 <a
id="fail" name="fail">Booting 'failsafe'
'failsafe' is a standard boot option in all Mandrake Linux systems.
Under normal circumstances, the system switches right into the preferred <a
class="int" href="aservice.html#system">runlevel during boot ('3' for
console, '5' for X). 'failsafe' on the other hand first boots into runlevel 1
(Single User Mode, see below), then tries to switch to runlevel 3 (console)
and then, if 5 is the default runlevel, into runlevel 5.
If the 'Linuxconf' administration suite is installed, it will be started in
console mode upon reaching runlevel 1. You will be presented with a runlevel
menu or the possibility to use 'Linuxconf' to do system maintenance tasks.
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<a
id="single" name="single">Single User Mode
Linux also provides two built-in rescue systems, one of them is the 'single
user mode', aka runlevel 1. This 'single user' is 'root'. There will only be a
minimum of processes running.
There are several ways to get into this runlevel:
-
From within a running system (as 'root'): init 1. Notice that
this command will shutdown almost everything on your machine. It's also a
popular way to simulate a reboot.
-
From the prompt of a boot loader: linux single or linux
init 1. You might also be dropped off here when using the 'failsafe'
boot option if the system can't go to runlevel 3.
There's no login required.
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<a
id="root" name="root">The root Shell
The single user mode still relies on a working 'init'. But what if 'init'
is corrupted or even missing? If you boot your system with this boot loader
option:
linux init=/bin/sh
only the kernel will be loaded into system memory and you will be dropped
almost immediately to a shell.
On a Mandrake Linux 8.1 system, you should add another option to turn off
devfsd, because otherwise you will run into trouble with hardware related
utilities:
linux init=/bin/sh devfs=nomount
Things you do not have initially in this shell:
- A national key map and font. The default ASCII font and an American key
map is used.
- A PATH containing '/sbin'. Execute all commands from that directory with
their full path, e.g. /sbin/fdisk -l /dev/hda instead of fdisk
-l /dev/hda.
- Mounts apart from the partition containing '/'. Only '/' is mounted,
nothing else (thus the name).
- A writable mount. Not only that just '/' is mounted, it is also mounted
read-only.
- init. Obviously ;-). There's no 'init', no 'shutdown' or 'halt'. You have
to sync and unmount all media you might have mounted during your root shell
session by yourself.
The first thing you should try is getting write access to your '/'
partition:
mount -o remount,rw /dev/device
Run mount to find out the name of device. Another
file system you want to mount is the virtual 'proc' file system, which
provides you and the system utilities with information about what's going on
in your system:
mount /proc /proc -t proc
From here on you should be able to do your repair tasks. Your main
objective should be getting init to work again, so that you can do further
repairs in single user mode.
Before leaving this shell, flush all buffers with
sync
unmount all mounts with
umount -a
and remount the '/' mount read-only again with
mount -o remount,ro /
Press <ALT> <CTRL> <DEL>
simultaneously to leave the shell and reboot the machine.
Notice:
- The flags of the 'mount' command are wrong and do not reflect
changes, e.g. a remount from read-only to read-write. Don't trust
them.
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There's quite a number of Linux distributions which run off a
removable medium (floppy, CD, ZIP) or a Windows partition. CD based
distributions often offer the added advantage of providing a graphical
interface.
You'll find a fairly updated list of those Linux distributions on <a
href="http://lwn.net/Distributions/">The LWN.net Linux Distribution
List.
Things to keep in mind when using a third party rescue
distribution:
-
Make sure it works. If you've downloaded a CD image, run
md5sum name.iso
and compare the resulting number with the one provided on the server you
downloaded the image from.
Floppy images are less prone to transmission CRC errors but media failures are
much more likely. Having put the image onto a floppy, run
cmp /dev/fd0 name.img
to make sure the image on the floppy and the image you've downloaded are
identical.
Boot your new rescue system to check if everything's OK.
-
Check if the distribution is actively maintained. Linux is a fast moving
target. If you can't mount your hard disk partition because of an out-of-date
file system driver on your rescue system, you're back to square one.
-
Have a look at the included software. If your partitions are formatted with
a less common file system like XFS or JFS, it might happen that the
distribution does not contain the necessary utilities. Use the content list of
the Mandrake Linux rescue system as a template.
The next two pages of this article will list some common (and some less
common) emergency scenarios and describe how to handle them.
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 <a
class="int" href="arecov3.html">Emergency scenarios I
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