How to make a custom ISO image with mkimage

From Alpine Linux

This document explains how to build a custom ISO image using the new mkimage scripts located in aports directory.

Prerequisites

First make sure we have the needed tools

apk add alpine-sdk alpine-conf syslinux xorriso squashfs-tools grub grub-efi doas

For efi you should add the following:

apk add mtools dosfstools grub-efi

Create a user (e.g. build) and add them to the abuild group:

adduser build -G abuild

Give the user root-like permissions:

vi /etc/doas.d/doas.conf

permit :abuild
permit persist :abuild

Change to the build user:

su - build

Then create signing keys (-i installs them in /etc/apk/keys which is required for later)

abuild-keygen -i -a

Tip: Make sure your public keys are placed in /etc/apk/keys/ (example: build-xxxxxxxx.rsa.pub)

ls /etc/apk/keys/

Clone (or update) the git repository.

git clone --depth=1 https://gitlab.alpinelinux.org/alpine/aports.git


Make sure the apk index is up to date (so apk can find the packages):

doas apk update

Make sure your /tmp is large enough. If you have the default 1GB /tmp, then create a local one with: 

mkdir -pv ~/tmp
export TMPDIR=~/tmp

Configuration

The mkimg scripts are shipped with pre-configured profiles.

The format is mkimg.$PROFILENAME.sh

In order to have a custom ISO, you should create your own mkimg.$PROFILENAME.sh script.

This is an example used to make a ZFS module, overlayfs (which enables r/w mode for /lib/modules), a serial console output and some other useful apks to build a simple NAS: 

export PROFILENAME=nas
cat << EOF > ~/aports/scripts/mkimg.$PROFILENAME.sh
profile_$PROFILENAME() {
        profile_standard
        kernel_cmdline="unionfs_size=512M console=tty0 console=ttyS0,115200"
        syslinux_serial="0 115200"
        kernel_addons="zfs"
        apks="\$apks iscsi-scst zfs-scripts zfs zfs-utils-py
                cciss_vol_status lvm2 mdadm mkinitfs mtools nfs-utils
                parted rsync sfdisk syslinux util-linux xfsprogs
                dosfstools ntfs-3g
                "
        local _k _a
        for _k in \$kernel_flavors; do
                apks="\$apks linux-\$_k"
                for _a in \$kernel_addons; do
                        apks="\$apks \$_a-\$_k"
                done
        done
        apks="\$apks linux-firmware"
}
EOF

Set the script as executable:

chmod +x ~/aports/scripts/mkimg.$PROFILENAME.sh

Making packages available on boot

A package may be made available in the live system by defining the generation of an apkovl which contains a corresponding /etc/apk/world file, and adding that overlay definition to the mkimg-profile, e.g. with `apkovl="genapkovl-mkimgoverlay.sh"`

Note that to *use* your added apks, you have to install them, with apk add.

The definition may be done as in the genapkovl-dhcp.sh example. Copy the relevant parts (including the rc_add lines) into a `genapkovl-mkimgoverlay.sh` file and add the package(s) that should be installed in the live system on separate lines in the file contents for /etc/apk/world.

cp ~/aports/scripts/genapkovl-dhcp.sh ~/aports/scripts/genapkovl-mkimgoverlay.sh

Edit the file to add: 

...
mkdir -p "$tmp"/etc/apk
makefile root:root 0644 "$tmp"/etc/apk/world <<EOF
alpine-base
<apk1-service>
<apk2-service>
EOF

...
rc_add <apk1-service> boot
rc_add <apk2-service> boot
...

in the relevant locations.

Then edit the profile build script above and add: 

apkovl="aports/scripts/genapkovl-mkimgoverlay.sh"

immediately after the last apks= line.

Create the ISO

mkimage.sh [--tag RELEASE] [--outdir OUTDIR] [--workdir WORKDIR] [--arch ARCH] [--profile PROFILE] [--hostkeys] [--simulate] [--repository REPO] [--extra-repository REPO] [--yaml FILE] mkimage.sh --help options: --arch Specify which architecture images to build (default: x86_64) --hostkeys Copy system apk signing keys to created images --outdir Specify directory for the created images --profile Specify which profiles to build --repository Package repository to use for the image create --extra-repository Add repository to search packages from --simulate Don't execute commands --tag Build images for tag RELEASE --workdir Specify temporary working directory (cache) --yaml known profiles: ali rpi uboot base minirootfs standard vanilla extended virt xen

Tip: You can use the --repository option multiple times, which is very useful when mixing local and official repositories. The --extra-repository option is there only for backward-compatibility.

Create an iso directory in your home dir:

mkdir -p ~/iso

Then create the actual ISO. In this example we will use the edge version x86_64:

sh aports/scripts/mkimage.sh --tag edge \ --outdir ~/iso \ --arch x86_64 \ --repository https://dl-cdn.alpinelinux.org/alpine/edge/main \ --profile $PROFILENAME


Notes:

  • If you want to make a customized installer, you need to create .default_boot_services which will cause mkinitfs to create the defaults for the live image.
  • Several parts of this doc can be automated with a script, like the repository/arch/outdir settings.

Those steps are left to you and your imagination :)

Testing your ISO image

QEMU is useful for a quick test of your newly created ISO image. This ISO build process has been tested to work and generate an ISO with auto-installed APKs as of Nov 20, 2023 with Alpine edge.

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