SKUF - Suckless Kexec Using Fileshare

Ever wanted to be able to boot Linux[^1] over the network using an Ethernet cable but without setting up a PXE? Now you only need a SMB server that can be installed anywhere!

SKUF Network Boot System allows you to boot the Arch Linux[^2] operating system on a computer connected to network via Ethernet using USB flash drive (150MB minimum) and a SMB file share.

[!CAUTION] The only supported distribution is Arch Linux™. Other shitty systems like Debian, Ubuntu, Manjaro, Fedora, openSUSE, etc. are NOT supported and NEVER WILL BE.

Requirements

Two computers in the same network:

Server:

• [x] Running SMB file server
• [x] Your user on the SMB server has a password. Users without password or anonymous access are not supported

Client:

• [x] Connected to network via Ethernet cable. Wireless is not supported.
• [x] A USB stick/CD/DVD with the skuflinux image (you can also use Ventoy)
• [x] Brain not poisoned with beer so you have enough brain cells to read this manual

[!CAUTION] Prebuilt binaries and ISO images will NEVER be available due to possible security risks. Read the build instructions carefully.

Scheme of work

[!NOTE] The example illustrates how the server and client work together.
Server — a computer with the SMB server running.
Client — a computer that will boot the system from the server over the network using a cable

You have a USB flash drive/CD/DVD with an ISO image of skuflinux on it. You have two PCs in your room/college/office. First one is the one you will be sitting at. The other one is running SMB server with a directory that you have write access to. That directory contains filesystem image with the Arch Linux distribution and the skuf package installed.

Step 1: Loading kernel and initramfs from SMB server

After booting from USB drive with skuflinux you will be prompted to enter SMB server address and port, user credentials and path to filesystem image. Now SKUF script will do the following:

• Obtain an IP address using dhcpcd
• Mount the SMB directory (read-only)
• Mount the image volume with Arch Linux (read-only)
• Generate an encrypted string with your answers to the questions asked earlier
• Load kernel and initramfs from a previously mounted Arch Linux image into RAM
• Unmount SMB and image volume with Arch Linux
• Execute kexec

Step 2: Re-mounting SMB and running system

Now when the kernel and initramfs of your Arch Linux were loaded from SMB server, SKUF mounts system image again:

• The newly booted system obtaining IP address again
• The previously encrypted string contained your answers to the questions. It was passed to the kernel command line (/proc/cmdline) in encrypted form, and will now be decrypted, so you don't have to write it all over again.
• Mounting the SMB directory again (read/write)
• Mounting the image with Arch Linux again (read/write)
• Once everything mounted, SKUF executes switch_root and system is booted. Congratulations!

Building

[!CAUTION] The only supported distribution is Arch Linux™. Other shitty systems like Debian, Ubuntu, Manjaro, Fedora, openSUSE, etc. are NOT supported and NEVER WILL BE.

Required packages

• arch-install-scripts
• archiso
• base
• base-devel
• binutils
• clang or gcc
• musl
• linux-api-headers
• kernel-headers-musl
• patch (optional, for build_rootfs_tar.sh, if patches present)

Build instructions

Clone this repository using git:

git clone https://github.com/BiteDasher/skuf
cd skuf
./switch-tag latest

Tune encryption obfuscation and encryption password (see Customization instructions):

vim tune.password
vim tune.crypt

Setup defaults for ISO (optional):

vim defaults

Run configuraion sripts:

./tune_crypt.sh
./tune_password.sh
./setup_defaults.sh

Build SKUF:

./build_rootfs_tar.sh
./build_package.sh
./setup_repo.sh
./build_iso.sh
./create_image.sh SIZE_IN_GIGABYTES additional_packages
# For sparse file, use ./create_image.sh -s

[!NOTE] Basic installation of Arch Linux without GUI or any additional software takes about 1 GB.

Done! 💪🎉 Now write skuflinux-smth.iso to your USB drive, put arch.ext4 into your directory on SMB server and try SKUF Network Boot System.

Customisation instructions

Password tuning

String for /proc/cmdline is encrypted using OpenSSL. You need to specify encryption password and number of iterations in the tune.password file in following format:

ITERATIONS_COUNT PASSWORD

[!NOTE] For an example, see the tune.passwordX file

Obfuscation tuning

String that is encrypted through OpenSSL is eventually turned into a base64 string. You can obfuscate this string by swapping these symbols. Write pairs of letters or numbers in the following format to the tune.crypt file:

A B
X Y
I O
0 1
4 8

[!NOTE] For an example, see the tune.cryptX file

Defaults setup

When you booted up the skuflinux ISO image from your media device, you will be asked questions like: SMB server address, SMB server port, SMB protocol version and so on. Edit the defaults file if you want to preset them manually.

Table of main SKUF variables:

Table of auxiliary SKUF variables:

[!NOTE] "Mandatory" variables are marked with an asterisk at the end.

Presets using bootloader (iMac with wireless keyboard)

If you need to change any presets before booting into SKUF, you can change the bootable kernel parameters through the bootloader. This can be useful when you have, for example, an iMac with a wireless keyboard that only works in EFI applications. The way to change kernel parameters depends on the bootloader:

• syslinux: hit Tab and start typing
• grub2: select entry in menu and click e to open editor
• systemd-boot: click e and start typing

1. Set presets using variables

You can specify presets using one or more kernel variables like this:

skuf.samba_username="Username with spaces"
skuf.SAMBA_PASSWORD='Password \' with \\ character escaping'
skuf.VOLUME_PATH=Path\ with\ spaces\ and\ \"quotes\'
skuf.skip=1

The syntax of the variable is as follows: skuf. + any variable from SKUF variable table in lower or upper case.

[!NOTE]

• To escape " inside "double quotes", use \"
  
• To escape ' inside 'single quotes', use \'
  
• To escape ", ', (whitespace) in case of missing quotation marks, use \", \', \ respectively
  
• In all cases above, to escape \ themselves, use \\
  

2. Set presets using curly or square brackets at the end of kernel parameters

[!IMPORTANT] Variables set via this method will be prioritized over every variable you specified in method №1

Curly brackets allow you to set several variables at once:

{username;password;address;;;;volume\\path;volume \; file;;;;;;/kernel;/initramfs}

The syntax corresponds to the order of the main variables from the SKUF variable table separated from each other by semicolons.

[!NOTE]

• To escape ;, use \;
  
• To escape \ themselves, use \\
  

Square brackets allow you to set only username and password at the same time:

[username;password;with;semicolons]

[!NOTE] Unlike curly brackets, there's no escaping required.

• In case you did not specify skuf.skip in the kernel parameters, but you did specify {...} or [...], script will attempt to apply skuf.skip=1, but only if you have set (or have already set) the 7 mandatory variables listed above.

• Setting skuf.skip=1 will omit all variable checks.

• The skuf.skip preset in the kernel parameters will have a higher priority than same SKIP preset set in the defaults file or {...}/[...].

Tips and Tricks

• It is highly recommended to have an individual folder with a volume file in it for each user on the SMB server. With this setup, there will be fewer possible caching issues (e.g. cache=singleclient).

• You can place a swap file next to the Arch Linux image volume so you can use it on your system. The swap file will be connected over the network as a loop device.

• You can use Plymouth in step 2. Add splash to EXTRA_KERNEL_OPTS to the defaults file, also don't forget to add HOOKS=(... plymouth ...) to the skuf_src/mkinitcpio.conf and install plymouth package.

• In step 1, you can write @u@ and @fu@ in the path to the client(your) directory, in the path to the image volume file and in the swap file. If you login as [email protected], @u@ will be john and @fu@ will be [email protected].

• After building the ISO image and creating a file system image with Arch Linux you can execute sudo ./clean.sh to remove unnedeed files.

• You can execute custom post-install script/binary inside chroot during image creation via ./create_image.sh. To do this, specify the file location via the POST_INSTALL environment variable.

• Password for root and test users in arch.ext4 is 0000

• If you enter something incorrectly while entering SMB address, kernel path, etc. at step 1 and fall into the interactive shell, write reboot -f. No, you cannot restart the script. Train your attention.

• If the client computer has UEFI, you can install SKUF on a FAT32 EFI partition so you don't have to use a USB flash drive/CD/DVD. To do this, mount skuflinux-smth.iso somewhere (like /mnt), then copy /mnt/skuf/boot/x86_64/{vmlinuz-linux,initramfs-linux.img} to FAT32 EFI partition and execute efibootmgr -c -d /dev/sdX -p Y -u 'initrd=\initramfs-linux.img' -l '\vmlinuz-linux' -L 'SKUF' where /dev/sdX is the target disk and Y is the target FAT32 EFI partition number.

Demonstration

https://github.com/BiteDasher/skuf/assets/48867887/79a5b40d-7a48-4046-a857-aa300a57e137

Afterword

Huge thanks to the Arch Linux development team for their awesome distribution, archiso and mkinitcpio utilities. They made the creation of this project much easier.

[^1]: The registered trademark Linux® is used pursuant to a sublicense from LMI, the exclusive licensee of Linus Torvalds, owner of the mark on a world-wide basis. [^2]:Copyright © 2002-2024 Judd Vinet, Aaron Griffin and Levente Polyák. The Arch Linux name and logo are recognized trademarks. Some rights reserved.