UEFI machines were given syslinux.efi, which is unsigned: with Secure
Boot on (the default on much server hardware) the firmware refuses it
("Access Denied", seen on an OVMF lab VM), so such a machine could not
PXE-boot at all. They now get Debian's signed shim (bootx64.efi), which
loads the signed network GRUB (grubx64.efi); GRUB reads
grub/grub.cfg-<mac>, else grub/grub.cfg-default, and loads kernels
only as shim verifies them. Rack controllers write the GRUB menus from
the pxelinux menus they already get (same entries: kernel and initrd,
local disk via exit, power off via halt), so both always agree; BIOS
machines keep pxelinux. install.sh, install-rack.sh and bootstrap.sh
install shim-signed and grub-efi-amd64-signed and lay out the files.
Secure Boot covers the kernel; the initrd and the squashfs it fetches
are not covered by it.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>