Virtualizing without virtualizing

Kapil Hari Paranjape

1  Introduction

Nowadays one reads a lot about virtualization. The Gazette even ran an article on it. Most of these reports talk about Xen, Vserver, OpenVZ, Qemu, User Mode Linux or co-Linux. Each of those modern technologies has its place no doubt, but this article will concentrate on the humble chroot way of running programs in a virtual environment.

Here is the EULA1 that you must agree to if you want to read further:

Here some possible scenarios where one might want to run programs in a virtual context in spite of these (admittedly rather stringent) conditions:

2  Ask the super

Your super-user avatar (SU) must carry out the following steps for you to enter the virtual context.

2.1  Allocate disk space

Either there is already enough disk space on some mounted partition or you have an unused partition. You will need this empty space to appear in some fixed location like /srv/schroot. SU can use mount (with the --bind option in the former case) to set this up.

2.2  Create a new Debian installation

Do not reach for that Debian boot CD! Here is a way that does not destroy your uptime record. SU should install debootstrap

 apt-get install debootstrap

and run it

 debootstrap --include=iceweasel,mozplugger lenny /srv/schroot

SU can of course replace iceweasel and mozplugger by any comma-separated list of programs which need to be installed. It is nice to add a local mirror at the tail end of the debootstrap invocation so as to get a faster download.

2.3  The main program

Since you run Debian installing schroot is as easy as SU running:

 apt-get install schroot

You also need to configure schroot. There are a number of rather interesting options. The following configuration stanza seemed to be just right:

 description=Debian lenny (testing)

If you are planning to use a 32-bit chroot under a 64-bit system then you need to have personality=linux32 as part of this configuration. The above stanza is placed in the file schroot.conf in the configuration directory /etc/schroot/. It says that SU allows the user luser to use schroot to run under the directory /src/schroot after various standard setup and startup scripts have been executed.

You should also look through the setup script 10mount in the setup.d sub-directory of the above directory as you may need to create some additional mounts. For example, adding the line

 do_mount "-o rw,bind" "/dev/snd"    "${CHROOT_PATH}/dev/snd"

to 10mount together with the creation of the dev/snd subdirectory of /srv/schroot ensures that the (ALSA) sound devices are accessible in the chroot; which is extremely important for flash!

That completes the setup that the super-user needs to do. After this luser can go ahead and play in this newly created “sand-box”.

3  All play and no work

The authorized user luser can execute a shell by running

 schroot -p -c lenny

The shell will normally run in an environment where the users home directories, /tmp and /dev will be mounted from the base system. Hence it should be possible to execute commands with need the X window environment as well. (The -p option given above is required to preserve the environment that includes the DISPLAY variable).

Another way to run a command like iceweasel directly is

 schroot -p -c lenny iceweasel

Note that each such command creates a new schroot “session”. To re-use an already created session, you must save the session identifier and use it. For example, you can start a new session, without any command

 SCHROOT_SESSION=$(schroot -b -c lenny)

If you then issue the command

 schroot -p --chroot $(SCHROOT_SESSION) iceweasel

to get the weasel up and running in that session. If you run this command another time, you will not create a new session.

4  Philosophy

The chroot command has existed for “eons”, but it was often felt that it is “for the super-user only”. By using schroot it becomes quite safely accessible to the regular user of the system. Using this kind of minimal virtualisation is certainly not in the same league security-wise as the “real” virtualisation techniques but has no overheads (except disk-usage); I hope the article demonstrates that schroot is at least as easy to set up.

Using schroot is a good solution to the frequently asked question:

How do I run the late-esht version of <name your favourite rapidly developing application> on Debian?

In my opinion, the above solution is to be preferred over running a mixed stable/testing version of Debian. Even backports are slightly worse, as a mixed stable/backports environment is not what the packages are being tested in by most developers.

The motivation to write this article came from a discussion I saw in the letters to Linux Weekly News where people said that Debian was hindering those who want to run 32-bit programs on a 64-bit system. If indeed mixed library setups are well packaged and maintained, then that is easier than the solution herein. All the same, Debian does have this solution!

The title of this article is inspired by John Archibald Wheeler, one of the most fascinating physicists of the 20th century, who passed away recently.

This is a dyslexic acronym for “Lookout — Advanced Experimental Usage”
Mark Shuttleworth: Benevolent Dictator For Life for Ubuntu
or boys as the case may be

This document was translated from LATEX by HEVEA.