Boot Management

Warewulf uses iPXE to for network boot by default. As a tech preview, support for GRUB is also available, which adds support for secure boot.

Booting with iPXE

$digraph G{ node [shape=box]; compound=true; edge [label2node=true] bios [shape=record label="{BIOS | boots from DHCP/next-server via TFTP}"] subgraph cluster0 { label="iPXE boot" iPXE; ipxe_cfg [shape=record label="{ipxe.cfg|generated for each node}"]; iPXE -> ipxe_cfg [label="http"]; } bios->iPXE [lhead=cluster0,label="iPXE.efi"]; kernel [shape=record label="{kernel|ramdisk (root fs)|wwinit overlay}|extracted from node container"]; ipxe_cfg->kernel[ltail=cluster0,label="http"]; }$

Booting with GRUB

Support for GRUB as a network bootloader (replacing iPXE) is available in Warewulf as a technology preview.

digraph G{
node [shape=box];
compound=true;
edge [label2node=true]
bios [shape=record label="{BIOS | boots from DHCP/next-server via TFTP}"]

bios->shim [lhead=cluster1,label="shim.efi"];
subgraph cluster1{
label="Grub boot"
shim[shape=record label="{shim.efi|from ww4 host}"];
grub[shape=record label="{grubx64.efi | name hardcoded in shim.efi|from ww4 host}"]
shim->grub[label="TFTP"];
grubcfg[shape=record label="{grub.cfg|static under TFTP root}"];
grub->grubcfg[label="TFTP"];
}
kernel [shape=record label="{kernel|ramdisk (root fs)|wwinit overlay}|extracted from node container"];
grubcfg->kernel[ltail=cluster1,label="http"];
}

Instead of the iPXE starter a combination of shim and GRUB can be used with the advantage that secure boot can be used. That means that only the signed kernel of a distribution can be booted. This can be a huge security benefit for some scenarios.

In order to enable the grub boot method it has to be enabled in warewulf.conf.

warewulf:
  grubboot: true

Nodes which are not known to Warewulf are booted with the shim/grub from the Warewulf server host.

Secure boot

digraph foo {
node [shape=box];
subgraph boot {
"EFI" [label="EFI",row=boot];
"Shim" [label="Shim",row=boot];
"Grub" [label="Grub",row=boot];
"Kernel" [label="kernel",row=boot];
EFI -> Shim[label="Check for Microsoft signature"];
Shim -> Grub[label="Check for Distribution signature"];
Grub->Kernel[label="Check for Distribution or MOK signature"];
}
}

If secure boot is enabled at every step a signature is checked and the boot process fails if this check fails. The shim typically only includes the key for a single operating system, which means that each distribution needs separate shim and grub executables. Warewulf extracts these binaries from the containers. If the node is unknown to Warewulf or can’t be identified during the TFTP boot phase, the shim/grub binaries of the host in which Warewulf is running are used.

Install shim and efi

shim.efi and grub.efi must be installed in the container for it to be booted by GRUB.

# wwctl container shell leap15.5
[leap15.5] Warewulf> zypper install grub2 shim

# wwctl container shell rocky9
[rocky9] Warewulf> dnf install shim-x64.x86_64 grub2-efi-x64.x86_64

These packages must also be installed on the Warewulf server host to enable node discovery using GRUB.

http boot

Modern EFI systems have the possibility to directly boot per http. The flow diagram is the following:

digraph G{
node [shape=box];
efi [shape=record label="{EFI|boots from URI defined in filename}"];
shim [shape=record label="{shim.efi|replaces shim.efi with grubx64.efi in URI|extracted from node container}"];
grub [shape=record label="{grub.efi|checks for grub.cfg|extracted from node container}"]
kernel [shape=record label="{kernel|ramdisk (root fs)|wwinit overlay}|extracted from node container"];
efi->shim [label="http"];
shim->grub [label="http"];
grub->kernel [label="http"];
}

Warewulf delivers the initial shim.efi and grub.efi via http as taken directly from the node’s assigned container.