SSH returns: no matching host key type found. Their offer: ssh-rsa

The version of OpenSSH included in 16.04 disables ssh-dss. There's a neat page with legacy information that includes this issue: http://www.openssh.com/legacy.html

In a nutshell, you should add the option -oHostKeyAlgorithms=+ssh-rsa to the SSH command:

ssh -oHostKeyAlgorithms=+ssh-rsa root@192.168.8.109

You can also add a host pattern in your ~/.ssh/config so you don't have to specify the key algorithm every time:

Host nas
  HostName 192.168.8.109
  HostKeyAlgorithms=+ssh-rsa