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+# Build-time Customization
+
+You can customize RancherOS before building your own binary distribution of the OS. That means, you can call RancherOS <Whatever>OS and use your own version and other stuff.
+
+## os
+
+https://github.com/rancher/os
+
+In the root of the project you can find these files: `build.conf`, `build.conf.amd64`, `build.conf.arm`. They set the customizable options for the build.
+
+`build.conf` sets architecture independent options and all of them are **required**, that means the values should be set and below are the current defaults for these options:
+
+- IMAGE_NAME=rancher/os - installer docker image name
+- VERSION=v0.4.4-dev - version
+- DFS_IMAGE=rancher/docker:v1.10.2 - docker-from-scratch image (for the skeleton root filesystem)
+- SELINUX_POLICY_URL=https://github.com/rancher/refpolicy/releases/download/v0.0.1/policy.29 - the default compiled SELinux policy
+- HOSTNAME_DEFAULT=rancher - default hostname (if non was provided by DHCP or configured by cloud-config)
+- OS_IMAGES_ROOT=rancher - system services docker images' name prefix (like in `rancher/os-console`)
+- OS_SERVICES_REPO=https://raw.githubusercontent.com/rancher/os-services - URL where to find an `index.yml` file listing the "external" system services (like `debian-console`)
+- OS_RELEASES_YML=https://releases.rancher.com/os/releases.yml - URL to release index for upgrades and rollbacks
+
+`build.conf.{arch}` set the options for that particular architecture, only
+- COMPILED_KERNEL_URL=https://github.com/rancher/os-kernel/releases/download/Ubuntu-4.2.0-28.33-rancher/linux-4.2.8-ckt3-rancher-x86.tar.gz (**optional**) - compiled and packaged Linux kernel with modules and firmware
+- DOCKER_BINARY_URL=https://get.docker.com/builds/Linux/x86_64/docker-1.10.2 (**required**) -, URL where to get the docker binary built for that particular architecture
+
+
+## os-images
+
+https://github.com/rancher/os-images
+
+In os-images, `build.conf` sets the following options:
+
+- VERSION=v0.4.4-dev - OS version for which we're building the system service images
+- OS_IMAGES_ROOT=rancher - system services docker images' name prefix (like in `rancher/os-console`)
+- OS_BASE_URL_amd64=https://github.com/rancher/os-base/releases/download/v2015.11.1-1/os-base_amd64.tar.xz - os-base for amd64 tarball download URL
+- OS_BASE_SHA1_amd64=098a64ddb42381fc62ba515870aa8dc6ce19002f - os-base for amd64 tarball sha1 hash
+- OS_BASE_URL_arm=https://github.com/rancher/os-base/releases/download/v2015.11.1-1/os-base_arm.tar.xz - os-base for arm tarball download URL
+- OS_BASE_SHA1_arm=1da38279861aee631a5d41a7ac9835e4f996d843 - os-base for arm tarball sha1 hash