Editing Bootable DVDs as ISO images

 When someone says ISO, I reach for my Q*Bert

A friend recently asked how to go about changing the contents of an install DVD with freely available tools that didn't have a size limit. This was something I hadn't explicitly done before and sounded like quite a fun thing to try (you may not entirely agree with the use of the word 'fun' there) so I thought I'd look into it and try to hack my Win7 install DVD a bit. For this I used Ubuntu (though I'm guessing almost any Linux install would work), dd, mount, mkisofs and a script called geteltorito.pl (available here). I was going to use ISO Master but hit problems which I will get to later. If you're of the Windows persuasion, I'm not sure how to do this using free tools, however there's nothing to stop you creating a live boot CD/USB stick of a Linux distro, going into that and just going into some area on your existing HD.

Anyway, the first step was to create an ISO image of the DVD. This was fairly easy using:

 
markwslater@markwslater-System-Product-Name:~$ dd if=/dev/cdrom of=~/win7_image.iso
6298216+0 records in
6298216+0 records out
3224686592 bytes (3.2 GB) copied, 229.392 s, 14.1 MB/s
markwslater@markwslater-System-Product-Name:~$ ls -ltrh ~/win7_image.iso 
-rw-rw-r-- 1 markwslater markwslater 3.1G Mar  5 23:26 /home/markwslater/win7_image.iso

So we now have the ISO image which we could (in theory) start messing with through various tools. Apparently (though I didn't confirm this) most of the free ones or trials for Windows have an upper limit on the size of ISO you can mess with (around CD size by all accounts). Looking around I happened across ISO Master and thought this would do the job. Unfortunately, plugging the above ISO image in only ended up with a README that said:
 

This disc contains a "UDF" file system and requires an operating system
that supports the ISO-13346 "UDF" file system specification.

Which was not the most helpful. So, after a bit of searching I found the following thread that explained how to do things with basic Linux tools. The key thing is to preserve the master boot record of the ISO and recreate it with this intact. The aforementioned geteltorito.pl perl script did a grand job of this:

 
markwslater@markwslater-System-Product-Name:~$ ./geteltorito.pl  win7_image.iso > ~/boot.bin
Booting catalog starts at sector: 22 
Manufacturer of CD: Microsoft Corporation
Image architecture: x86
Boot media type is: no emulation
El Torito image starts at sector 734 and has 8 sector(s) of 512 Bytes
Image has been written to stdout ....
markwslater@markwslater-System-Product-Name:~$

It's then quite easy to mount the existing ISO image (read only), copy this elsewhere, change the permissions and do what's necessary:

 
mkdir windvd
sudo mount -t auto -o loop win7_image.iso /home/markwslater/windvdcp -r windvd windvd_rw
chmod u+w windvd_rw -R
echo stuff > windvd_rw/my_file.txt

So all that remains is to recreate the ISO with the boot.bin file created above:

 
mkisofs -udf -b boot.bin -no-emul-boot -hide boot.bin -relaxed-filenames -joliet-long -D -o ~/new_win7.iso  ~/windvd_rw

Burn this to a DVD however you wish and bob's-your-uncle, you have recreated the win7 install DVD after messing with it :)