7.8. GRUB and everything related

The directory /var/lib/cobbler/grub_config contains GRUB boot loader (version 2.02) configuration files.

The directory structure is exactly synced (e.g. via cobbler sync) to the TFTP (or http/www for http network boot) directory and must be kept as is.

7.8.1. Additional dependencies

If you wish to generate GRUB2 bootloaders in the EFI format please install the dependencies according to the arches you wish to boot with your Cobbler installation: grub2-ARCH-efi-modules.

7.8.2. The command “cobbler mkloaders”

This command can create a bootable GRUB2 bootloader in the EFI format. Thus it collects all modules and creates a bootable GRUB2 bootloader. The folder where this is executed is not relevant.

To build GRUB bootloaders for other architectures install the packages and then execute the command against the newly installed directories. openSUSE has enabled you to do this but other distros may not decide to do this. If your distro does not enable you to do this you need to enable yourself for this. For this you need advanced GRUB knowledge, thus this is not part of the tutorial.

This command must be ran after every GRUB2 package update.

The command can be manipulated by changing the settings of Cobbler. The following are being used:

• bootloaders_dir

• grub2_mod_dir

• bootloaders_formats

• bootloaders_modules

• syslinux_dir

• syslinux_memdisk_folder

• syslinux_pxelinux_folder

• bootloaders_shim_folder

• bootloaders_shim_file

• bootloaders_ipxe_folder

7.8.3. Current workflow

1. Check the settings for above mentioned keys.

2. Create a bootable grubx64.efi loader via cobbler mkloaders

3. In /etc/cobbler/settings.yaml grubconfig_dir has to be set to /var/lib/cobbler/grub_config

4. cobbler sync automatically populates the GRUB configuration directory now in the TFTP root folder

5. On your DHCP server, point option 67 (filename) to grubx64.efi (assuming you have configured the other options already)

When you want to use cloud init with the new subiquity installer in Ubuntu 20.04, please keep in mind that the nocloud source has to be quoted in GRUB, otherwise it won’t work. For syslinux however, the nocloud source mustn’t be quoted! That said, currently you can’t use cloud init profiles for Ubuntu 20.04 simultaneously in both Syslinux and GRUB.

7.8.4. IMPORTANT FILES

7.8.4.1. config/grub

7.8.4.1.1. grub.cfg

This file in the main TFTP directory is a fallback for broken firmware. Normally GRUB should already set the prefix to the directory where it has been loaded from (GRUB subdirectory in our case). It is known for (specific versions?) KVM and ppc64le that GRUB may end up loading this as first grub.cfg. We simply set prefix="grub" and manually load the main config file grub/grub.cfg.

7.8.4.1.2. grub/grub.cfg

This is the main entry point for all architectures. We always load this config file.

7.8.4.1.3. grub/grub/local_*.cfg

This are the architecture specific config files providing local (hard disk) boot entries. These may need adjusting over the time, depending how distributions name their *.efi executable for local boot.

7.8.4.1.4. grub/grub/system/*

Empty directory where Cobbler will sync machine specific configuration (typically setting local boot or an (auto-)install menu entry). These are named after the mac address of a machine, e.g.: grub/system/52:54:00:42:07:04 This config file is tried to be loaded from the main grub.cfg.

7.8.4.1.5. grub/grub/system_link.*

Empty directory where Cobbler will create symlinks, named after the Cobbler name of the machine and it links to above described mac address file in ../system/${mac} This is only for easier reading and debugging of machine specific GRUB settings.

7.8.4.2. /var/lib/cobbler/loaders

This directory holds network bootloaders (or links to them) and is also synced to /srv/tftp root directory 1 to 1.

It creates GRUB executables for each installed grub2-$arch via: grub2-mkimage and links in the corresponding GRUB2 modules and other supported bootloaders (pxelinux.0, …)

If you have installed e.g. a new GRUB or Syslinux version, you should re-run cobbler mkloaders to build new GRUB executables. For other, static or already compiled/linked bootloaders like, shim, pxelinux.0 or a precompiled, signed grub.efi executable, it is enough to call cobbler sync now (we store links to these now).

The GRUB specific files generated/linked via cobbler mkloaders are also described here:

7.8.4.2.1. .cobbler_postun_cleanup

Filled up with generated grub2-mkimage binaries and created links.

This is needed in postun cobbler.spec section to remove things again. This is the only, not synced file.

7.8.4.2.2. grub/grub.0

• 32 bit PXE (x86 legacy) GRUB executable.

• grub2-mkimage generated.

• This can/should be used instead of pxelinux.0. You then get the full grub boot process.

• The bootloader is named grub.0, because pxelinux.0 can chain boot this grub executable via network. But it (or specific versions?) wants bootloaders with a filename ending on .0.

7.8.4.2.3. grub/{grubaa64.efi,grub.ppc64le,grubx64.efi}

Also grub2-mkimage generated, architecture specific GRUB executables. These, can directly be network booted on the corresponding/matching architecture. Please have a look at the dhcpd.conf template for getting an idea how architecture differing (via DHCP request network packets) works.

On grub-${arch} package updates, please call cobbler mkloaders to get up-to-date executables. The names of these executables are derived from GRUB2 sources. These are the default names as they should get generated on all distributions by default. These map to ${grub-cpu}-${grub-platform} as seen below the modules directory structure. Unfortunately this does not map 1 to 1.

7.8.4.2.4. grub/{arm64-efi,i386-pc,powerpc-ieee1275,x86_64-efi}

Links to architecture specific GRUB modules. From these grub2-mkimage generates above executables.

These directories (where the links point to) have to be named exactly like this. GRUB may download missing/needed modules from /srv/tftp/${prefix}/${grub-cpu}-${grub-platform} on the fly as needed.

E.g. using the grub.cfg command: hello, will end up in downloading hello.mod then doing automatically an insmod hello…

7.8.4.2.5. grub/{grub.efi,shim.efi}

• Links to precompiled from distribution provided and signed shim and GRUB EFI executables.

• By default shim.efi is used in UEFI (x86 at least) case.

• shim.efi automatically tries to load grub.efi.

• Module loading via network using a signed grub.efi loader, does not work.

• All GRUB modules need grub.cfg and later sourced config files must be present in the signed grub.efi executable.

• For example the “tr” GRUB module was not part of SLES 12 and therefore the reforming of the ${mac} address to the previous pxelinux.0 style, e.g.: 52:54:00:42:56:58 -> 01-52-54-00-42-56-58 does not work. But this is overhead anyway, so we now use the plain mac address as filenames for system specific grub configuration.