Starting up Ubuntu Server inside VirtualBox

If you are testing an Ubuntu Server installation with VirtualBox you'll get some trouble to boot it up, here you can find how to solve it

After you installed Ubuntu Server from the CD inside a VirtualBox you will get the following error:

PANIC: CPU too old for this kernel.

This is because Ubuntu Server has been compiled with PAE support, but this feature is missing in VirtualBox.

Solution is to change the "linux-server" kernel for a simpler one like "linux-i386".

1. Boot from Ubuntu Server CD and select "Rescue a broken system"

2. When it's asking about the device to use as root select the partition where the / filesystem is, in my case is /dev/sda1.

3. Now select "Execute a shell in /dev/sda1" and you get the linux prompt.

4. (Optional) If you have a system with multiple partitions you need to mount them with commands like from an alternative console, use ALT-F2 keys:

# mount /dev/sda2 /target/boot
# mount /dev/sda4 /target/var

This is especially important if you use a separate partition for /boot.

5. Now change the kernel with the following commands:

# apt-get update
# apt-get install linux-386
# apt-get remove linux-server

6. Now type exit and reboot the system.