Steve's blog

We've had a wishlist bug (#551951) open against debian-cd for some time, asking for the creation of "isohybrid" CD images for i386 and amd64. These are special in that as well as the normal CD-based ISO9660 filesystem they also contain a valid-looking DOS-style partition table. Thus, if you simply copy one of these images raw to a USB stick a normal PC BIOS will boot the image directly. This would be a neat feature, making it much easier for people to use standard Debian installer images on their USB sticks without having to follow a lot more instructions.

Support for this feature was added in syslinux/isolinux quite a while back (thanks to H Peter Anvin and the other syslinux contributors), but this depended on post-processing images with the isohybrid program. Due to the special way that we make our CD and DVD images to produce jigdo images as well, that wouldn't work for us: the jigdo files would no longer match the ISO files. Bugger...! Recently, along came more help in the form of Thomas Schmitt (author of xorriso) and George Danchev, its Debian maintainer. Recent versions of xorriso support creating isohybrid images directly, which is very handy. We've worked together in the last few months on porting my old JTE code (which creates our jigdo images) from cdrkit to xorriso, and after a lot of testing and debugging we now have things working fully. I've added a small amount of code in debian-cd to use the new xorriso features, and for the last couple of weeks all of the i386, amd64 and i386-amd64 multi-arch CDs and DVDs have been built as hybrids.

It's been a delight working with Thomas and George - they're enthusiastic, helpful and friendly. Thanks, guys!

What does this mean for end users?

Instead of having to specially prepare USB sticks for the installer, you can now simply use dd to write the image straight to the raw stick, e.g.:

# dd if=debian-testing-i386-netinst.iso of=/dev/sdX

The USB stick should now boot directly, showing exactly the same bootup screen as if you'd written the image directly to a CD. Yay!

I'm expecting to be using this code from now on for new releases, i.e. d-i beta and RC releases and for Squeeze itself when we get there. I'm not planning on backporting this code into the Lenny branch of debian-cd as it's a lot of work and I don't want to destabilise things there.

What's next?

We need to clear up some of the maintenance around the code as it stands; I'm planning on moving libjte inside my existing jigit package, as that's a sensible place to put it. That will come after the Squeeze release, though.

We need to finalise boot support for other architectures in xorriso; I added support for them into mkisofs/genisoimage a long time ago and we've already moved some of that into xorriso.

Thomas and George have also been working hard to give me faster/better size estimation code, useful for making debian-cd run faster.

Finally: I'd like to add support into xorriso for creating the nasty HFS hybrid images that are needed for booting Macs. The code that does this in cdrkit is probably some of the worst that I've ever worked with, and I'd like to get away from it. If only Apple hadn't stupidly built their proprietary platform around this shit and had used open standards instead. :-(

Once we have these bits in xorriso, I'll be able to move production of all of the CDs across from genisoimage to xorriso. Very nearly there for being able to drop the last remnants of Schily-ware from the Debian archive...