Tools for modifying Ubuntu Live ISOs
Project description
A tool to "edit" Ubuntu live server CDs
There are a few reasons the Ubuntu live server ISOs as shipped may not quite be what you want. Here are some examples:
-
You want to make an ISO that does a completely automated install
-
You want packages that are not in the package repository by default to be available during install, even when there is no network (or only a very isolated network).
-
You want to add an argument to the default kernel command line.
-
You want to inject a new version of the subiquity snap for testing.
This script aims to help you making modified versions of the distributed ISOs with changes such as those above.
Some of the things it does probably work on desktop installer ISOs too but I haven't thought very hard about that side of things yet.
Dependencies
This script is pretty Linux-dependent and requires xorriso
and
mksquashfs
to be available on $PATH
. Some actions require the
python3-debian
package to be installed and gpg
command to be
available.
It needs to be run as root (although possibly using FUSE variants for all the mounts would allow it to run as a regular user, maybe).
General invocation
The basic idea behind this tool is that you tell it where to find the source ISO, where to put the modified ISO and a list of actions that make up the modifications. So an invocation always looks somewhat like this:
# livefs-edit $source.iso $dest.iso [actions]
Actions can be specified two ways: on the command line or in a YAML file. Each action has a name and many of them take arguments.
On the command line, actions and arguments are specified like:
--action-name arg1 arg2 --next-action-name
Arguments that are interpreted as boolean interpret 'on', 'yes', 'true' (case insensitively) as true.
Alternatively (if shell quoting starts to get painful), the actions
can be passed as a YAML file, using the --action-yaml
flag:
# livefs-edit $source.iso $dest.iso --action-yaml examples/example.yaml
The YAML file should be a list of mappings. Each mapping names the
actions with name
and lists any arguments by name, for example:
- name: add-cmdline-arg
arg: autoinstall
persist: false
- name: shell
- name: add-packages-to-pool
packages:
- casper
- valgrind
Directory structure
This script does all its work in a temporary directory. Within that
directory the original ISO is mounted at old/iso
and what will be
packed into the new ISO is present at new/iso
(the script uses a lot
of overlayfs
mounts to avoid copying large amounts of data around,
and also only repack things when there are changes).
Many actions require a writable emulation of the root filesystem that
the installer will run in. By default this is created at rootfs
in
the main temporary directory, but this can be customized.
In general, things from the original ISO are mounted in old/
and are
either read-only or should be treated as such. Writable versions for
the new ISO live in new/
(mostly).
Actions
setup-rootfs
argument: target
(default: "rootfs"
)
This action sets up a writable emulation of the root filesystem that
the installer will run in at the directory named by target
. Changes
to this rootfs will be present in the rootfs used by installer on the
modified ISO.
Many actions will do this implicitly but it may be clearer to be
explicit about the target directory name if later shell
or cp
actions refer to paths in the rootfs.
shell
argument: command
(default: null
)
Runs a shell (bash) in the main temporary directory. If command
is
present, this is the command that is run. If not, an interactive shell
is run. If the shell (command or interactive) exits with a non-zero
code, that aborts the run.
cp
argument: source
argument: dest
Copy a file. If source
or dest
are relative they are assumed to be
relative to the main temporary directory. So something like this:
--cp /my/custom/initrd new/iso/casper/initrd
to replace the initrd.
inject-snap
argument: snap
argument: channel
(default: "stable"
)
Inject the passed snap into the rootfs the installer runs in. This is used to test new versions of subiquity. If there is an assert alongside the snap, this will be copied into the ISO too and the snap set up to track the passed channel, otherwise it is installed unasserted.
add-snap-from-store
argument: snap_name
argument: channel
(default: "stable"
)
A wrapper around `--inject-snap`` that downloads the specified snap from the store first.
edit-squashfs
argument: name
argument: add_sys_mounts
(default: true
)
Mount the squashfs named name
at new/{name}
and arrange for it be
repacked if there are any changes before the new ISO is made.
add_sys_mounts
controls whether the usual chroot setup stuff is done
(mounting /dev, /proc/ etc, setting up /etc/resolv.conf).
add-cmdline-arg
argument: arg
argument: persist
(default: true
)
Add an argument to the default kernel command line. If persist
is
true, it will be present on the default kernel command line of the
installed system as well.
add-autoinstall-config
argument: autoinstall_config
Add the provided autoinstall config to the ISO so it is used by default. This also adds "autoinstall" to the default kernel command line.
autoinstall_config
is the path to a YAML file which can contain
either the autoinstall config directly or a cloud-init user-data file
(in which case it can contain other configuration for the live
installer session, such as ssh keys to be used for the installer
user).
add-debs-to-pool
argument: list of deb files
Add the passed deb files to the repository on the CD so that they are available for installation while the installer is running, even if the install is done offline.
This will generate a new Ed25519 GPG key, sign the package repository
on the ISO with it, arrange for the public part to end up in
/etc/apt/trusted.gpg.d/custom-iso-key.gpg
in the installed system,
and then throw the private part away. You should be aware of this
change to the default apt configuration! Deleting this file in an
autoinstall late-command
would be a reasonable thing to do, unless
you want the option of using the ISO as an apt repository later on.
add-packages-to-pool
argument: list of package names
This is a wrapper around add-debs-to-pool
which takes package names
rather than deb files. It downloads the listed packages and any others
needed to satisfy their dependencies from the main Ubuntu archive and
passes them to add-debs-to-pool
. Do read the note about GPG in the
description of that action.
unpack-initrd
argument: target
(default: "new/initrd"
)
Unpack the initrd using unmkinitramfs into target
(contents will
likely end up in subdirectories called things like early
, early2
and main
, at least on amd64) and arrange for these to be repacked
into a replacement initrd for the modified ISO if any changes are made.
Project details
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