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What to do when good disks go bad
Carla Schroder
(dworks-at-bratgrrl.com)
Consultant,
Tux Computing
23 Oct 2003
This article shows how to access a non-booting Linux
system with a Knoppix CD, get read-write permissions on
configuration files, create and manage partitions and filesystems,
and copy files to various storage media and over the network. You
can use Knoppix for hardware and system configuration detection
and for creating and managing partitions and filesystems. You can
do it all from Knoppix's excellent graphical utilities, or from
the command line.
Knoppix, which is a complete bootable Linux on a CD, has become
my rescue disk of choice. An excellent introduction to Knoppix
appeared in developerWorks a few months ago ("Knoppix
gives bootable, one-disk Linux" by Cameron Laird).
My old mainstays were Tom's Root Boot, "The most GNU/Linux
on one floppy disk," and Peter Anvin's SuperRescue CD, "An
overfeatured rescue CD." Both are first-rate Linux rescue
disks. One of my favorite show-off tricks is to do a complete
bare-metal system rebuild, using only a Tom's Root Boot disk and
an Internet connection.
As CD-ROMs became standard on PCs, I wore out several
SuperRescue CDs. However, it is based on Red Hat 7.2, which is a
great Red Hat but is also an old Red Hat. So, 7.2 does not have
the hardware support, such as USB or wireless, found in later
distributions.
A star is born
Knoppix, the hot new kid on the block, offers
some great features:
First-rate hardware detection
and support, including PCMCIA, USB, and wireless
Latest and greatest Debian and
KDE
Fast booting, usually around
two minutes
Commercially produced disks that can be purchased for a
minimal price
That last bit didn't become important to me until I moved to an
area with no high-speed Internet. Dial-up only, which made 700 MB
downloads rather impractical.
Knoppix incorporates the best of Debian, KDE, and its own
system utilities. In this article, we'll look at how to do things
both from the command line and using graphical utilities. Be sure
to use values appropriate for your systems, such as partition
numbers, filenames, and network hosts.
Getting started
If you want to run KDE or any other
graphical desktop or window manager, you'll want at least 96 MB of
RAM and a Pentium equivalent or better. In text mode, without X,
it needs 20 MB to be happy and will even run on an old 486. It
will use your existing Linux swap file.
Boot options
After booting up the Knoppix CD, it will pause
at a command prompt for 30 seconds, then continue booting. There
are a large number of boot options; hit F2 to see them. For
example, this is how I boot Knoppix 3.2 on one of my test systems:
knoppix lang=us knoppix wheelmouse knoppix desktop=icewm
The default keyboard mapping is German, so I always boot with
knoppix lang=us. By default, Knoppix boots to the KDE
desktop. knoppix 2 boots to text mode, without
starting X. failsafe starts up with minimal hardware
detection.
Knoppix runs entirely in memory, or if you want to sound
"leet," in a ramdisk. In fact you can see the ramdisk in
the file tree. Remember, sometimes it will be slow, because it
must fetch executables from the CD, which is slower than a hard
drive. It can be installed to a hard disk, but that is a topic for
another day.
Rescuing a non-booting Linux system
This is the most common
scenario. Something goes haywire, and boom, no boot. No problem:
boot up Knoppix and find all your local partitions nicely
iconicized on the KDE desktop. (Or cruise the file tree to /mnt.)
Click on the correct icon, and there are all your files. But they
are wisely mounted read-only. Again, no problem: right-click the
desktop icon to bring up a nice menu with a "Change
read/write mode" option. This mounts the filesystem on the
partition as read/write. Now you can edit any file.
The default user is knoppix. For operations that
require root privileges, you need to su to root and
assign a root password:
knoppix@ttyp0[knoppix]# su
root@ttyp0[knoppix]# passwd
To mount a filesystem read/write from the command line:
root@ttyp0[knoppix]# mount -t reiserfs -o rw /dev/hda5
/mnt/hda5
To unmount:
root@ttyp0[knoppix]# umount /mnt/hda5
If you get an error message "Could not unmount device,
device is busy," something is reading the filesystem. Close
files and cd out of the filesystem.
How do you know what mountpoint and filesystem to specify? Just
read /etc/fstab:
root@ttyp0[knoppix]# cat /etc/fstab
...
# Added by
Knoppix
/dev/hda5 /mnt/hda5 reiserfs noauto,users,exec 0 0
Hardware detection
Before going on a mad config file editing
spree, it often pays to examine hardware information. Knoppix
excels at this, as it has the latest editions of Linux's excellent
hardware and system utilities: fdisk, lspci, iwconfig, ifconfig,
dmesg, /proc, and so forth. (Checking hardware information is also
handy for testing a system for Linux compatibility before you buy
it. Sound cards, softmodems, and wireless NICs are especially
troublesome; manufacturers often change the chipsets without
changing the model numbers, and you need to know the chipsets to
determine if Linux drivers are available. The Knoppix CD also
contains a number of sound files, for quick sound testing,
starting with "OpenMusic" on the welcome screen.)
fdisk -l displays
all partitions on all hard drives.
lspci -v gives
detailed information about every device and chipset connected to
the PCI bus.
cat /proc/cpuinfo
tells exactly what CPU is installed.
ifconfig displays,
and also manipulates, network interface settings. Most commonly
Ethernet cards and ppp, the modem interface.
iwconfig is like
ifconfig, but for wireless network cards.
dmesg is interesting. man dmesg
isn't all that helpful if you're not a kernel hacker. Just using
dmesg | grep <device> is a useful
troubleshooting and system discovery tool. To see everything, run
dmesg with no options.
And of course KDE provides a nice GUI to see all this; go to
System > Info Center.
Rescuing data files
Usually the first rescue chore is to
copy data files off of the troubled drive. This is my favorite
method when there are large numbers of files to copy: install a
second hard drive, then boot Knoppix, then copy files from the old
disk to the new disk. Even if you don't have nice hot-swappable
drives or removable drive cages, it takes just a couple of minutes
to pop the case open and hook one up. Do you have a brand new
blank drive, or an old one that needs to be wiped clean and
reformatted? No problem, do the disk preparation from Knoppix.
Partitioning and formatting
First, install the second hard
drive. Then boot Knoppix and open a root shell. If there are
partitions already on the second disk, simply re-format whatever
ones you need. Note that SCSI drives are designated sd,
while IDE drives are hd. This command displays the
existing disk partitions; be sure to use values appropriate for
your system:
root@ttyp0[knoppix]# fdisk -l /dev/hdb
To format a disk partition:
root@ttyp0[knoppix]# mkfs.ext2 -c /dev/hdb1
This creates a plain-vanilla ext2 filesystem. -c
checks for bad blocks. Of course, you can make it anything you
like: ext3, ReiserFS, whatever:
root@ttyp0[knoppix]# mke2fs -j -c
/dev/hdb1
root@ttyp0[knoppix]# mkreiserfs /dev/hdb1
What, no partitions? First, here's how to create them the
command-line way, with fdisk. It's medium-safe to
futz with fdisk, as changes are not written to disk until you give
the command to do so. So, you can try different options and
preview the partition table before committing to any changes. This
sequence of commands creates a single partition:
root@ttyp0[knoppix]# fdisk /dev/hdb
Type "m" at any time to display a table of fdisk
commands. Then, type "n" to create a new partition. Now,
type "p" to create a primary partition. Hit Enter twice
to accept the defaults. Or, if you don't want to use the whole
disk, hit Enter once to accept the default starting point, then
select the size you want:
+1000M
Hit "p" at any time to preview the new partition
table. When everything looks good, press "w" to write
the changes to disk. By default, fdisk creates a
"type 83" partition, which means Linux. To see a list of
partition types, press "l". To change the partition
type, hit "t". Want to delete a partition? Easy as pie:
press "d" and follow the prompts.
QTParted
Even easier is firing up KDE and using QTParted
(System > QTParted). QTParted creates, deletes, and
non-destructively moves and resizes partitions (even NTFS). So,
you can make room to copy your data without losing anything.
Copying files the GUI way
I like graphical file managers.
It's a lot simpler to drag and drop than to type out long command
strings. Click on the icons on the KDE desktop that represent your
source drive and the drive you want to copy them to. Each one
opens in its own file manager, for fast and easy drag and drop. Be
sure to make the destination drive writeable.
Copying files at the command line
Remember to create a
directory to move files into:
# mkdir /mnt/hdb1/home/carla/backup
# cp -r
/mnt/hda5/home/carla /mnt/hdb1/home/carla/backup
Cloning an entire drive
You'll need two hard drives the same
size, or a destination drive larger than the source drive. Make
sure no partitions are mounted on either drive. In this example
/dev/hda is the source drive, /dev/hdb is the destination drive.
The dd command makes an exact, byte-for-byte copy,
including the MBR (master boot record):
# dd if=/dev/hda of=/dev/hdb
Mounting confusion
Are you losing track of what's mounted,
and in what state? No problem, here comes /proc to
the rescue:
# cat /proc/mounts
This displays all mounted filesystems, the filesystem types,
read/write status, and other attributes. How many hard drives are
on the system? One of these will tell you (and remember, SCSI
drives are sd, IDE are hd):
# fdisk -l
or
# dmesg | grep hd
or
# dmesg | grep sd
Copying to CD
KDE and Knoppix make this easy. Assuming there
is a CD writer on the system, simply right-click on the desktop
icon for the partition containing your files, and you will see
"Create Data CD with K3b." Do File > New Project,
drag and drop the files you want to copy, and there you go. K3b is
very good at autodetecting and autoconfiguring your CD drives; it
should do it all for you. If something goes awry, please refer to
the developerWorks article "Burning
CDs on Linux", which also teaches how to burn CDs from
the command line.
Copying to other media
Zip drives, floppy disks, and USB
storage devices will be automatically recognized by Knoppix, and
icons will be placed on the desktop. Simply make the drive you
want to copy files to writeable, then drag and drop until it's all
done.
Copying over the network
You can configure Knoppix to
connect to a network, just like any other Linux. Knoppix has its
own graphical configuration utility: on the main menu find Knoppix
> Network/Internet. Again Knoppix's excellent hardware
detection comes into play; it even works on wireless NICs
(assuming it's a wireless NIC that is supported in Linux!). Simply
answer a series of questions, and you're done.
It's just as easy from the command line. As root, run:
# netcardconfig
Once your network settings are configured, there are several
options for transferring files. cp is fine for
locally mounted filesystems. Copying files over an untrusted
network should be done with scp (secure copy), and in
fact Knoppix won't let you use anything else. scp
uses ssh for encrypted file transfer and lets you move files
without setting up NFS or Samba. You'll need an ssh server running
somewhere on the network to receive the files. This command copies
an entire directory:
# scp -rp /mnt/hda5/home/carla
192.168.1.5:/home/carla/tmp
SSH quickstart
What, you have no ssh server? If you really
do not yet have ssh installed, here is a quick-start guide to
running SSH. But before using it for even routine remote
administration tasks, you should study ssh in more depth. Note
also that there have been a number of important security patches
issued recently.
OpenSSH comes with all major Linux distributions, and yours
should already have it. (To find out, type locate sshd.)
If not, download and install it. It doesn't need to be on a
special machine; any Linux PC can run SSH. Start it up like so:
# /etc/init.d/ssh start
Then, all you need is for the same user to have accounts on
both machines. Using root is easiest, but potentially dangerous.
And, of course, you can create user accounts on Knoppix as needed,
with useradd and passwd. Then run the
scp command as in the example above, and there you
go.
The first time you connect, you'll get a "The authenticity
of host X can't be established...are you sure you want to continue
connecting?" message. Answer "yes." It will ask for
the root password of the SSH server, and then you're home free. To
move files as a non-root user:
# scp -rp /mnt/hda5/home/carla
carla@192.168.1.5:/home/carla/tmp
Open a root shell on the host system
This lets you operate
on the host system, as though you were logged into it directly.
Identify the partition the host system is on, then open a Knoppix
root shell and mount it:
root@ttyp0[knoppix]# mount /dev/hda1
/mnt/hda1
root@ttyp0[knoppix]# chroot /mnt/hda1
root@Knoppix:/
Knoppix glitches
Running programs from a CD can cause some
odd troubles, especially on notebooks. It is common for power
management to mess up a Knoppix session: when you wake up the
machine, Knoppix doesn't respond to commands anymore. The only
cure I know is to disable power management, or simply hit the
power switch and start over.
Sometimes Knoppix gets stuck during boot, and hangs partway
into KDE (or any X session) startup. Switch to the first virtual
console (Ctrl + Alt + F1) to see the live system messages; this
should tell you where the problem is. Hit Alt + F5 to get back to
the default X session. For example, on one of my test systems it
got stuck doing SCSI detection. Why? Who knows. I disabled SCSI
detection by adding knoppix no scsi to the boot
command, and that took care of it.
Knoppix creative ferment
Linux has always inspired amazing
creativity. Knoppix has taken off in a big way, and there are
dozens of inventive Knoppix-inspired projects, such as Inside
Security Rescue Toolkit, OpenGroupware Knoppix, Damn Small Linux,
and Overclockix. See the Resources
section below for more information and links to how-tos for making
your own customized Knoppix distribution.
Resources
As always, man pages, while not
exactly riveting reading, contain complete command and option
listings. The following man pages are most useful in rescue
operations:
man fdisk
man mkfs
man fstab
man ifconfig
man mount
man proc
man dmesg
man iwconfig
man chroot
man scp
man sshd
man dd
man lspci
"Knoppix
gives bootable, one-disk Linux" (developerWorks,
February 2003) by Cameron Laird is an excellent introduction to
Knoppix.
Knoppix.org is the
official site, by the author of Knoppix, Klaus Knopper.
The primo Knoppix resource, Knoppix.net,
includes user forums, Wikis, how-tos, and links to the dozens of
projects inspired by Knoppix -- such as making your own
customized Live Linux CD, running Knoppix on a hard drive,
creative uses of USB pen drives, tiny Linuxes, and more.
Knoppix
Customizations are unoffical Knoppix modifications, including
Oralux, with a number of braille drivers, the Norwegian-language
SkoleKNOPPIX, and many more.
The tutorial "Build
a Linux test network" (developerWorks, May 2003)
includes good GRUB instructions; refer to this for help with
curing boot problems.
"Burning
CDs on Linux" (developerWorks, April 2003) shows
how to burn CDs from the command line.
If you're new to Linux development, check out "Basic
tasks for new Linux developers" (developerWorks,
January 2003), one of a number of introductory articles on the
"Speed-start
your Linux app" page, developerWorks' resource
for developing Linux applications on IBM middleware.
The Debian
GNU/Linux Network Administrator's Manual is an invaluable
resource.
We also recommend Wireless
LAN resources for Linux, a comprehensive guide to wireless
networking. This site covers everything you need to know about
wireless networking on Linux, so please refer to this if you have
trouble making a wireless connection during your rescue
operations.
Before Knoppix, Carla's lifesavers were Tom's
Root Boot (or tomsrtbt) and Peter
Anvin's SuperRescue CD.
The searchable CERT Coordination Center at Carnegie Mellon
University has vulnerability
information for various implementations of SSH and other
software. A great resource!
You'll find all the answers in the Secure
Shell FAQ.
OpenSSH is a free
and open source implementation of ssh.
Members of the IBM Linux Technology Center contribute to a
number of device-related projects for Linux, including the APC
Modem (Mwave) driver, BlueDrekar
Middleware transport driver, the Hotplug
Project, and USB on
Linux, among others. You'll find a complete
list of Linux Technology Center-supported projects on the LTC
Web site.
You'll find more
Linux articles and tutorials in the developerWorks
Linux zone.
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