rancherOS/docs/os/configuration/custom-kernels/index.md

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title: Custom Kernels in RancherOS

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Custom Kernels

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Changing the Kernel in RancherOS

By default, RancherOS ships with the kernel provided by the os-kernel repository. Swapping out the default kernel can by done by building your own custom RancherOS ISO.

1. Create a clone of the main RancherOS repository to your local machine with a git clone.

   $ git clone https://github.com/rancher/os.git
   

2. In the root of the repository, the "General Configuration" section of Dockerfile.dapper will need to be updated. Using your favorite editor, replace the appropriate KERNEL_URL value with a URL of your compiled custom kernel tarball. Ideally, the URL will use HTTPS.

   Dockerfile.dapper file

   # Update the URL to your own custom kernel tarball
   ARG KERNEL_URL_amd64=https://github.com/rancher/os-kernel/releases/download/Ubuntu-4.4.0-23.41-rancher/linux-4.4.10-rancher-x86.tar.gz
   ARG KERNEL_URL_arm64=https://github.com/imikushin/os-kernel/releases/download/Estuary-4.1.18-arm64-3/linux-4.1.18-arm64.tar.gz
   

   
   

   Note: KERNEL_URL settings should point to a Linux kernel, compiled and packaged in a specific way. You can fork os-kernel repository to package your own kernel.

   Your kernel should be packaged and published as a set of files of the following format:

   <kernel-name-and-version>.tar.gz is the one KERNEL_URL should point to. It contains the kernel binary, core modules and firmware:

   boot/
        vmlinuz-<kernel-version>
   lib/
       modules/
               <kernel-version>/
                                ...
       firmware/
                ...
   

   build.tar.gz contains build headers to build additional modules (e.g. using DKMS): it is a subset of the kernel sources tarball. These files will be installed into /usr/src/<os-kernel-tag> using the kernel-headers-system-docker and kernel-headers services.

   extra.tar.gz contains extra modules and firmware for your kernel and should be built into a kernel-extras service:

   lib/
       modules/
               <kernel-version>/
                                ...
       firmware/
                ...
   

3. After you've replaced the URL with your custom kernel, you can follow the steps in building your own custom RancherOS ISO.

Packaging a Kernel to be used in RancherOS

We build the kernel for RancherOS at the os-kernel repository. You can use this repository to help package your own custom kernel to be used in RancherOS.

1. Create a clone of the os-kernel repository to your local machine using git clone.

   $ git clone https://github.com/rancher/os-kernel.git

1. In the ./scripts/build-common file, update the KERNEL_URL and KERNEL_SHA1. KERNEL_URL points to Linux kernel sources archive, packaged as .tar.gz or .tar.xz. KERNEL_SHA1 is the SHA1 sum of the kernel sources archive.

./scripts/build-common file

   #!/bin/bash
   set -e

   : ${KERNEL_URL:="https://github.com/rancher/linux/archive/Ubuntu-3.19.0-27.29.tar.gz"}
   : ${KERNEL_SHA1:="84b9bc53bbb4dd465b97ea54a71a9805e27ae4f2"}
   : ${ARTIFACTS:=$(pwd)/assets}
   : ${BUILD:=$(pwd)/build}
   : ${CONFIG:=$(pwd)/config}
   : ${DIST:=$(pwd)/dist}

1. After you've replaced the KERNEL_URL and KERNEL_SHA1, run make in the root os-kernel directory. After the build is completed, a ./dist/kernel directory will be created with the freshly built kernel tarball and headers.

   $ make
   $ cd dist/kernel
   $ ls
   build.tar.gz                     extra.tar.gz    <name_of_kernel>.tar.gz

The build.tar.gz and extra.tar.gz files are used to build the rancher/os-extras and rancher/os-headers images for your RancherOS release - see https://github.com/rancher/os-images and https://github.com/rancher/os-services for how to build the images and make them available.