How can I boot from a root filesystem (maybe squashfs or dd image)?

[NiKiZe]

You could always dd your disk to an iSCSI device and boot from that with iPXEs sanboot command.

But my guess that you have something small-ish that you want to boot and if so I would recommend creating an initramfs with your whole disk.

Below is psuedo command to create such a file of your fs

Code:

find . | cpio -H newc -o | gzip > /ramfs.gz

The choices here depends on a few things
* Do you want it to work when network cable is disconnected for whatever reason
* Do you have the RAM to hold your entire system
* Do you want to be able to store data
* Do you want to have multiple machines running with the same image

Since you have already tried squashfs my guess is that you have the RAM and want it to work even if the machine goes "offline"

If so if you post what you have with your squashfs approach so far, as in what is the files you have, the iPXE script you are using, and what the actuall issue is (error message) when you use this I'm sure we can figure it out, but it's always hard to start helping from "nothing".

it might be as easy as adding your squashfs image with initrd, but first adding cpio headers to it, if your are booting in pcbios mode (not efi) than it is possible to have iPXE add those headers for you.