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+# The DragonFly virtual kernels 
+
+
+
+***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 -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.
+
+
+
+