Block IP addresses in Linux with iptables

Most system administrators will already be familiar with iptables. It is around for quite a while and is enabled by default within the Linux kernel. We can use iptables to block one, multiple IP addresses, or even full networks. This may come in handy when you get repeating port scans or see failed login attempts in your log files. Time to get started and block some IP addresses!

Check existing iptables configuration

The first step is to validate existing iptables rules. We will use an empty ruleset for test purposes.

iptables -L

Manually blocking a single IP address

The first option to permanently block an IP address is by creating a rule in the INPUT chain. This way traffic is no longer allowed from that particular IP address.

iptables -I INPUT -s 192.168.1.100 -j DROP

Although this option works great, it might not scale very well. You might even get a very long list of IP addresses to block after a while. Let’s have a look at ipset.

Using blacklists with iptables and ipset

Another option is creating a blacklist. This way we can add multiple systems we no longer want to connect to our systems.

Install ipset utility

Most Linux systems do not have the ipset utility installed by default. So first install that toolkit.

CentOS

yum install ipset

You may need to install the epel-release package first.

Debian and Ubuntu

apt install ipset

Creating a blacklist

With the newly installed ipset utility we create a new list to block IP addresses. We name it blacklist to show clearly its purpose.

Create blacklist with ipset utility (once)

ipset create blacklist hash:ip hashsize 4096

Note: if you want to block based on networks, use hash:net.

After the blacklist is created, we can use the set in iptables. It is related to the -match-set option.

Set up iptables rules. Match with blacklist and drop traffic:

iptables -I INPUT -m set --match-set blacklist src -j DROP
iptables -I FORWARD -m set --match-set blacklist src -j DROP

These commands will add the blacklist (or set) to the INPUT and FORWARD chains. As this is a blacklist, the related policy is to drop traffic. No output will be displayed when entering the commands.

Adding IP addresses to block

Next step is adding actual IP address to the list. Add a specific IP address to your newly created blacklist:

ipset add blacklist 192.168.1.100

Show details

To confirm the blacklist contains the IP address, use the ipset list command.

Test rules and activate rules on reboot

When setting up a blacklist like this, always test it. You want to be sure that the blacklist is enforced in your specific configuration. Also, make sure it still works after a reboot of the system.

To save and restore iptables rules, use the package iptables-persistent. As the name implies, this makes the iptables rules persistent across reboots.

apt install iptables-persistent

To also store ipset rules, create a small systemd service file: /etc/systemd/system/save-ipset-rules.service

# ipset save/restore script (see https://linux-audit.com/blocking-ip-addresses-in-linux-with-iptables/)

[Unit]  
Description=ipset persistent rule service  
Before=netfilter-persistent.service  
ConditionFileNotEmpty=/etc/iptables/ipset

[Service]  
Type=oneshot  
RemainAfterExit=yes  
ExecStart=/sbin/ipset -exist -file /etc/iptables/ipset restore  
ExecStop=/sbin/ipset -file /etc/iptables/ipset save

[Install]  
WantedBy=multi-user.target

This script helps to save and restore the ipset rules. You may need to create the /etc/iptables/ipset file.

/sbin/ipset -file /etc/iptables/ipset save

Combining ipset and IPv6

If you want to use IPv6 addresses, create the related database with the ‘inet6’ family.

ipset create blacklist6 hash:net hashsize 4096 family inet6

Then create the ip6tables rule:

ip6tables -I INPUT -m set --match-set blacklist6 src -j DROP

Happy blocking!

Take the next step!

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