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Update custom installation docs
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Signed-off-by: David Cassany <[email protected]>
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Elemental installed OS images can be customized in different ways.

One option is to remaster container OS images by simply using a docker build.
SLE Micro images are regular container images, so it is absolutely possible to create
SL Micro images are regular container images, so it is absolutely possible to create
a new image using a Dockerfile based on SLE Micro. See [Build Custom OS Images](/custom-images.md)
section for further details on that possibility.
section for further details, this is the preferred option.

Alternatively, it is also possible to provide additional resources within the installation
media so that during installation, or eventually at boot time, additional binaries such as
drivers or extra configuration files can be included.
Alternatively, it is also possible to provide additional resources and configuration
within the installation media so that during installation, or eventually at boot time,
additional binaries such as drivers or extra configuration files can be included.

This section focuses on how to customize the installation process from a given OS image.

## Customize installation ISO and installation process
## Customization options

In order to adapt the installation ISO a simple approach is to append extra configuration
files into the ISO root in an analog way the registration yaml configuration file
is added.
Elemental installation can be customized in three different non-exclusive ways. First,
including additional cloud-config files into the installed system, second, by including
additional cloud-config file into the installation media, and finally, by adding a custom
Elemental client configuration file (`/etc/elemental/config.yaml`).

### Common customization pattern
1. The additional cloud-config files included into the installed system are useful to run
custom operations at boot time. See the [Cloud Config Reference](cloud-config-reference.md).

Elemental installation can be customized in three different non-exclusive ways. First, including
some custom Elemental client configuration file, second, by including additional cloud-init files to execute at
boot time, and finally, by including `cloud-init` files such as installation hooks or boot stages evaluated during
the live system boot itself.
2. The additional cloud-config files into the installation media are useful to run custom operations
at install time or to customize the installation environment to match certain specific needs.

A common pattern is to combine the three ways described above. This pattern will allow you to add custom steps during the installation and add `cloud-init` files to be evaluated at boot time.
3. A custom Elemental client configuration file is, by default, located at `/etc/elemental/config.yaml`
and it can also be splitted into multiple yaml files under `/etc/elemental/config.d` directory.
See the configuration file
[reference](https://rancher.github.io/elemental-toolkit/docs/customizing/general_configuration/).

Additional config files can be added dynamically boot time by generating the ISO via a [SeedImage](/seedimage-reference.md) including custom cloud-config data.
A common pattern is to combine the three ways described above depending on the specific needs.

To apply this pattern, the following files need to be included in the ISO or generated at boot time:
### Adding additional cloud-config files within the installed OS

1. A [configuration file](https://rancher.github.io/elemental-toolkit/docs/customizing/general_configuration/)
for the elemental client. The file must be named `config.yaml` and located by default in `/etc/elemental`.
This path can be configured as part of the installation parameters of a [MachineRegistration](/machineregistration-reference) resource.
In order to include additional cloud-config files during the installation they need
to be added to the installation data into the MachineRegistration resource. The most simple
way to achieve it is by adding the desired cloud-config directly as part of the MachineRegistration [devoted
section](machineregistration-reference.md#configcloud-config). See the example below

2. The additional `cloud-init` files to be included into the installed system. They
allow to perform custom operations at boot time.

3. The installation hooks are evalutated at install time. They allow to perform custom operations
during the installation process (include extra software, set additional disks...). The same
way `cloud-init` files can be included to perform custom operations during the live installation
media boot time.

#### Custom Elemental client configuration file

[Elemental client](https://github.com/rancher/elemental-toolkit/blob/main/docs/elemental.md) `install`, `upgrade` and `reset` commands can be configured with a [custom configuration file](https://rancher.github.io/elemental-toolkit/docs/customizing/general_configuration/) located by default in `/etc/elemental/config.yaml`.
If you have multiple yaml files, you need to add them in the `/etc/elemental/config.d` directory.

A simple example to set hooks location could be:

```yaml
extra-partitions:
- size: 10240
fs: ext4
label: EXTRA_PARTITION
```yaml showLineNumbers
apiVersion: elemental.cattle.io/v1beta1
kind: MachineRegistration
metadata:
name: my-nodes
namespace: fleet-default
spec:
...
config:
...
cloud-config:
stages:
boot:
- name: "Setting root user password"
users:
root:
passwd: root
```
The above example sets an additional extra partition during the installation.
#### Adding additional cloud-init files within the installed OS
In order to include additional cloud-init files during the installation they need
to be added to the installation data into the MachineRegistration resource. More specific
the `config-urls` field is used for this exact purpose. See [MachineRegistration reference](/machineregistration-reference) page.
Alternatively, cloud-config files path can also be referenced explicitly so the configuration is not
extrictly needed to live within the MachineRegistration resource itself. The `config-urls` section of the
MachineRegistration is used for this exact purpose. See
[MachineRegistration reference](/machineregistration-reference) page.

`config-urls` is a list of string literals where each item is an HTTP URL or a local path pointing to a
cloud-init file. The local path is evaluated during
cloud-config file. The local path is evaluated during
the installation, hence it must exists within the installation media, commonly an ISO image.

By default, Elemental live systems mount the ISO root at `/run/initramfs/live` which is also the default path set for `config-url` in `MachineRegistrations`:
Expand Down Expand Up @@ -106,165 +105,83 @@ According to that, the ISO for the example above is expected to include the `/oe
`/run/initramfs/live` is a readonly mountpoint and it's not an appropriate path for dynamically generated content at ISO boot.
:::

#### Adding installation hooks or boot stages for the live system

[Elemental client](https://github.com/rancher/elemental-cli) `install`, `upgrade` and `reset` procedures include four different hooks:

* `before-install`: executed after all partition mountpoints are set.
* `after-install-chroot`: executed after deploying the OS image and before unmounting the associated loop filesystem image. Runs chrooted to the OS image.
* `after-install`: executed just after the after-install-chroot hook. It is not chrooted.
* `post-install`: executed as the very last step before ending the installation, partitions are still mounted, the loop devices for the image is not.

Hooks are provided as `cloud-init` stages. Equivalent hooks exist for `reset` and `upgrade` procedures.
They are loaded from the `/iso-config` folder in ISO filesystem root. In fact, hooks are regular `cloud-init` stages with the
only difference that Elemental client parses them during `install`, `upgrade` or `reset` actions, rather than boot time.
Note any boot stage included this way will be executed during the live installation media boot.

Hooks are evaluated during `install`,`reset` and `upgrade` action from `/oem`, `/system/oem` and `/usr/local/cloud-config` paths,
however for the live ISOs there is an additional the path `/run/initramfs/live/iso-config` included. Note the `/run/initramfs/live`
prefix is the mount point of the ISO filesystem of the Elemental Live ISO once booted.

### Adding extra driver binaries into the ISO example

This example is covering the case in which extra driver binaries are included into the ISO
and during the installation they are installed over the OS image.
### Adding additional cloud-config files within the installation media

For that use case the following files are required:
Adding additional cloud-config files within he installtion media might be required to configure the
installation environment (e.g. setting the network connectivity to register) or to provide some
[installation hooks](cloud-config-reference.md#elemental-client-cloud-config-hooks) to run custom
logic during the installation itself.

* additional binaries to install (they could be in the form of RPMs)
* additional hooks file to copy binaries into the persistent storage and to install them

Let's create an `overlay` directory to create the directory tree that needs to be
added into the ISO root. In that case the `overlay` directory could contain:

```yaml showLineNumbers
overlay/
data/
extra_drivers/
some_driver.rpm
iso-config/
install_hooks.yaml
```

The `overlay/iso-config/install_hooks.yaml` could be as:
In Elemental the [SeedImage](seedimage-reference.md) resources are the responsibles of handling the installation
media. So the simplest place to include additional cloud-config data is within the installation media is by
adding it to the [`cloud-config` section](seedimage-reference.md#seedimagespec-reference). By doing so the
given cloud-config will be already evaluated form the very first boot of the installation media and also
honored during the installation phase in case some install hook is provided. Consider the following example:

```yaml showLineNumbers
name: "Install extra drivers"
stages:
before-install:
# Preload data to the persistent storage
# During installation persistent partition is mounted at /run/cos/persistent
- commands:
- rsync -a /run/initramfs/live/data/ /run/cos/persistent
after-install-chroot:
# extra_drivers folder is at `/usr/local/extra_drivers` from the OS image chroot
- commands:
- rpm -iv /usr/local/extra_drivers/some_driver.rpm
apiVersion: elemental.cattle.io/v1beta1
kind: SeedImage
metadata:
name: custom-seed
namespace: fleet-default
spec:
...
cloud-config:
stages:
post-install:
- name: "Run custom script after installation"
commands:
- |
echo "This is a custom script"
echo "For instance, this could be used to handle extra drives for an LVM group"
boot:
- name: "Add proxy setup for the installation media"
files:
- path: /etc/sysconfig/proxy
permissions: 0664
content: |
PROXY_ENABLED="yes"
HTTP_PROXY=http://<MY_PROXY>:<MY_PORT>
HTTPS_PROXY=https://<MY_PROXY>:<MY_PORT>
NO_PROXY="localhost, 127.0.0.1"
```

Note the installation hooks only cover installation procedures, for upgrades equivalent
`before-upgrade` and/or `after-upgrade-chroot` should be defined.

### Adding extra LVM volume group disks during the installation

This example is covering the setup of an host with multiple disks and some of them used
as part of an LVM setup.

As an example, we have an host with three disks (`/dev/sda`, `/dev/sdb`
and `/dev/sdc`).

The first disk is used for a regular Elemental installation
and the other remaining two are used as part of a LVM group where arbitrary logical volumes
are created, formatted and mounted at boot time via an extended `fstab` file.

For this example, the following files are required:

* additional `clout-init` files included in the installed system
* additional installation hooks to prepare the LVM volumes during the installation

Let's create an `overlay` directory to create the directory tree that needs to be
added into the ISO root. In that case the `overlay` directory could contain:
### Custom Elemental client configuration file

```yaml showLineNumbers
overlay/
oem/
lvm_volumes_in_fstab.yaml
iso-config/
lvm_volumes_hook.yaml
```
[Elemental client](https://github.com/rancher/elemental-toolkit/blob/main/docs/elemental.md) `install`, `upgrade` and `reset` commands can be configured with a [custom configuration file](https://rancher.github.io/elemental-toolkit/docs/customizing/general_configuration/) located by default in `/etc/elemental/config.yaml`.
If you have multiple yaml files, you need to add them in the `/etc/elemental/config.d` directory.

The installation hook `overlay/iso-config/lvm_volumes_hook.yaml`:
A simple example to set hooks location could be:

```yaml showLineNumbers
name: "Create LVM logic volumes over some physical disks"
stages:
post-install:
- name: "Create physical volume, volume group and logical volumes"
if: '[ -e "/dev/sdb" ] && [ -e "/dev/sdc" ]'
commands:
- |
# Create the physical volume, volume group and logical volumes
pvcreate /dev/sdb /dev/sdc
vgcreate elementalLVM /dev/sdb /dev/sdc
lvcreate -L 8G -n elementalVol1 elementalLVM
lvcreate -l 100%FREE -n elementalVol2 elementalLVM
# Trigger udev detection
if [ ! -e "/dev/elementalLVM/elementalVol1" ] || [ ! -e "/dev/elementalLVM/elementalVol2" ]; then
sleep 10
udevadm settle
fi
# Ensure devices are already available
[ -e "/dev/elementalLVM/elementalVol1" ] || exit 1
[ -e "/dev/elementalLVM/elementalVol2" ] || exit 1
# Format logical volumes with a known label for later use in fstab
mkfs.xfs -L eVol1 /dev/elementalLVM/elementalVol1
mkfs.xfs -L eVol2 /dev/elementalLVM/elementalVol2
extra-partitions:
- size: 10240
fs: ext4
label: EXTRA_PARTITION
```

The LVM devices are created and formatted as desired. This is a good
example of an installation hook, as this setup is only needed once, at installation
time. As an alternative, the same action could be done on first boot, however it would
require a more sophisticated logic to ensure it's only applied once at first boot.

Finally, the boot time `cloud-init` files contain the mount points settings and trigger the
action of mounting those mountpoints. The Elemental OS `fstab` file is ephemeral and it's
dynamically created at boot time. That's why it doesn't exist during the installation and
can't be used in an installation hook.

Here's an example of `overlay/oem/lvm_volumes_in_fstab.yaml`:
The above example sets an additional extra partition during the installation. In order to make it available at
at installation time this could be done my adding the extra file as part of the SeedImage resource cloud-config
as it is described in the previous section of this page. Consider the example:

```yaml showLineNumbers
name: "Mount LVM volumes"
stages:
initramfs:
- name: "Extend fstab to mount LVM logical volumes at boot"
commands:
- |
echo "LABEL=eVol1 /usr/local/eVol1 xfs defaults 0 0" >> /etc/fstab
echo "LABEL=eVol2 /usr/local/eVol2 xfs defaults 0 0" >> /etc/fstab
```

:::note
The `initramfs` stage is the last stage before switching to the actual root tree.
At this stage, the `/etc/fstab` file already exists and can be adapted before
switching root. Once running in the final root tree, SystemD will handle the rest of the initialization and apply it.
:::

This cloud-init file should be included into the `/oem` directory on the installed
system. `/oem` is a mount point for the OEM partition. In order to include extra files,
they should be listed as `config-urls` within the Registration CRD at the
management cluster.

### Repacking the ISO image with extra files

Assuming an `overlay` folder was created in the current directory containing all
additional files to be appended, the following `xorriso` command adds the extra files:

```bash showLineNumbers
xorriso -indev elemental.x86_64.iso -outdev elemental.custom.x86_64.iso -map overlay / -boot_image any replay
apiVersion: elemental.cattle.io/v1beta1
kind: SeedImage
metadata:
name: custom-seed
namespace: fleet-default
spec:
...
cloud-config:
stages:
boot:
- name: "Add Elemental client configuration file"
files:
- path: /etc/elemental/config.d/extra-partition.yaml
permissions: 0664
content: |
extra-partitions:
- size: 10240
fs: ext4
label: EXTRA_PARTITION
```

For that a `xorriso` equal or higher than version 1.5 is required.
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