Using User-mode-linux with pbuilder¶

It is possible to use user-mode-linux by invoking pbuilder-user-mode-linux instead of pbuilder. pbuilder-user-mode-linux doesn’t require root privileges, and it uses the copy-on-write (COW) disk access method of User-mode-linux which typically makes it much faster than the traditional pbuilder.

User-mode-linux is a somewhat less proven platform than the standard Unix tools which pbuilder relies on (chroot, tar, and gzip) but mature enough to support pbuilder-user-mode-linux since its version 0.59. And since then, pbuilder-user-mode-linux has seen a rapid evolution.

The configuration of pbuilder-user-mode-linux goes in three steps:

Configuration of user-mode-linux
Configuration of rootstrap
Configuration of pbuilder-uml

Configuring user-mode-linux¶

user-mode-linux isn’t completely trivial to set up. It would probably be useful to acquaint yourself with it a bit before attempting to use rootstrap or pbuilder-user-mode-linux. For details, read /usr/share/doc/uml-utilities/README.Debian and the user-mode-linux documentation. (It’s in a separate package, user-mode-linux-doc.)

user-mode-linux requires the user to be in the uml-net group in order to configure the network unless you are using slirp.

If you compile your own kernel, you may want to verify that you enable TUN/TAP support, and you might want to consider the SKAS patch.

Configuring rootstrap¶

rootstrap is a wrapper around debootstrap. It creates a Debian disk image for use with UML. To configure rootstrap, there are several requirements.

Install the rootstrap package.
TUN/TAP only: add the user to the uml-net group to allow access to the network
```
adduser dancer uml-net
```
TUN/TAP only: Check that the kernel supports the TUN/TAP interface, or recompile the kernel if necessary.
Set up /etc/rootstrap/rootstrap.conf. For example, if the current host is 192.168.1.2, changing following entries to something like this seems to work.
```
transport=tuntap
interface=eth0
gateway=192.168.1.1
mirror=http://192.168.1.2:8081/debian
host=192.168.1.198
uml=192.168.1.199
netmask=255.255.255.0
```
Some experimentation with configuration and running rootstrap ~/test.uml to actually test it would be handy.

Using slirp requires less configuration. The default configuration comes with a working example.

Configuring pbuilder-uml¶

The following needs to happen:

Install the pbuilder-uml package.

Set up the configuration file /etc/pbuilder/pbuilder-uml.conf in the following manner. It will be different for slirp.

MY_ETH0=tuntap,,,192.168.1.198
UML_IP=192.168.1.199
UML_NETMASK=255.255.255.0
UML_NETWORK=192.168.1.0
UML_BROADCAST=255.255.255.255
UML_GATEWAY=192.168.1.1
PBUILDER_UML_IMAGE="/home/dancer/uml-image"

Also, it needs to match the rootstrap configuration.

Make sure BUILDPLACE is writable by the user. Change BUILDPLACE in the configuration file to a place where the user has access.
Run pbuilder-user-mode-linux to create the image.
Try running pbuilder-user-mode-linux build.

Considerations for running pbuilder-user-mode-linux¶

pbuilder-user-mode-linux emulates most of pbuilder, but there are some differences.

pbuilder-user-mode-linux does not support all options of pbuilder properly yet. This is a problem, and will be addressed as specific areas are discovered.
/tmp is handled differently inside pbuilder-user-mode-linux. In pbuilder-user-mode-linux, /tmp is mounted as tmpfs inside UML, so accessing files under /tmp from outside user-mode-linux does not work. It affects options like --configfile, and when trying to build packages placed under /tmp.

Parallel running of pbuilder-user-mode-linux¶

To run pbuilder-user-mode-linux in parallel on a system, there are a few things to bear in mind.

The create and update methods must not be run when a build is in progress, or the COW file will be invalidated.
If you are not using slirp, user-mode-linux processes which are running in parallel need to have different IP addresses. Just trying to run the pbuilder-user-mode-linux several times will result in failure to access the network. But something like the following will work:
```
for IP in 102 103 104 105; do
  xterm -e pbuilder-user-mode-linux build --uml-ip 192.168.0.$IP \
    20030107/whizzytex_1.1.1-1.dsc &
done
```
When using slirp, this problem does not exist.

Using pbuilder-user-mode-linux as a wrapper script to start up a virtual machine¶

It is possible to use pbuilder-user-mode-linux for other uses than just building Debian packages. pbuilder-user-mode-linux will let a user use a shell inside the user-mode-linux pbuilder base image, and pbuilder-user-mode-linux will allow the user to execute a script inside the image.

You can use the script to install ssh and add a new user, so that it is possible to access inside the user-mode-linux through ssh.

Note that it is not possible to use a script from /tmp due to the way pbuilder-user-mode-linux mounts a tmpfs at /tmp.

The following example script may be useful in starting a sshd inside user-mode-linux.

#!/bin/bash

apt-get install -y ssh xbase-clients xterm
echo "enter root password"
passwd
cp /etc/ssh/sshd_config{,-}
sed 's/X11Forwarding.*/X11Forwarding yes/' /etc/ssh/sshd_config- > /etc/ssh/sshd_config

/etc/init.d/ssh restart
ifconfig
echo "Hit enter to finish"
read