Adding gpxe to a GRUB menu as an item may prove to be another great way to boot a rescue system. Note that I didn't try this yet (only booting installation media with b.k.org).
This service is great but as it does not verify any data so you basically just download data via http and run it as your operating system. Just install a live system on your usb pen and use it or run a live system and setup the network boot environment.
http://grml.org for example can create a pxe boot environment from the live system, just run grml-terminalserver and it will start a dhcp/nfs/tftp server and provide all necessary data.
P.S: I am involved with grml so take it with a grain of salt
True. However, that should be a motivation to make that service more secure. But it does not make the idea flawed. It's very convenient to be able to boot any distribution just like that, without preparing USB sticks or CDs.
I guess it could be made pretty secure with good old code signing. Most distributions already sign their packages, but as this concerns the booting process it could be somewhat more difficult.
Is it possible to boot a generic OS (not necessarily linux or windows) from USB by simply loading a live-CD image file?
Use case: one USB drive with custom MBR, one text configuration file and one or more liveCD images. On boot-up, the user selects one of the images and loads that generic OS.
DOS (MS-DOS, FreeDOS, DR-DOS, ...), Windows 95/98/ME and boot loaders (Syslinux, grub, grub4dos, gujin, gag, mbldr, ...) boots, other operating systems must be aware of the memory mapping.
With SuperGrub2Disk http://www.supergrubdisk.org/wiki/SuperGRUB2Disk you just have to add a iso file into a specific folder and it will automatically be added to the bootloader menu. But AFAIK it does currently not support the memdisk solution.
This isn't possible in the generic case. Once the OS loads, its kernel will look for a root filesystem somewhere, and the ISO isn't one of the usual places it will look (ie: a partition).
I recently tried to do something like this to install Debian (already having Ubuntu on this computer) without using a USB drive or blank CD. Didn't succeed. So I'm glad to see this submission; I'll probably try it out in the near future.
Try network boot. Setup a pxe environment and you should be good to go.
Another solution is use debootstrap to install Debian or Ubuntu to a plain directory. Be aware that you have to make some adjustments to the system (like installing a bootloader/kernel/adjust fstab etc.) after executing debootstrap. You can also use grml-debootstrap which automatically executes the needed steps to make a system bootable.
/shameless plug