After cloning a working Debian virtual machine, the new system would not bring up its Ethernet interface. This is because I checked VirtualBox's dialog, "Reset MAC address for network card" and got a new hardware Ethernet address.

Trying "ifup eth0" didn't work. I did an "ip addr" and saw that the only configured interface was now called eth1.

This is simple to solve. Edit the file /etc/udev/rules.d/70-persistent-net.rules and move the new MAC address (like "00-0F-35-01..." and so on) from the eth1 line back to eth0, erasing the previous address in the eth0 line. Then delete the remainder of the eth1 line, save, and reboot.

You should now have your eth0 back.

Full instructions, particularly for "headless" installation, are here.

First we download the RPM for RedHat Enterprise Linux 5 (RHEL 5) which is the equivalent to Centos from the virtualbox.org site

After downloading (and renaming the file to eliminate everything after the .rpm) we install as follows but get an error:

# rpm -ivh VirtualBox-2.1.0_(version).rpm
...
No precompiled module for this kernel found! ...
Compilation of kernel module FAILED! ...
Please consult
   /var/log/vbox-install.log
to find out why the kernel module does not compile.
Most probably the kernel sources are not found.
Install them and execute
   <strong>/etc/init.d/vboxdrv setup</strong>
as root.

To rectify this:

# yum install SDL kernel-devel kernel-headers
# KERN_DIR=/usr/src/kernels/2.6.18-whatever-your-current-kernel
# export KERN_DIR
# /etc/init.d/vboxdrv setup

Then add users to the vboxusers group:

# /usr/sbin/usermod -a -G vboxusers yourusername

and you should be able to launch VirtualBox from the Applications / System Tools menu.

Have fun!