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For my company’s hard disk-based backup system I needed the ability to automount disk drives by their device name into a standard directory structure.

One possible approach would be to add some lines like these to fstab:

/dev/sda1       /mnt/sda1       ext3    defaults,noauto 0       0

This may be good enough in some cases, though it wasn’t sufficient for me, when there were dozens of device nodes which could get mounted eventually.

So I basically wanted something that would allow me to just access a directory, while the underlying disk was mounted automatically, then having it unmounted automatically if not in use, but still being dynamic in it’s nature so it would auto-adjust.

Now there’s a simple trick using an AutoFS feature called “executable maps”, which would allow me to achive this all.

The idea is, that all devices (let’s say /dev/sda1, /dev/sda2, /dev/sdb1, /dev/sdc1 as an example) will get mounted to /mnt/disks/[devicename].

First make sure, that AutoFS is installed. On Debian for example, it is installed like this:

apt-get install autofs

Then create a file called /etc/auto.disks with the following lines therein:

#!/bin/bash

# $1 is passed-over from automount
# key refers to the mount point we are looking for
key="$1"

# default mount options
opts="-fstype=ext3,rw"

# if a block device exists at /dev/[key]
# pass it back to automount
[ -b /dev/${key} ] && { echo "$opts \"; echo -e "t:/dev/${key}"; }

Don’t forget to chmod 755 /etc/auto.disks.

This script will create an automounter map dynamically as soon as it passed
a device node. It it finds it (e.g. while looking up /dev/sda1, which exists), it’ll
return the map to automount, which will cause the device node to be mounted.

In my case, the script didn’t need to be very sophisticated as I only have ext3-formatted disks, but it’s easy to script it for automatic file system recognition.

Btw, the script can be tested like this to see if it’s actually working:

satyr:~# bash /etc/auto.disks sda1
-fstype=ext3,rw
        :/dev/sda1
satyr:~# bash /etc/auto.disks sdx1

The first command returns the map for an existing device node /dev/sda1, while the second command returns nothing as /dev/sdx1 doesn’t exist on the system.

Now set AutoFS to use the executable map for /mnt/disks directory. Add this line to /etc/auto.master:

/mnt/disks  /etc/auto.disks --timeout=360

This will cause AutoFS to examine the executable map on all requested sub directories beneath /mnt/disks. So if you’re going to access /mnt/disks/sda1, /mnt/disks/sda2, /mnt/disks/sdb1, /mnt/disks/sdc1, the block devices corresponding to the directories are mounted automatically — as long as the devices exist of course.

The timeout value designates after how much time (of inactivity) an automounted file system expires and get’s unmounted.

Bootable “El Torito” CD-ROMs usually use a boot loader such as isolinux. The loader will allow to boot any particular disk image located inside the iso9660 filesystem.

This makes it very easy to change a particular boot image. It’s merely a matter of picking the right file, changing it and burning a new CD-ROM.

Now there are also CD-ROMs which do not rely upon a boot loader or images inside the iso9660 filesystem.
Instead, the bootable image is stored within the first sectors of the CD-ROM, referenced only from the boot catalog by it’s sector offset. This actually means, that the boot image is hidden and neither visible from the iso9660 filesystem nor loaded from there.

This drawing should get you the basic idea (have a look at the El Torito Specification for further Details):

               +-------------------------+
    Sector 0:  |          SYSTEM         |
               |-------------------------|
    Sector 16: |      Primary Volume     |
               |-------------------------|
    Sector 17: |    Boot Record Volume   |-------+
               |-------------------------|       |
               |                         |       |
               |-------------------------|       |
               | Set Termination Volume  |       |
               |-------------------------|       |
               |     Boot Catalog        | <-----+
               |                         |-------+
               |-------------------------|       |
               |  Bootable Disk/Loader   | <-----+
               |-------------------------|
               |   CD-ROM File System    |
               +-------------------------+

So to change the boot image, it needs to be extracted from the CD-ROM first.

This is where geteltorito comes in, a perl utility by Rainer Krienke, which serves exactly this purpose.

Luckily enough I found this one before I started writing an image dumper on my own…