Iptables useful deny rules

Block Incoming Request From IP 1.2.3.4

The following command will drop any packet coming from the IP address 1.2.3.4:
/sbin/iptables -I INPUT -s {IP-HERE} -j DROP
/sbin/iptables -I INPUT -s 1.2.3.4 -j DROP

You can also specify an interface such as eth1 via which a packet was received:
/sbin/iptables -I INPUT -i {INTERFACE-NAME-HERE} -s {IP-HERE} -j DROP
/sbin/iptables -I INPUT -i eth1 -s 1.2.3.4 -j DROP

Please note that when the “!” argument is used before the interface name, the sense is inverted:
/sbin/iptables -I INPUT ! -i {INTERFACE-NAME-HERE} -s {IP-HERE} -j DROP
/sbin/iptables -I INPUT ! -i eth1 -s 1.2.3.4 -j DROP

If the interface name ends in a “+”, then any interface which begins with this name will match. If this option is omitted, any interface name will match:
/sbin/iptables -I INPUT -i {INTERFACE-NAME-HERE}+ -s {IP-HERE} -j DROP
/sbin/iptables -I INPUT -i br+ -s 1.2.3.4 -j DROP

You can replace -I INPUT (insert) with -A INPUT (append) rule as follows:
/sbin/iptables -A INPUT -s 1.2.3.4 -j DROP
/sbin/iptables -i eth1 -A INPUT -s 1.2.3.4 -j DROP

How Do I Block Subnet (xx.yy.zz.ww/ss)?
Use the following syntax to block 10.0.0.0/8 on eth1 public interface:
# /sbin/iptables -i eth1 -A INPUT -s 10.0.0.0/8 -j DROP

How Do I Block and Log Dropped IP Address Information?
You can turn on kernel logging of matching packets with LOG target as follows:
# /sbin/iptables -i eth1 -A INPUT -s 10.0.0.0/8 -j LOG –log-prefix “IP DROP SPOOF A:”

The next rule will actually drop the ip / subnet:
# /sbin/iptables -i eth1 -A INPUT -s 10.0.0.0/8 -j DROP

How Do I View Blocked IP Address?
Simply use the following command:
# /sbin/iptables -L -v

OR
# /sbin/iptables -L INPUT -v

OR
# /sbin/iptables -L INPUT -v -n

Sample outputs:

Chain INPUT (policy ACCEPT 3107K packets, 1847M bytes)
pkts bytes target prot opt in out source destination
0 0 DROP all — br+ any 1.2.3.4 anywhere
0 0 DROP all — !eth1 any 1.2.3.4 anywhere
0 0 DROP all — !eth1 any 1.2.3.4 anywhere
How Do I Search For Blocked IP Address?
Use the grep command as follows:
# /sbin/iptables -L INPUT -v -n | grep 1.2.3.4

How Do I Delete Blocked IP Address?
First, you need to display blocked IP address along with line number and other information, enter:
# iptables -L INPUT -n –line-numbers
# iptables -L INPUT -n –line-numbers | grep 1.2.3.4

Sample outputs:

num pkts bytes target prot opt in out source destination
1 0 0 DROP 0 — * * 116.199.128.1 0.0.0.0/0
2 0 0 DROP 0 — * * 116.199.128.10 0.0.0.0/0
3 0 0 DROP 0 — * * 123.199.2.255 0.0.0.0/0
To delete line number 3 (123.199.2.255), enter:
# iptables -D INPUT 3

Verify the same, enter:
# iptables -L INPUT -v -n

You can also use the following syntax:
# iptables -D INPUT -s 1.2.3.4 -j DROP

How Do I Save Blocked IP Address?

If you are using Redhat / RHEL / CentOS / Fedora Linux, type the following command:
# iptables -D INPUT -s 1.2.3.4 -j DROP
##########################
#////// command to save iptables ///////#
##########################
# /sbin/service iptables save
# less /etc/sysconfig/iptables
# grep ‘1.2.3.4’ /etc/sysconfig/iptables

For all other Linux distributions use the iptables-save command to dump the contents of an IP Table to a file:
# iptables-save > /root/myfirewall.conf

Please not that you need to run the ‘iptables-save’ or ‘service iptables save’ as soon as you add or delete the ip address.

A Note About Restoring Firewall
To restore your firewall use the iptables-restore command to restore IP Tables from a file called /root/myfirewall.conf, enter:
# iptables-restore < /root/myfirewall.conf

How Do I Block Large Number Of IP Address or Subnets?

You need to write a shell script as follows:

#!/bin/bash
_input=”/root/blocked.ip.db”
IPT=/sbin/iptables
$IPT -N droplist
egrep -v “^#|^$” x | while IFS= read -r ip
do
$IPT -A droplist -i eth1 -s $ip -j LOG –log-prefix “IP BlockList ”
$IPT -A droplist -i eth1 -s $ip -j DROP
done < “$_input”
# Drop it
$IPT -I INPUT -j droplist
$IPT -I OUTPUT -j droplist
$IPT -I FORWARD -j droplist