SSH banner warnings are essential when organizations or companies want to present a strict warning to deter unauthorized individuals from accessing a server.

These warnings appear just before the password prompt, ensuring that unauthorized users attempting to log in are made aware of the potential consequences. These consequences often include legal actions that unauthorized users may face if they proceed with access.

It should be noted that a banner warning does not act as a barrier to prevent unauthorized access. The warning banner serves only as a deterrent. To actively block unauthorized access, additional SSH configurations must be implemented.

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With that in mind, let's proceed with setting up a custom SSH warning banner.

Step 1: Configure SSH Warning Banner

Begin by opening the /etc/ssh/sshd_config configuration file using your preferred text editor. In this example, we will use vim.

<code>$ sudo vim /etc/ssh/sshd_config</code>

Find the Banner none line. Here, you need to define the path to the file that will contain your custom SSH warning.

Remove the comment symbol (if necessary) and provide the path to your custom banner file. For demonstration purposes, we'll use /etc/mybanner.

<code>Banner /etc/mybanner</code>

Save the changes and close the file.

Step 2: Create SSH Warning Banner

Next, create the file where you’ll define your custom banner message — in this case, /etc/mybanner.

<code>$ sudo vim /etc/mybanner</code>

Paste the following banner content, or modify it according to your preference.

<code>------------------------------------------------------------
------------------------------------------------------------
<p>Authorized access only!</p>
<p>If you are not authorized to access or use this system, disconnect now!</p>
<hr>
<p>------------------------------------------------------------</p></code>

Save and exit the file.

To activate the new configuration, restart the SSH service:

<code>$ sudo systemctl restart sshd</code>

Step 3: Testing SSH Warning Banner

To verify the banner, attempt to connect to the remote server. As shown below, the warning appears just before the password prompt, serving as a reminder to unauthorized users.

<code>$ ssh user@remote-server-ip</code>

Step 4: Setting a MOTD Banner

For displaying a MOTD (Message Of The Day) banner after login, edit the /etc/motd file.

<code>$ sudo vim /etc/motd</code>

Enter your desired MOTD message. We have included some ASCII art for visual appeal.

<code> <strong>_<strong>                   <em>       </em><br>
|<em>   </em>|                 (<em>)     | |<br>
| | </em></strong>  _</strong> <em> <strong> </strong></em>  <em> </em> <strong> | |<em> 
| |/ </em> \/ </strong>| '<em> ` </em> | | '_ | <strong>|
| |  </strong>/ (<strong>| | | | | | | | | | |<em> 
_/\</em></strong>|_<strong>|<em>| |</em>| |<em>|</em>|<em>| |</em>|\</strong>|</code>

Once done, save the file and restart the SSH service again.

<code>$ sudo systemctl restart sshd</code>

The MOTD will now display immediately after logging in, as illustrated below.

That’s all. You should now be able to configure a custom SSH warning banner on your server to alert unauthorized users against accessing the system.

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