMDROPMSG "
   $IPT -A $SPAMLIST -s $ipblock -j DROP
done
 
$IPT -I INPUT -j $SPAMLIST
$IPT -I OUTPUT -j $SPAMLIST
$IPT -I FORWARD -j $SPAMLIST
fi
 
# Block sync
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW  -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: Drop Sync "
$IPT -A INPUT -i ${PUB_IF} -p tcp ! --syn -m state --state NEW -j DROP
 
# Block Fragments
$IPT -A INPUT -i ${PUB_IF} -f  -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: Fragments Packets "
$IPT -A INPUT -i ${PUB_IF} -f -j DROP
 
# Block bad stuff
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL FIN,URG,PSH -j DROP
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL ALL -j DROP
 
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: NULL Packets "
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL NONE -j DROP # NULL packets
 
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,RST SYN,RST -j DROP
 
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: XMAS Packets "
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags SYN,FIN SYN,FIN -j DROP #XMAS
 
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: Fin Packets Scan "
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags FIN,ACK FIN -j DROP # FIN packet scans
 
$IPT  -A INPUT -i ${PUB_IF} -p tcp --tcp-flags ALL SYN,RST,ACK,FIN,URG -j DROP
 
# Allow full outgoing connection but no incomming stuff
$IPT -A INPUT -i ${PUB_IF} -m state --state ESTABLISHED,RELATED -j ACCEPT
$IPT -A OUTPUT -o ${PUB_IF} -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
 
# Allow ssh
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 22 -j ACCEPT

# default action can be DROP or REJECT
DACTION="DROP"
$IPT -A INPUT -p tcp --dport 80 -i eth0 -m state --state NEW -m recent --set
$IPT -A INPUT -p tcp --dport 80 -i eth0 -m state --state NEW -m recent --update --seconds ${SECONDS} --hitcount ${BLOCKCOUNT} -m limit --limit 5/m --limit-burst 7 -j LOG --log-level 4 --log-prefix "ipt: Rate "
$IPT -A INPUT -p tcp --dport 80 -i eth0 -m state --state NEW -m recent --update --seconds ${SECONDS} --hitcount ${BLOCKCOUNT} -j ${DACTION}
 
# Allow http / https (open port 80 / 443)
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 80 -j ACCEPT
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 443 -j ACCEPT
 
# allow incomming ICMP ping pong stuff
$IPT -A INPUT -i ${PUB_IF} -p icmp --icmp-type 8 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p icmp --icmp-type 0 -m state --state ESTABLISHED,RELATED -j ACCEPT
 
# Allow port 53 tcp/udp (DNS Server)
$IPT -A INPUT -i ${PUB_IF} -p udp --dport 53 -m state --state NEW,ESTABLISHED,RELATED -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p udp --sport 53 -m state --state ESTABLISHED,RELATED -j ACCEPT
 
$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 53 -m state --state NEW,ESTABLISHED,RELATED  -j ACCEPT
#$IPT -A OUTPUT -o ${PUB_IF} -p tcp --sport 53 -m state --state ESTABLISHED,RELATED -j ACCEPT
 
# Open port 110 (pop3) / 143
#$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 110 -j ACCEPT
#$IPT -A INPUT -i ${PUB_IF} -p tcp --destination-port 143 -j ACCEPT

##### Add your rules below ######
#
# 
##### END your rules ############
 
# Do not log smb/windows sharing packets - too much logging
$IPT -A INPUT -p tcp -i ${PUB_IF} --dport 137:139 -j REJECT
$IPT -A INPUT -p udp -i ${PUB_IF} --dport 137:139 -j REJECT
 
# log everything else and drop
$IPT -A INPUT -j LOG --log-prefix "ipt: in "
$IPT -A FORWARD -j LOG --log-prefix "ipt: fwd "
$IPT -A INPUT -j DROP

exit 0

Logging with rsyslog

In this case I am logging with rsyslog and want to have a dedicated log for iptables.
Therefore the file /etc/rsyslog.d/iptables.conf will be created:

# :msg, startswith, "ipt: " -/var/log/iptables.log
# & ~

The first line writes all messages with “ipt:” to the defined log file.
The second line discards the messages that were matched in the previous line and therefore are not used for later matches.

Links

IP Tables Firewall Tutorial !
CentOS IP Tables
Securing SSH (just because I found it)

Other useful stuff

Install the iptables-services package:
yum install iptables-services

Enable the service at boot-time:
systemctl enable iptables

Managing the service
systemctl [stop|start|restart] iptables

Saving your firewall rules can be done as follows:
service iptables save
or
/usr/libexec/iptables/iptables.init save