The Management Console
The UML management console is a low-level interface to the kernel,
somewhat like the i386 SysRq interface. Since there is a full-blown
operating system under UML, there is much greater flexibility possible
than with the SysRq mechanism.
There are a number of things you can do with the mconsole interface:
get the kernel version
add and remove devices
halt or reboot the machine
send SysRq commands
pause and resume the UML
make online backups without shutting down the UML
receive notifications of events of interest from within UML
monitor the internal state of the UML
You need the mconsole client (uml_mconsole) which is present in CVS
(/tools/mconsole) in 2.4.5-9um and later, and will be in the RPM in 2.4.6.
You also need CONFIG_MCONSOLE (under 'General Setup') enabled in UML.
When you boot UML, you'll see a line like:
mconsole initialized on /home/jdike/.uml/umlNJ32yL/mconsole
If you specify a unique machine id one the UML command line, i.e.
you'll see this
mconsole initialized on /home/jdike/.uml/debian/mconsole
That file is the socket that uml_mconsole will use to communicate
with UML. Run it with either the umid or the full path as its argument:
host% uml_mconsole debian
host% uml_mconsole /home/jdike/.uml/debian/mconsole
You'll get a prompt, at which you can run one of these commands:
This takes no arguments. It prints the UML version.
OK Linux usermode 2.4.5-9um #1 Wed Jun 20 22:47:08 EDT 2001 i686
There are a couple actual uses for this. It's a simple no-op which
can be used to check that a UML is running. It's also a way of
sending an interrupt to the UML. This is sometimes useful on SMP
hosts, where there's a bug which causes signals to UML to be lost,
often causing it to appear to hang. Sending such a UML the mconsole
version command is a good way to 'wake it up' before networking has
been enabled, as it does not do anything to the function of the UML.
These take no arguments. They shut the machine down immediately, with
no syncing of disks and no clean shutdown of userspace. So, they are
pretty close to crashing the machine.
"config" adds a new device to the virtual machine or queries the configuration of an existing device.
Currently the ubd and network drivers support pulling devices. It takes one
argument, which is the device to add, with the same syntax as the
kernel command line.
(mconsole) config eth1=mcast
Querying the configuration of a device is handy when you don't know
before the boot what host device the UML device will attach to. This
is a problem with attaching consoles and serial lines to host pty or
pts devices. You have no way of knowing how to access them without
parsing the kernel messages. So, the syntax for this is the same as
above, except you don't specify a configuration
(mconsole) config ssl0
(mconsole) config ubd0
This is supported by the console, serial line, and ubd drivers. As
yet, the network drivers don't support this.
"remove" deletes a device from the system. Its argument is just the
name of the device to be removed. The device must be idle in
whatever sense the driver considers necessary. In the case of the ubd
driver, the removed block device must not be mounted, swapped on, or
otherwise open, and in the case of the network driver, the device must be down.
(mconsole) remove ubd3
(mconsole) remove eth1
This takes one argument, which is a single letter. It calls the
generic kernel's SysRq driver, which does whatever is called for by
that argument. See the SysRq documentation in Documentation/sysrq.txt
in your favorite kernel tree to see what letters are valid and what
"help" returns a string listing the valid commands and what each one does.
This invokes the Ctl-Alt-Del action on init. What exactly this ends
up doing is up to /etc/inittab. Normally, it reboots the machine.
With UML, this is usually not desired, so if a halt would be better,
then find the section of inittab that looks like this
# What to do when CTRL-ALT-DEL is pressed.
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
and change the command to halt.
This puts the UML in a loop reading mconsole requests until a 'go'
mconsole command is recieved. This is very useful for making backups of
UML filesystems, as the UML can be stopped, then synced via 'sysrq s', so
that everything is written to the filesystem. You can then copy the
filesystem and then send the UML 'go' via mconsole.
Note that a UML running with more than one CPU will have problems after
you send the 'stop' command, as only one CPU will be held in a mconsole
loop and all others will continue as normal. This is a bug, and will
This resumes a UML after being paused by a 'stop' command. Note that
when the UML has resumed, TCP connections may have timed out and if the
UML is paused for a long period of time, crond might go a little crazy,
running all the jobs it didn't do earlier.
This takes a string as its argument, and will cause the UML to printk
the string so that it ends up in the kernel message log. This is
intended for use in honeypots by allowing the UML-specific stuff in
the kernel log to be replaced with messages that don't expose the
machine as being a UML.
This takes a filename as its argument. It will return the contents of
the corresponding /proc file inside the UML. Example:
(mconsole) proc uptime
will return the contents of the UML's /proc/uptime.
It is possible to make a backup of a UML's data without shutting it
down. The idea is to pause it, make it flush out its data, copy the
filesystem to a safe place, and then resume it. This should usually
take seconds, while shutting down and rebooting the UML could take
minutes. The exact procedure is this:
(mconsole) sysrq s
host% # Copy the UML's filesystem someplace safe
By causing UML to flush its data out to disk, the 'sysrq s' will cause
the filesystem to be a clean image. Of course, no guarantees are made
for process data which hadn't been written back to the kernel, but the
filesystem itself won't need an fsck if it's booted.
The mconsole interface also provides a mechanism for processes inside
a UML to send messages to an mconsole client on the host. The
procedure is this:
A common use for this mechanism is to have an rc script inside UML
send a message out that the UML has booted to a certain stage, and
that something on the host which depends on that can proceed.
However, this is a completely general mechanism which can be used to
communicate any information at all to the host.
Create a unix socket and pass that to UML on the command line as the
mconsole notification socket
A /proc/mconsole file will be created inside UML
Anything that is written to it will be turned into an mconsole
notification which your mconsole client should be listening for on the
There is a demo mconsole notification client in the utilities tarball
in mconsole/notify.pl. This is only a demo, and as such, isn't very
useful by itself. It should be customized to fit into whatever
environment you are setting up.