User-Mode Linux

Debian GNU/Linux testing/unstable (none) tty0

(none) login: root  <ENTER>
Linux (none) 2.6.14.3-bs3 #7 Fri Dec 16 17:47:00 CET 2005 i686 GNU/Linux

The programs included with the Debian GNU/Linux system are free software;
the exact distribution terms for each program are described in the
individual files in /usr/share/doc/*/copyright.

Debian GNU/Linux comes with ABSOLUTELY NO WARRANTY, to the extent
permitted by applicable law.

(none):~# ps -ef <ENTER>
UID        PID  PPID  C STIME TTY          TIME CMD
root         1     0  0 22:56 ?        00:00:00 init [2]
root         2     1  0 22:56 ?        00:00:00 [ksoftirqd/0]
root         3     1  0 22:56 ?        00:00:00 [watchdog/0]
root         4     1  0 22:56 ?        00:00:00 [events/0]
root         5     1  0 22:56 ?        00:00:00 [khelper]
root         6     1  0 22:56 ?        00:00:00 [kthread]
root         7     6  0 22:56 ?        00:00:00 [kblockd/0]
root         8     6  0 22:56 ?        00:00:00 [pdflush]
root         9     6  0 22:56 ?        00:00:00 [pdflush]
root        11     6  0 22:56 ?        00:00:00 [aio/0]
root        10     1  0 22:56 ?        00:00:00 [kswapd0]
root        12     1  0 22:56 ?        00:00:00 [kjournald]
root       299     1  0 22:56 ?        00:00:00 /sbin/syslogd
root       305     1  0 22:56 ?        00:00:00 /sbin/klogd
root       343     1  0 22:56 ?        00:00:00 /usr/sbin/exim4 -bd -q30m
root       348     1  0 22:56 ?        00:00:00 /usr/sbin/inetd
daemon     361     1  0 22:56 ?        00:00:00 /usr/sbin/atd
root       364     1  0 22:56 ?        00:00:00 /usr/sbin/cron
root       379     1  0 22:56 tty0     00:00:00 /bin/login --
root       380   379  0 22:56 tty0     00:00:00 -bash
root       384   380  0 22:57 tty0     00:00:00 ps -ef

(none):~# df -h <ENTER>
Filesystem            Size  Used Avail Use% Mounted on
/dev/ubda            1008M  264M  694M  28% /
tmpfs                 768M     0  768M   0% /tmp
tmpfs                  14M     0   14M   0% /dev/shm

(none):~# halt <ENTER>

Broadcast message from root (tty0) (Sun Jan 15 22:57:17 2006):

The system is going down for system halt NOW!

Pretty cool, eh? It's your very own Debian 3.1 sandbox to make or break as you like. You can ignore warnings about hwclock and tty0, as these are normal for most UML systems because some hardware features are not supported by UML kernels. Feel free at this point to change the root password to anything you like.

Next, let's set up networking. You need two free static IP addresses, one for each side of a tunnel that will be created by the UML utilities we compiled earlier. I use 192.168.1.100 and 192.168.1.101 here. Use anything appropriate for your local network. To get started, boot up your Debian UML again, and use the following command:

cd /tmp/UML-Demo
./vmlinux-2.6.14.3-bs3  ubd0=Debian1.cow,Debian-3.1-x86-root_fs
-->root=/dev/ubda eth0=tuntap,,,192.168.1.100

After our Debian guest system is booted, log in as root again and modify the network configuration as follows.

Edit the /etc/network/interfaces file to contain only the following lines:

auto lo
iface lo inet loopback
auto eth0
iface eth0 inet static
 address 192.168.1.101
 netmask 255.255.255.0
 gateway 192.168.1.1

Enter a hostname of your choice in the /etc/hostname file and, finally, copy your resolver settings from /etc/resolv.conf on the host system to the guest Debian system. Halt the guest system and reboot.

After the guest system is booted, you will be able to ping it from anywhere on your network. I would suggest doing a couple things to your newly networked Debian system. First, install OpenSSH, and then update all installed packages to current versions. To do so, execute the following commands and answer the simple questions when asked:

apt-get install openssh-server
apt-get upgrade

The possibilities at this point are wide open. Any network service or application can be run under this guest Debian install. You can use UML to test applications across many kernel versions and Linux distributions all on one box. You can place the filesystem image and COW file on a USB thumbdrive, giving you a stable development environment across all the computers you use. User-mode Linux makes it easy and painless to test system changes that otherwise might make a system unbootable.

Okay, you know you've always wanted to do it. Now, here is your chance. Bring up a new standalone guest Debian UML system, and do an rm -rf /. If you are like me, your fingers start to curl under as you even consider typing that command. To begin, boot up the new Debian guest using the following command (notice we are using a different COW file, because we do not want to disturb the nice networked setup we created previously):

cd /tmp/UML-Demo
./vmlinux-2.6.14.3-bs3  ubd0=DangerDanger.cow,Debian-3.1-x86-root_fs
-->root=/dev/ubda

After our doomed friend boots up, let 'em have it. Make sure you double- (perhaps even triple-) check that you are still typing in the guest Debian system (Listing 2)!