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.

Also as a tech preview, Warewulf may also use iPXE to boot a dracut initramfs as an initial stage before loading the container image.

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 dracut

Some systems, typically due to limitations in their BIOS or EFI firmware, are unable to load container image of a certain size directly with a traditional bootloader, either iPXE or GRUB. As a workaround for such systems, Warewulf can be configured to load a dracut initramfs from the container and to use that initramfs to load the full container image.

Warewulf provides a dracut module to configure the dracut initramfs to load the container image. This module is available in the warewulf-dracut subpackage, which must be installed in the container image.

With the warewulf-dracut package installed, you can build an initramfs inside the container.

dnf -y install warewulf-dracut
dracut --force --no-hostonly --add wwinit --kver $(ls /lib/modules | head -n1)

Set the node’s iPXE template to dracut to direct iPXE to fetch the node’s initramfs image and boot with dracut semantics, rather than booting the node image directly.

Note

Warewulf iPXE templates are located at /etc/warewulf/ipxe/ when Warewulf is installed via official packages. You can learn more about how dracut booting works by inspecting its iPXE template at /etc/warewulf/ipxe/dracut.ipxe.

wwctl node set wwnode1 --ipxe dracut

Note

The iPXE template may be set at the node or profile level.

Alternatively, to direct GRUB to fetch the node’s initramfs image and boot with dracut semantics, set a GrubMenuEntry tag for the node.

Note

Warewulf configures GRUB with a template located at /etc/warewulf/grub/grub.cfg.ww. Inspect the template to learn more about the dracut booting process.

wwctl node set wwnode1 --tagadd GrubMenuEntry=dracut

Note

The GrubMenuEntry variable may be set at the node or profile level.

During boot, warewulfd will detect and dynamically serve an initramfs from a node’s container image in much the same way that it can serve a kernel from a container image. This image is loaded by iPXE (or GRUB) which directs dracut to fetch the node’s container image during boot.

The wwinit module provisions to tmpfs. By default, tmpfs is permitted to use up to 50% of physical memory. This size limit may be adjustd using the kernel argument wwinit.tmpfs.size. (This parameter is passed to the size option during tmpfs mount. See tmpfs(5) for more details.)

Warning

Kernel overrides are not currently fully supported during dracut initramfs boot.

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.