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Kernel command line switches

This is a list of the UML-specific command line arguments, plus a few generic ones which deserve mention here.
--version
This causes UML to print its version and exit.
--help
This causes UML to print a usage message and exit.
con
con=channel attaches one or more UML consoles to the named channel. The format of the channel is described here.
debug
Starts up the kernel under the control of gdb. See the kernel debugging tutorial and the debugging session pages for more information. Another form of this switch is debug=go which is the same as debug except that the kernel runs instead of stopping at the beginning of start_kernel.
debugtrace
Causes the tracing thread to pause until it is attached by a debugger and continued. This is mostly for debugging crashes early during boot, and should be pretty much obsoleted by the debug switch.
dsp
dsp=host dsp tells the UML sound driver what the filename of the host dsp is so that it can relay to it. The default is "/dev/sound/dsp".
eth
ethn=host interface enables a virtual ethernet device inside UML. See the networking HOWTO for more information on setting up UML networking.
fakehd
Causes the ubd device to put its partition information in /proc/partitions under the device name "hd" rather than "ubd". Again, this is to fake out installation procedures which are overly picky in their sanity-checking.
fake_ide
fake_ide causes the ubd driver to install realistic-looking entries into /proc/ide. This is useful for convincing some distribution installation procedures to run inside UML.
gdb
gdb=channel , when used with debug, attaches gdb to the named channel. The format of the channel is described here.
gdb-pid
gdb-pid=pid , when used with debug, specifies the pid of an already running debugger that UML should attach to. This can be used to debug UML with a gdb wrapper such as emacs or ddd, as well as with debuggers other than gdb. See the debugging page for more information.
honeypot
honeypot causes UML to rearrange its address space in order to put process stacks in the same location as on the host. This allows stack smash exploits to work against UML just as they do against the host. This option enables jail, since it is most unlikely that a honeypot UML should run without it enabled.
initrd
initrd=image sets the filename of the initrd image that UML will boot from.
iomem
iomem=name,file makes file available to be mapped by a driver inside UML. See this page for more information.
jail
jail enables protection of UML kernel memory from UML processes. This is disabled by default for performance reasons. Without it, it is fairly simple to break out of UML by changing the right pieces of UML kernel data.
mconsole
mconsole=notify:socket asks the mconsole driver to send the name of its socket to the Unix socket named by this switch. This is intended for the use of scripts which want to know when they can start using the mconsole and what socket they should send commands to.
mem
mem=size controls how much "physical" memory the kernel allocates for the system. The size is specified as a number followed by one of 'k', 'K", 'm', 'M", which have the obvious meanings. This is not related to the amount of memory in the physical machine. It can be more, and the excess, if it's ever used, will just be swapped out.
mixer
mixer=host mixer tells the UML sound driver what the filename of the host mixer is so that it can relay to it. The default is "/dev/sound/mixer".
ncpus
ncpus=number tells an SMP kernel how many virtual processors to start. Currently, this has no effect because SMP isn't enabled.
ssl
ssl=channel attaches one or more UML serial lines to the named channel. The format of the channel is described here.
root
root=root device is actually used by the generic kernel in exactly the same way as in any other kernel. If you configure a number of block devices and want to boot off something other than ubd0, you would use something like:
root=/dev/ubd5
ubd
ubd=number causes the ubd device to take over a different major number than the one assigned to it. This is useful for making it appear to be an "hd" device.
ubd
ubdn=filename is used to associate a device with a file in the underlying filesystem. Usually, there is a filesystem in the file, but that's not required. Swap devices containing swap files can be specified like this. Also, a file which doesn't contain a filesystem can have its contents read in the virtual machine by running dd on the device. n must be in the range 0 to 7. Appending an 'r' to the number will cause that device to be mounted read-only.
ubd
ubdn=cow-file,backing-file is used to layer a COW file on another, possibly readonly, file. This is useful in a number of ways. See this page for all the details.
umid
umid=name is used to assign a name to a virtual machine. This is intended to make it easy for UIs to manage multiple UMLs. Currently, the only effect of this is that UML writes its tracing thread pid in /tmp/uml/name.
uml_dir
uml_dir=directory sets the directory in which UML will put the umid directory, which in turn will contain the pid file and mconsole socket.
umn
umn=ip-address sets the ip address of the host side of the slip device that the umn device configures. This is necessary if you want to set up networking, but your local net isn't 192.168.0.x, or you want to run multiple virtual machines on a network, in which case, you need to assign different ip addresses to the different machines. See the networking tutorial for more information.
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