# The DragonFly virtual kernels 
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***Obtained from [vkernel(7)](http://leaf.dragonflybsd.org/cgi/web-man?command=vkernel&section=7) written by Sascha Wildner, added by Matthias Schmidt***

The idea behind the development of the vkernel architecture was to find an elegant solution to debugging of the kernel and its components. It eases debugging, as it allows for a virtual kernel being loaded in userland and hence debug it without affecting the real kernel itself. By being able to load it on a running system it also removes the need for reboots between kernel compiles.

The vkernel architecture allows for running DragonFly kernels in userland.

## Supported devices 

A number of virtual device drivers exist to supplement the virtual kernel.

<!-- XXX: why do they only support 16 devices? is this really true? -->

### Disk device 

The vkd driver allows for up to 16 [vn(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=vn&section=4) based disk devices.  The root device will be `vkd0`.

### CD-ROM device 

The vcd driver allows for up to 16 virtual CD-ROM devices.  Basically this is a read only `vkd` device with a block size of 2048.

### Network interface 

The vke driver supports up to 16 virtual network interfaces which are

associated with [tap(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=tap&section=4) devices on the host.  For each `vke` device, the per-interface read only [sysctl(3)](http://leaf.dragonflybsd.org/cgi/web-man?command=sysctl&section=3) variable `hw.vkeX.tap_unit` holds the unit number of the associated [tap(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=tap&section=4) device.

## Setup a virtual kernel environment 

A couple of steps are necessary in order to prepare the system to build and run a virtual kernel.

### Setting up the filesystem 

The vkernel architecture needs a number of files which reside in `/var/vkernel`.  Since these files tend to get rather big and the `/var` partition is usually of limited size, we recommend the directory to be created in the `/home` partition with a link to it in `/var`:

    

    % mkdir /home/var.vkernel
    % ln -s /home/var.vkernel /var/vkernel

Next, a filesystem image to be used by the virtual kernel has to be created and populated (assuming world has been built previously):    

    # dd if=/dev/zero of=/var/vkernel/rootimg.01 bs=1m count=2048
    # vnconfig -c vn0 /var/vkernel/rootimg.01
    # disklabel -r -w vn0s0 auto
    # disklabel -e vn0s0      # add 'a' partition with fstype `4.2BSD' size could be '*'
    # newfs /dev/vn0s0a
    # mount /dev/vn0s0a /mnt

If instead of using `vn0` you specify `vn` to `vnconfig`, a new `vn` device will be created and a message saying which `vnX` was created will appear. This effectively lifts the limit of 4 vn devices.

Assuming that you build your world before, you can populate the image now.  If you didn't build your world see [chapter 21](../updating-makeworld.html).

    # cd /usr/src
    # make installworld DESTDIR=/mnt
    # cd etc
    # make distribution DESTDIR=/mnt

Create a fstab file to let the vkernel find your image file.

    

    # echo '/dev/vkd0s0a      /       ufs     rw      1  1' >/mnt/etc/fstab
    # echo 'proc              /proc   procfs  rw      0  0' >>/mnt/etc/fstab

Edit `/mnt/etc/ttys` and replace the console entry with the following line and turn off all other gettys.

    # console "/usr/libexec/getty Pc"         cons25  on  secure

Then, unmount the disk.

    # umount /mnt
    # vnconfig -u vn0

### Compiling the virtual kernel 

In order to compile a virtual kernel use the VKERNEL kernel configuration file residing in `/usr/src/sys/config` (or a configuration file derived thereof):
    

    # cd /usr/src
    # make -DNO_MODULES buildkernel KERNCONF=VKERNEL
    # make -DNO_MODULES installkernel KERNCONF=VKERNEL DESTDIR=/var/vkernel

### Enabling virtual kernel operation 

A special sysctl(8), `vm.vkernel_enable`, must be set to enable vkernel operation:

    # sysctl vm.vkernel_enable=1

To make this change permanent, edit `/etc/sysctl.conf`

## Setup networking 

### Configuring the network on the host system 

In order to access a network interface of the host system from the vkernel, you must add the interface to a [bridge(4)](http://leaf.dragonflybsd.org/cgi/web-man?command=bridge&section=4) device which will then be passed to the `-I` option:

    

    # kldload if_bridge.ko
    # kldload if_tap.ko
    # ifconfig bridge0 create
    # ifconfig bridge0 addm re0       # assuming re0 is the host's interface
    # ifconfig bridge0 up

 **Note** : You have to change `re0` to the interface of your host machine.

 
 
 
## Run a virtual kernel 

Finally, the virtual kernel can be run:

    # cd /var/vkernel
    # ./boot/kernel/kernel -m 64m -r /var/vkernel/rootimg.01 -I auto:bridge0

You can issue the reboot(8), halt(8), or shutdown(8) commands from inside a virtual kernel.  After doing a clean shutdown the reboot(8) command will re-exec the virtual kernel binary while the other two will cause the virtual kernel to exit.