PXE-booting with iPXE

UEFI

iPXE supports UEFI and so do we. This requires the ipxe.efi boot image.

Instructions how to copy the necessary grub files to the tftp server: https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/7/html/installation_guide/chap-installation-server-setup#sect-network-boot-setup-uefi (alternatively one can also copy the files from the /boot/efi/EFI/redhat/ directory of an installed system)

[root@ tmp]# cd /tmp
[root@ tmp]# curl -OL http://repos.psi.ch/rhel9/iso/rhel-baseos-9.1-x86_64-dvd/BaseOS/Packages/shim-x64-15.6-1.el9.x86_64.rpm
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100  471k  100  471k    0     0  92.0M      0 --:--:-- --:--:-- --:--:-- 92.0M
[root@lx-repos-01  tmp]# curl -LO http://repos.psi.ch/rhel9/iso/rhel-baseos-9.1-x86_64-dvd/BaseOS/Packages/grub2-efi-x64-2.06-46.el9.x86_64.rpm
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 1333k  100 1333k    0     0  8550k      0 --:--:-- --:--:-- --:--:-- 8550k
[root@ tmp]# rpm2cpio shim-x64-15.6-1.el9.x86_64.rpm | cpio -dimv
./boot/efi/EFI/BOOT/BOOTX64.EFI
./boot/efi/EFI/BOOT/fbx64.efi
./boot/efi/EFI/redhat/BOOTX64.CSV
./boot/efi/EFI/redhat/mmx64.efi
./boot/efi/EFI/redhat/shim.efi
./boot/efi/EFI/redhat/shimx64-redhat.efi
./boot/efi/EFI/redhat/shimx64.efi
9232 blocks
[root@ tmp]# rpm2cpio grub2-efi-x64-2.06-46.el9.x86_64.rpm | cpio -dimv
./boot/efi/EFI/redhat/grubx64.efi
./boot/grub2/fonts
./boot/grub2/fonts/unicode.pf2
./boot/grub2/grubenv
./boot/loader/entries
./etc/dnf/protected.d/grub2-efi-x64.conf
./etc/grub2-efi.cfg
./etc/grub2.cfg
9612 blocks
[root@ tmp]# ls boot/efi/EFI/redhat/
BOOTX64.CSV  grubx64.efi  mmx64.efi  shim.efi  shimx64.efi  shimx64-redhat.efi
[root@ tmp]# ls -la boot/efi/EFI/redhat/
total 6088
drwxr-xr-x. 2 root root     122 Mar  1 16:16 .
drwxr-xr-x. 4 root root      32 Mar  1 16:16 ..
-rwx------. 1 root root     108 Jun  7  2022 BOOTX64.CSV
-rwx------. 1 root root 2524792 Aug 25  2022 grubx64.efi
-rwx------. 1 root root  856528 Jun  7  2022 mmx64.efi
-rwx------. 1 root root  946736 Jun  7  2022 shim.efi
-rwx------. 1 root root  946736 Jun  7  2022 shimx64.efi
-rwx------. 1 root root  938808 Jun  7  2022 shimx64-redhat.efi
[root@ tmp]#

Process

When a system PXE boots, the DHCP server will boot either pxelinux in legacy mode or grub on UEFI. It is still possible to load ipxe from either of these. With the right option, the boot process is pointed to sysdb.psi.ch and one of the following boot images:

ipxe.efi for systems using UEFI
ipxe.lkrn for all other systems

The system downloads the image and executes it.

The image contains a small iPXE script, which makes iPXE retrieve its actual configuration from a web service running (usually) on sysdb.psi.ch. Specifically, it queries the URL https://sysdb.psi.ch/ipxe/v1/config?mac=<MAC>, where <MAC> is the MAC address of the interface used by iPXE.

The web service on sysdb.psi.ch will generate the iPXE configuration on the fly, depending on whether the system is supposed to be reinstalled and if so, which distribution it is supposed to use. The menu offers other options as well, e.g. an interactive iPXE shell and a memory test.

Sequence Diagram of the UEFI Boot Process

Sequence Diagram of the BIOS (Legacy) Boot Process

Building the iPXE boot image

The steps for building an iPXE image are

Clone git@git.psi.ch:linux-infra/ipxe-build.git
Change into the new directory
Run the refresh-ipxe.sh script. This will check out the iPXE source code mirror from git.psi.ch.
Run the build.sh script. This will use the named configurations in the ipxe-build repository to compile two iPXE images: src/bin/ipxe.lkrn (legacy boot) and src/bin-x86_64-efi/ipxe.efi (UEFI).

As described above, we use an iPXE boot image with an embedded script looking roughly like the following

#!ipxe

dhcp && goto dhcp_succeeded

shell
#prompt for network info

:dhcp_succeeded

chain http://sysdb.psi.ch/ipxe/v1/config?mac=${netX/mac}

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