Just did a clean install of Ubuntu 20.04.4 LTS and fully patched it before installing xymon. For my specs (overkill) I gave it 8 cores, 16GB, and 90GB drive.

SSH into your server and run the following to get the latest package and start the install
sudo apt-get update
Sudo apt-get install apache2 rrdtool librrd-dev libpcre3-dev libssl-dev ldap-utils libldap2-dev fping
sudo apt-get -y install xymon
You’ll be prompt with a pink configuration screen asking you to provide a static IP. (highly recommended)

Enter the IP you wish to use and press ENTER.
That’s it, xymon server is installed but we still need to grant user access via Apache. To verify xymon is installed load the server ip in your browser http://<YOURSERVERIP> and you should be greeted with Apache2 Ubuntu Default Page.

Now let’s see what happens if we try to load xymon. Try http://<YOURSERVERIP>/xymon. You’ll see a forbidden page. Which is good, means it working we just need to grant access to it.

Go back to your ssh session to your server and let’s edit xymon.conf in Apache2.
sudo nano /etc/apache2/conf-enabled/xymon.conf
Since my server is protected internally already I’m just going to be granting everyone access. Not the best practice/recommended way but you can follow xymons install guide to secure it. https://xymon.sourceforge.io/xymon/help/install.html (Configuring your webserver section)
https://xymon.sourceforge.io/xymon/help/install.html
Change the 3 sections circled in red below to
Allow From All
Satisfy Any

After

Ctrl + C to exit and save it.
Restart Apache to load our changes
sudo systemctl restart apache2.service
Now let’s see what happens when we load http://<YOURSERVERIP>/xymon

It’s works. That’s it!