Previously, I praised Debian for supporting an installation method ("hard disk booting") that only requires an existing filesystem and Grub, and can be kicked off just by downloading two files and asking Grub to boot from them. This is really convenient because you can install a full system without a CD (or USB key, or any media), without PXE (or configuring any other hosts on your LAN), and with a download that is quite small.
Well, it turns out that Ubuntu supports this installation method too (which is, now that I think about it, not surprising). It just doesn't seem to be advertised anywhere! (I suppose that instead of praising Debian I should praise the Debian documentation.) I just used it to install Ubuntu with encrypted LVM on my Thinkpad X61s, a machine which would otherwise be nontrivial to reinstall because it has no optical drive.
Here are the links to the downloads (the "netboot" installer): i386, amd64. Download initrd.gz (disk image) and linux (the kernel). Make sure you put them somewhere Grub knows how to get to (i.e. not to a networked or encrypted volume; /boot is a good place). Then do the following to boot into your new installer (more complete instructions from Debian):
- Restart your computer and wait for Grub to load.
- Find some existing boot entry and press e to edit it.
- Edit the root line to make sure that it corresponds to the partition where you downloaded the files. It might already be correct.
- Edit the kernel line so it reads kernel /boot/wherever/you/put/linux (on recent grubs, you may have to use linux instead of kernel).
- Edit the initrd line so it reads initrd /boot/wherever/you/put/initrd.gz
- Press b to boot.
Enjoy your new installer!
Update, 19 Jun 2008: I have documented this procedure in the Ubuntu Wiki here.