Listing 1. Debian 3.1 UML Guest Boot Demonstration
Debian GNU/Linux testing/unstable (none) tty0 (none) login: root <ENTER> Linux (none) 18.104.22.168-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  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-22.214.171.124-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-126.96.36.199-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)!
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