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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