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Block SSH connections by origin of IP address

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Introduction

If you’re exposing services to the internet, you’ll notice a lot of connection attempts. To block those bots and scripts trying to login to your machine, you should use fail2ban.

However, you can also limit the range of allowed origins of the IP addresses. The company MaxMind provides a database of IP addresses and their origin contries. You can configure your machine in such a way that only certain country codes are allowed.

Install geoip client

Start by installing the geoip client and database by using this apt command:

sudo apt-get install geoip-bin geoip-database

This database is updated automatically, when you’ve got your machine configured for auto updates.

ipfilter script

The next step is to save this script to your machine in /usr/local//usr/local/bin/ipfilter.sh:

Edit the script to your needs, e.g. by limiting the number of allowed countries. Now make this script executable:

chmod +x /usr/local/bin/ipfilter.sh

Testing

It is time to test it. Try the command with a known IP in America and one from a local network or known IP from the allowed countries:

> /usr/local/bin/ipfilter.sh
Usage:  ipfilter.sh  <ip>
> /usr/local/bin/ipfilter.sh 8.8.8.8
> echo $?
1
> /usr/local/bin/ipfilter.sh 192.168.1.1
> echo $?
0

Notice the different exit codes of the script. If the IP is from a country that is allowed or if it is from a local network, it will exit with 0, otherwise 1. We can use this script now to configure a filter for sshd in the /etc/hosts.allow and /etc/hosts.deny files.

Add to /etc/hosts.allow:

sshd: ALL: aclexec /usr/local/bin/ipfilter.sh %a

```<figure class="wp-block-pullquote">

> As documented in the `hosts_options(5)` man page, the standard output is redirected to `/dev/null`, so that there's no chance for you to get the output from `echo`. And as you want the exit status to be taken into account, you should use `aclexec` instead of `spawn`. Indeed the man page says for `aclexec`:  "The connection will be allowed or refused depending on whether the command returns a true or false exit status."
> 
> <cite>https://unix.stackexchange.com/a/149057/298669</cite></figure> 

I've previously used  "spawn" instead of  "aclexec" but the IPs weren't blocked. There were still connection attempts in the fail2ban log. By using aclexec, the exit code will be properly used for filtering.

Add to /etc/hosts.deny:

sshd: ALL


Please note the trailing newline. If this is the last entry in the hosts file, you'll need to add a newline. Otherwise the role won't be active.

Do a reboot of your machine and try to connect. You should still be able to connect 😉 Otherwise you'll need to revert this changes locally, since you've successfully blocked yourself from accessing that machine.

You can have a look at the /var/log/auth.log and will see entries like this for example (blocking an IP from China/CN):

Sep 30 12:10:32 raspberrypi root: DENY sshd connection from 222.186.30.76 (CN) Sep 30 12:10:32 raspberrypi sshd[886]: aclexec returned 1 Sep 30 12:10:32 raspberrypi sshd[886]: refused connect from 222.186.30.76 (222.186.30.76)


This should reduce the amount of blocked SSH connections attempts significantly, if configured to a smaller selection of countries.
Last updated on Jan 06, 2022 20:48 CET
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