If the VirtualBox kernel module (vboxdrv) refuses to load, i.e. you get an "Error inserting vboxdrv: Invalid argument", check (as root) the output of the dmesg command to find out why the load failed. Most probably the kernel disagrees with the version of the gcc used to compile the module. Make sure that you use the same compiler as used to build the kernel.

If you have configured a virtual machine to use the host's CD/DVD drive, but this does not appear to work, make sure that the current user has permission to access the corresponding Linux device file (/dev/hdc or /dev/scd0 or /dev/cdrom or similar). On most distributions, the user must be added to a corresponding group (usually called cdrom or cdrw).

On older Linux distributions, if your CD/DVD device has a different name, VirtualBox may be unable to find it. On older Linux hosts, VirtualBox performs the following steps to locate your CD/DVD drives:

In other words, you can try to set VBOX_CDROM to contain a list of your CD/DVD devices, separated by colons, for example as follows:

export VBOX_CDROM='/dev/cdrom0:/dev/cdrom1'

On modern Linux distributions, VirtualBox uses the hardware abstraction layer (hal) to locate CD and DVD hardware.

The previous instructions (for CD and DVD drives) apply accordingly to floppy disks, except that on older distributions VirtualBox tests for /dev/fd* devices by default, and this can be overridden with the VBOX_FLOPPY environment variable.

If the experimental CD/DVD writer support is enabled with an incorrect VirtualBox, host or guest configuration, it is possible that any attempt to access the CD/DVD writer fails and simply results in guest kernel error messages (for Linux guests) or application error messages (for Windows guests). VirtualBox performs the usual consistency checks when a VM is powered up (in particular it aborts with an error message if the device for the CD/DVD writer is not writable by the user starting the VM), but it cannot detect all misconfigurations. The necessary host and guest OS configuration is not specific for VirtualBox, but a few frequent problems are listed here which occurred in connection with VirtualBox.

Special care must be taken to use the correct device. The configured host CD/DVD device file name (in most cases /dev/cdrom) must point to the device that allows writing to the CD/DVD unit. For CD/DVD writer units connected to a SCSI controller or to a IDE controller that interfaces to the Linux SCSI subsystem (common for some SATA controllers), this must refer to the SCSI device node (e.g. /dev/scd0). Even for IDE CD/DVD writer units this must refer to the appropriate SCSI CD-ROM device node (e.g. /dev/scd0) if the ide-scsi kernel module is loaded. This module is required for CD/DVD writer support with all Linux 2.4 kernels and some early 2.6 kernels. Many Linux distributions load this module whenever a CD/DVD writer is detected in the system, even if the kernel would support CD/DVD writers without the module. VirtualBox supports the use of IDE device files (e.g. /dev/hdc), provided the kernel supports this and the ide-scsi module is not loaded.

Similar rules (except that within the guest the CD/DVD writer is always an IDE device) apply to the guest configuration. Since this setup is very common, it is likely that the default configuration of the guest works as expected.

On Linux, VirtualBox makes use of a custom version of Mozilla XPCOM (cross platform component object model) for inter- and intra-process communication (IPC). The process VBoxSVC serves as a communication hub between different VirtualBox processes and maintains the global configuration, i.e. the XML database. When starting a VirtualBox component, the processes VBoxSVC and VirtualBoxXPCOMIPCD are started automatically. They are only accessible from the user account they are running under. VBoxSVC owns the VirtualBox configuration database which normally resides in ~/.VirtualBox. While it is running, the configuration files are locked. Communication between the various VirtualBox components and VBoxSVC is performed through a local domain socket residing in /tmp/.vbox-<username>-ipc. In case there are communication problems (i.e. a VirtualBox application cannot communicate with VBoxSVC), terminate the daemons and remove the local domain socket directory.

If USB is not working on your Linux host, make sure that the current user is a member of the vboxusers group. On older hosts, you need to make sure that the user has permission to access the USB filesystem (usbfs), which VirtualBox relies on to retrieve valid information about your host's USB devices. The rest of this section only applies to those older systems.

As usbfs is a virtual filesystem, a chmod on /proc/bus/usb has no effect. The permissions for usbfs can therefore only be changed by editing the /etc/fstab file.

For example, most Linux distributions have a user group called usb or similar, of which the current user must be a member. To give all users of that group access to usbfs, make sure the following line is present:

# 85 is the USB group
none     /proc/bus/usb     usbfs      devgid=85,devmode=664    0    0

Replace 85 with the group ID that matches your system (search /etc/group for "usb" or similar). Alternatively, if you don't mind the security hole, give all users access to USB by changing "664" to "666".

The various distributions are very creative from which script the usbfs filesystem is mounted. Sometimes the command is hidden in unexpected places. For SuSE 10.0 the mount command is part of the udev configuration file /etc/udev/rules.d/50-udev.rules. As this distribution has no user group called usb, you may e.g. use the vboxusers group which was created by the VirtualBox installer. Since group numbers are allocated dynamically, the following example uses 85 as a placeholder. Modify the line containing (a linebreak has been inserted to improve readability)

DEVPATH="/module/usbcore", ACTION=="add",
    RUN+="/bin/mount -t usbfs usbfs /proc/bus/usb"

and add the necessary options (make sure that everything is in a single line):

DEVPATH="/module/usbcore", ACTION=="add",
    RUN+="/bin/mount -t usbfs usbfs /proc/bus/usb -o devgid=85,devmode=664"

Debian Etch has the mount command in /etc/init.d/mountkernfs.sh. Since that distribution has no group usb, it is also the easiest solution to allow all members of the group vboxusers to access the USB subsystem. Modify the line

domount usbfs usbdevfs /proc/bus/usb -onoexec,nosuid,nodev

so that it contains

domount usbfs usbdevfs /proc/bus/usb -onoexec,nosuid,nodev,devgid=85,devmode=664

As usual, replace the 85 with the actual group number which should get access to USB devices.

Other distributions do similar operations in scripts stored in the /etc/init.d directory.

Linux kernels including the grsec patch (see http://www.grsecurity.net/) and derivates have to disable PAX_MPROTECT for the VBox binaries to be able to start a VM. The reason is that VBox has to create executable code on anonymous memory.

When running a large number of VMs with a lot of RAM on a Linux system (say 20 VMs with 1GB of RAM each), additional VMs might fail to start with a kernel error saying that the vmalloc pool is exhausted and should be extended. The error message also tells you to specify vmalloc=256MB in your kernel parameter list. If adding this parameter to your GRUB or LILO configuration makes the kernel fail to boot (with a weird error message such as "failed to mount the root partition"), then you have probably run into a memory conflict of your kernel and initial RAM disk. This can be solved by adding the following parameter to your GRUB configuration:

uppermem 524288