How to extract the bootloader from an iso?

Question

I found a project on GitHub that emulates a Windows XP operating system, and I have a problem with an iso.

• https://github.com/lrusso/VirtualXP/tree/main

I want to extract it, change things and recreate the iso.

The problem is that it doesn't extract the bootloader, so when I boot the new one with Qemu, it doesn't start the system.

So, it occurred to me that the bootloader could be extracted using dd, but it seems impossible.

isoinfo -d -i /mnt/sda2/windows/vxp/VirtualXP.iso
Setting input-charset to 'UTF-8' from locale.
CD-ROM is in ISO 9660 format
System id: 
Volume id: VirtualXP
Volume set id: 20210310_1500                                                                                                                  
Publisher id: MagicISO v5.2 COPYRIGHT (C) 2001-2006 MagicISO, Inc.
Data preparer id: MagicISO v5.2 COPYRIGHT (C) 2001-2006 MagicISO, Inc.
Application id: 
Copyright File id:                                     
Abstract File id:                                     
Bibliographic File id:                                     
Volume set size is: 0
Volume set sequence number is: 1
Logical block size is: 2048
Volume size is: 22250
El Torito VD version 1 found, boot catalog is in sector 22248

Joliet with UCS level 3 found.
No SUSP/Rock Ridge present
Eltorito validation header:
    Hid 1
    Arch 0 (x86)
    ID 'MagicISO Boot Record'
    Cksum 16 E7 OK
    Key 55 AA
    Eltorito defaultboot header:
        Bootid 88 (bootable)
        Boot media 0 (No Emulation Boot)
        Load segment 0
        Sys type 6
        Nsect 4
        Bootoff 56E9 22249

And when I create it again it looks like this, without it working in the emulator:

isoinfo -d -i /mnt/sda2/windows/vxp/vxp0.iso
Setting input-charset to 'UTF-8' from locale.
CD-ROM is in ISO 9660 format
System id: LINUX
Volume id: VXP0
Volume set id: 
Publisher id: 
Data preparer id: 
Application id: MKISOFS ISO9660/HFS/UDF FILESYSTEM BUILDER & CDRECORD CD/DVD/BluRay CREATOR (C) 1993 E.YOUNGDALE (C) 1997 J.PEARSON/J.SCHILLING
Copyright File id: 
Abstract File id: 
Bibliographic File id: 
Volume set size is: 1
Volume set sequence number is: 1
Logical block size is: 2048
Volume size is: 22935
El Torito VD version 1 found, boot catalog is in sector 79
CD-ROM uses ISO 9660:1999 relaxed format

Joliet with UCS level 3 found.
SUSP signatures version 1 found
Rock Ridge signatures version 1 found
Rock Ridge id 'RRIP_1991A'
Eltorito validation header:
    Hid 1
    Arch 0 (x86)
    ID ''
    Cksum AA 55 OK
    Key 55 AA
    Eltorito defaultboot header:
        Bootid 88 (bootable)
        Boot media 0 (No Emulation Boot)
        Load segment 0
        Sys type 0
        Nsect 4
        Bootoff 50 80

I tried with sector number 22249, and it didn't work:

dd if="$1" of=bootloader.bin bs=1024 skip=$sector count=1024

I changed the name of the bootloader to grldr (I imagine that by doing this I am just changing the name), currently the command to generate the iso, I am using the following:

mkisofs -V VXP0 -J -joliet-long -iso-level 4 -D -R -o vxp0.iso -b grldr -c boot/boot.catalog -no-emul-boot -boot-load-size 4 -boot-info-table vxp0

Answer 1

You can extract the bootloader from an ISO file through geteltorito included in genisoimage:

geteltorito -o bootloader VirtualXP.iso

In the manpage description:

   geteltorito  is a Perl script which extracts the initial/default El Torito boot image from
   a CD if one exists. It will not extract any of other boot images that are allowed  by  the
   El Torito standard.

Answer 2

I'm posting this as an answer instead of a comment in case it earns me the bonus, but I googled "Extract El Torito" and ended up at the:

Rufus Github Repository - Issue 63.

In case you don't know what RUFUS is see: Rufus - Create Bootable USB Drives the Easy Way. Apparently, issue 63 was a feature request now closed that involves the issue you're trying to resolve. About halfway down the request is this:

RUFUS FAQ Section - I'm trying to work with an ISO, but Rufus says it's unsupported.

I believe if you perform the steps outlined there, you'll end up with your desired results.

Summary of Steps

1. Create a USB based on FreeDOS in Rufus.
2. Mount the ISO file you created and look for the Virtual Disk Image that contains the DOS Boot Files
3. Extract those files Back to the root.
4. Modify any .bat files if needed.
5. After modification, dd the entire mounted image back to a USB

The El Torito Boot Standard

See the Phoenix's BIOS Manufacturers published standard on El Torito, specifically page 14 for what is actually happening. Quoting ISOBuster:

It is not simply possible to copy a bootable floppy image onto CD or DVD and then hope the medium is now bootable. The File System really needs to know and needs to provide special structures and volume descriptors (in accordance with the El Torito standard).

Answer 3

dd if="$1" of=bootloader.bin bs=1024 skip=$sector count=1024

You used the wrong block size. The CD-ROM operates on 2048-byte blocks, but the boot code size in the El Torito header is specified in 512-byte blocks for some reason. 4 x 512 bytes = 2048, or exactly 1 CD-ROM block. And the isoinfo results indicate the boot block seems to be the very last block in the image.

So:

dd if=VirtualXP.iso of=bootloader.bin bs=2048 skip=22249 count=1

or maybe just

tail -c 2048 VirtualXP.iso > bootloader.bin

Hopefully there are no binary offsets within the bootloader that would need to be changed to match your new ISO structure.