ixs' Vodkamelone

With the release of Fedora 9, anaconda has introduced a new consolidated boot image:

"Consolidated network booting ISO image, replacing old boot.iso, diskboot.img, and rescuecd.iso."

The problem with this approach is, that the 12MB diskboot.img file has disappeared, which could be used to boot a system with a small USB stick. Awesome! Thank you very much. ∗facepalm.jpg∗

But luckily, it is not very hard to recreate this file on the shell and write it onto an USB stick afterwards. This serves as another installment of my small article series announced earlier.

I started with the boot.iso image which I downloaded from http://download.fedoraproject.org/pub/fedora/linux/releases/9/Fedora/i386/os/images/boot.iso and loopback mounted on /mnt:

[root@localhost ~]# wget -q -c
http://download.fedoraproject.org/pub/fedora/linux/releases/9/Fedora/i386/os/images/boot.iso
[root@localhost ~]# mount boot.iso /mnt -o loop,ro
[root@localhost NotBackedUp]# ls /mnt/
images  isolinux
[root@localhost NotBackedUp]# 

Now the steps to create the USB pendrive image are quite simple. A file is created, formatted and made bootable with syslinux and the appropriate files from the boot.iso are copied over:

[root@localhost ~]# cd /tmp/
[root@localhost tmp]# mkdir pendrive
[root@localhost tmp]# cd pendrive/
[root@localhost pendrive]# dd if=/dev/zero of=diskboot.img bs=1M count=12
12+0 records in
12+0 records out
12582912 bytes (13 MB) copied, 0.0433931 seconds, 290 MB/s
[root@localhost pendrive]# mkdosfs diskboot.img 
mkdosfs 2.11 (12 Mar 2005)
[root@localhost pendrive]# syslinux diskboot.img 
[root@localhost pendrive]# mkdir mount
[root@localhost pendrive]# mount diskboot.img mount/ -o loop
[root@localhost pendrive]# cp -r /mnt/isolinux/∗ mount/
[root@localhost pendrive]# grep -v local mount/isolinux.cfg > mount/syslinux.cfg
[root@localhost pendrive]# rm -f mount/isolinux.∗
[root@localhost pendrive]# umount -f mount
[root@localhost pendrive]# rm -rf mount/
[root@localhost pendrive]# cd
[root@localhost ~]# umount -f /mnt
[root@localhost ~]#

After these steps, a diskboot.img file is located in /tmp/pendrive. This file can be written to an USB stick with a command such as dd if=diskboot.img of=/dev/sdb. Special care should be taken to write to the correct device (/dev/sdb in this case)and _not_ overwrite one of your own harddrives.

When working with disk images the admin is often faced with the problem of mounting the contained partitions inside of these images.

Such full-disk images can be the containers of virtual machines for xen as fount in /var/lib/xen/images or disk-images ripped from dying drives with the help of recovery tools such as dd_rescue.

Regardless of the origin, these images do contain everything needed to run a machine, a partition table, a boot sector, at least one, but often several partitions inside of one container.
Mounting these images through a normal loop mount does only result in the first image to be mounted, often not what is wanted.
In the past, suggestions ranged from imaging single partitions to using dd to "cut" the single partitions out of the full-disk image. Crude workarounds as we've had a perfect tool called device mapper for some time now.

Executing fdisk on the my test disk-image shows several partitions, among some ingorable warnings:

[root@workstation ~]# fdisk -l disk.img 
You must set cylinders.
You can do this from the extra functions menu.

Disk disk.img: 0 MB, 0 bytes
4 heads, 32 sectors/track, 0 cylinders
Units = cylinders of 128 ∗ 512 = 65536 bytes
Disk identifier: 0x00000000

   Device Boot      Start         End      Blocks   Id  System
disk.img1   ∗           1          32        2032   83  Linux
disk.img2              33         216       11776   83  Linux
disk.img3             217         705       31296   83  Linux
disk.img4             706         978       17472    5  Extended
disk.img5             706         859        9840   83  Linux
disk.img6             860         978        7600   83  Linux
[root@workstation ~]# 

Thus, we know we do have an image file containing several partitions.

Now, how to handle this?

Mounting this test-image gives an error in my case. Often though the first partition gets sucessfully mounted.

[root@workstation ~]# mount disk.img /mnt/ -t ext2 -o loop,ro
mount: wrong fs type, bad option, bad superblock on /dev/loop1,
       missing codepage or helper program, or other error
       In some cases useful info is found in syslog - try
       dmesg | tail  or so

[root@workstation ~]# 

The solution to this is called kpartx, a tool to map the partitions contained inside such an image into device-mapper block devices. This tools is contained in the kpartx RPM and can be easily installed by executing yum install kpartx if it is not already installed on your system.

Execution is quite simple. To list the contained partitions and see onto which loop-devices they would be mapped, the following command is needed:

[root@workstation ~]# kpartx -l disk.img 
loop0p1 : 0 4064 /dev/loop0 32
loop0p2 : 0 23552 /dev/loop0 4096
loop0p3 : 0 62592 /dev/loop0 27648
loop0p5 : 0 19680 /dev/loop0 90272
loop0p6 : 0 15200 /dev/loop0 109984
[root@workstation ~]# 

To actually map these partitions onto the loop-devices, call kpartx with the -a instead of the -l parameter:

[root@workstation ~]# kpartx -a -v disk.img 
add map loop0p1 : 0 4064 linear /dev/loop0 32
add map loop0p2 : 0 23552 linear /dev/loop0 4096
add map loop0p3 : 0 62592 linear /dev/loop0 27648
add map loop0p5 : 0 19680 linear /dev/loop0 90272
add map loop0p6 : 0 15200 linear /dev/loop0 109984
[root@workstation ~]# 

The partitions are then available in /dev/mapper/ to be mounted accordingly and the containing files to be accessed:

[root@workstation ~]# ls -all /dev/mapper/
total 0
drwxr-xr-x  2 root root     160 2008-06-11 15:06 .
drwxr-xr-x 13 root root    4180 2008-06-11 15:06 ..
crw-rw----  1 root root  10, 60 2008-06-11 13:28 control
brw-rw----  1 root disk 253,  0 2008-06-11 15:06 loop0p1
brw-rw----  1 root disk 253,  1 2008-06-11 15:06 loop0p2
brw-rw----  1 root disk 253,  2 2008-06-11 15:06 loop0p3
brw-rw----  1 root disk 253,  3 2008-06-11 15:06 loop0p5
brw-rw----  1 root disk 253,  4 2008-06-11 15:06 loop0p6
[root@workstation ~]# 
[root@workstation ~]# mount /dev/mapper/loop0p5 /mnt/ -o loop,ro
[root@workstation ~]# mount | grep '/mnt'
/dev/mapper/loop0p5 on /mnt type ext2 (ro,loop=/dev/loop1)
[root@workstation ~]# ls /mnt/
backup  conf.tar.gz     etc                  manifest.txt  tool
bin     default.list    factory_default.md5  rc.reboot     upgrade.list
cfg     default.tar.gz  lost+found           test
[root@workstation ~]# 

After the partitions have been unmounted, the device-mapper mappings can be removed by calling kpartx with the -d parameter. The image file can then be used again as it is not in use anymore by the device mapper.

[root@workstation ~]# umount /mnt/
[root@workstation ~]# kpartx -d -v disk.img 
del devmap : loop0p1
del devmap : loop0p2
del devmap : loop0p3
del devmap : loop0p5
del devmap : loop0p6
loop deleted : /dev/loop0
[root@workstation ~]# ls -al /dev/mapper/
total 0
drwxr-xr-x  2 root root     60 2008-06-11 15:16 .
drwxr-xr-x 13 root root   4080 2008-06-11 15:16 ..
crw-rw----  1 root root 10, 60 2008-06-11 13:28 control
[root@workstation ~]# 

I finally decided to try to breath some life into my weblog again and write some hopefully useful technical articles.

I'm currently planning on concentrating on stuff I find out fiddling with new toys (usually embedded stuff running Linux) or little hints or tricks I'm using when administrating systems.

The content of these articles might not all be new and many people already know about them, but I found it's mostly "advanced" administration stuff many people do not know about and can profit from these.

If there's anything which should be described in more detail or anything else, I'm available on the usual IRC networks (IRCnet, oftc, freenode etc.) as ixs.

The first article explains kpartx and how to use it to access partitions inside full-disk images.