Block IP addresses in Linux with iptables

Blocking IP addresses and subnets with ipset

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

Output of iptables rules

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-get 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

Iptables with blacklists to block some IP addresses

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.

Create iptables blacklist for blocking IP addresses

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.

Output of ipset command showing blacklist

In this screenshot, we can see the IP address is listed as a member of the set. Now traffic should be blocked.

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

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

Happy blocking!

One more thing...

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3 comments

  • TechTech

    Thank you that worked out fantastic.

    Reply
  • Stanislav PanayotovStanislav Panayotov

    I’m using this heavy duty bash script as root for some like 15 minutes:

    curl -s https://www.iblocklist.com/lists.php
    | grep -A 2 Bluetack
    | sed -n “s/.*value='(http:.*)’.*/1/p”
    | xargs wget -O –
    | gunzip
    | egrep -v ‘^#’ > blacklist
    grep -Eo ‘[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}’ blacklist > blacklist-ip
    grep -Eo ‘[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}[-][0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}’ blacklist > blacklist-ip-range
    sed -i -E “s/[-][0-9]{1,3}.[0-9]{1,3}.[0-9]{1,3}.///” blacklist-ip-range

    sort blacklist-ip | uniq -u > blacklist-ip-sorted
    sort blacklist-ip-range | uniq -u > blacklist-ip-range-sorted
    while read IPADDR
    do
    route add $IPADDR gw 127.0.0.1 lo &
    done < blacklist-ip-range-sorted

    while read IPADDR
    do
    route add $IPADDR gw 127.0.0.1 lo &
    done < blacklist-ip-sorted

    rm blacklist-ip
    rm blacklist-ip-sorted
    rm blacklist-ip-range
    rm blacklist-ip-range-sorted

    Reply

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