I run CentOS on my server, and I often find that my server is being attacked by other computers. Brute force SSH attacks, port scanning, viruses scanning for the ability to spread, things like that. In this article, I’ll show you how to block an IP address on Linux server using IPTables.

The First, I’ll assume you are already using iptables. If you need help setting that up, read this article.

How do I block an IP address ?

Example I want to block incoming request from IP 1.2.3.4, login as root and type the following command

# iptables -I INPUT -s 1.2.3.4 -j DROP

Where,
– I: Inserts the chain at the top of the rules.
– s: Match source IP address.
– j: Jump to the specified target chain when the packet matches the current rule.

To drop packets coming in on interface eth0 from 1.2.3.4, type the following command

# iptables -I INPUT -i eth0 -s 1.2.3.4 -j DROP

How do I block a subnet ?

Use the following syntax to block 10.0.0.0/8

# iptables -I INPUT -s 10.0.0.0/8 -j DROP

How do I save blocked IP address ?

To save blocked IP address to iptables config file, type the following command

# service iptables save

Or

# /etc/init.d/iptables save

How Do I Unblock An IP Address?

First, you need to display blocked IP address along with line number and other information, type the following command

# iptables -L INPUT -n --line-numbers
# iptables -L INPUT -n --line-numbers | grep 1.2.3.4

Sample outputs:

Chain INPUT (policy DROP)
num  target     prot opt source               destination
1    DROP       all  --  1.2.3.4              0.0.0.0/0
2    LOCALINPUT  all  --  0.0.0.0/0            0.0.0.0/0
3    ACCEPT     all  --  0.0.0.0/0            0.0.0.0/0
4    ACCEPT     udp  --  203.162.4.1          0.0.0.0/0           udp spts:1024:65535 dpt:53

To unblock 1.2.3.4 you must delete line number 1, enter:

# iptables -D INPUT 1

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Save Iptables rules

Rules created with the iptables command are stored in memory. If the system is restarted before saving the iptables rule set, all rules are lost. To save netfilter rules, type the following command as root:

# /etc/init.d/iptables save 

If you are using IPv6, enter:

# /etc/init.d/ip6tables save 

The above commands will write the current iptables configuration to /etc/sysconfig/iptables. The next time the system boots, the iptables init script reapplies the rules saved in /etc/sysconfig/iptables. You can also save the iptables rules to a separate file for distribution, backup or other purposes. Type the following command as root

# iptables-save > /root/iptables.rules

If you are using IPv6, enter:

# ip6tables-save  > /root/iptables.rules

Restore Iptables rules

To restore it use the command iptables-restore, type the following command as root:

# iptables-restore < /root/iptables.rules

If you are using IPv6, enter:

# ip6tables-restore < /root/iptables.rules

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