20.04 fails to boot via PXE (amd64) - "hidden .disk not found, and hard to discover"

[Robert Maerz]

The problem:
The following PXE cmdline fails to boot the 20.04 Desktop iso served via NFS but works flawlessly for 19.10 and earlier:
"imgargs vmlinuz initrd=initrd root=/dev/nfs boot=casper netboot=nfs nfsroot=<server_ip>:<iso_share_path> ip=dhcp splash quiet --"

During early boot, the only error message printed is:
"Unable to find a live file system on the network" with no further help on why the failure occoured or how to resolve it.

During debugging it was also found that the above message is printed for multiple failures during boot, which does not help to differientiate what's actually failing for what reason.

How to reproduce:
1. Download the official ubuntu desktop iso
2. Mount iso as loop on /mnt/iso
3. Copy (cp -r /mnt/iso/*) iso files to nfsroot location

No, do not use '*' use '.' as in cp -r /mnt/iso/. => as per documentation at https://wiki.ubuntu.com/LiveCDNetboot

4. Copy vmlinuz and initrd from iso to tftp location
5. Boot client PC
6. Watch boot fail

The solution (case specific):
In my case the underlying problem was caused by the .disk folder not being copied by cp to the nfsroot location. Once that was resolved, the above cmd line started working again.

Suggested fixes:
1. instead of .disk, use a folder which is not hidden by default and will hence be captured by normal copy operations
2. (significantly) improve error messages when currently the "no live file systems found" message would be triggered to better guide users to possible error causes and solutions
3. update documentation to point out importance of the .disk directory for PXE (and other) ways of booting

[Dimitri John Ledkov (xnox)]

Netboot UUID checks were added in focal
See https://launchpad.net/bugs/1847076

casper (1.428) focal; urgency=medium

  * Add UUID verification to netboot mounts. LP: #1847076

[Dimitri John Ledkov (xnox)]

Rejecting solution 1.

Improve casper error message is accepted.

Improve documentation is also accepted.

[Dimitri John Ledkov (xnox)]

Documentation:

I googled for "casper nfs" and that found https://wiki.ubuntu.com/LiveCDNetboot for me

Which explicitely says to use:

> sudo cp -a /mnt/. /srv/nfs/ubuntu-7.10-desktop

that copies the hidden directories.

Which documentation did you read when preparing the nfs mount?

[Launchpad Janitor (janitor)]

[Expired for casper (Ubuntu) because there has been no activity for 60 days.]