ssh: Could not resolve hostname server: Name or service not known

To connect to an ssh server in a terminal you need:

1. The call ssh to start the program
2. The user name, which in your case is root
3. An @ sign separating the user name from the server identification
4. The IP address or name of the server, which in your case is 10.0.2.15

Assembled, the command looks like:

ssh [email protected]

in general terms, ssh user@server.

Alternatively, you can use the -l option to directly specify the login name and skip the @ syntax:

ssh 10.0.2.15 -l root

As WooJoo stated, you need to tell it a valid server to connect to. If you wanted to use the form $ ssh root@server you can, but you would need to have server as an entry in /etc/hosts or your dns server (which is not the case or you would not have had an error), or an entry in a file called config located typically at /home/username/.ssh/config.

A sample /etc/hosts entry would look like:

# Sample /etc/hosts file
127.0.0.1    localhost
127.0.1.1    computerhostnamehere

10.0.2.15    server

and a sample /home/username/.ssh/config could be as simple as:

Host server
  HostName 10.0.2.15
  User root

This would get you the basic functionality you are looking for. There are many more options available for placing in the ~/.ssh/config file.

See man ssh for more options :)