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| -rw-r--r-- | docs/BUILDING_IMAGES.md | 139 |
1 files changed, 70 insertions, 69 deletions
diff --git a/docs/BUILDING_IMAGES.md b/docs/BUILDING_IMAGES.md index b11afa3..dcae4bb 100644 --- a/docs/BUILDING_IMAGES.md +++ b/docs/BUILDING_IMAGES.md @@ -9,12 +9,14 @@ SPDX-License-Identifier: LGPL-2.1-or-later In many scenarios OS installations are shipped as pre-built images, that require no further installation process beyond simple `dd`-ing the image to -disk and booting it up. When building such "golden" OS images for +disk and booting it up. +When building such "golden" OS images for `systemd`-based OSes a few points should be taken into account. Most of the points described here are implemented by the [`mkosi`](https://github.com/systemd/mkosi) OS image builder developed and -maintained by the systemd project. If you are using or working on another image +maintained by the systemd project. +If you are using or working on another image builder it's recommended to keep the following concepts and recommendations in mind. @@ -24,28 +26,25 @@ Typically the same OS image shall be deployable in multiple instances, and each instance should automatically acquire its own identifying credentials on first boot. For that it's essential to: -1. Remove the - [`/etc/machine-id`](https://www.freedesktop.org/software/systemd/man/machine-id.html) - file or write the string `uninitialized\n` into it. This file is supposed to - carry a 128-bit identifier unique to the system. Only when it is reset it - will be auto-generated on first boot and thus be truly unique. If this file - is not reset, and carries a valid ID every instance of the system will come +1. Remove the [`/etc/machine-id`](https://www.freedesktop.org/software/systemd/man/machine-id.html) + file or write the string `uninitialized\n` into it. + This file is supposed to carry a 128-bit identifier unique to the system. + Only when it is reset it will be auto-generated on first boot and thus be truly unique. + If this file is not reset, and carries a valid ID every instance of the system will come up with the same ID and that will likely lead to problems sooner or later, - as many network-visible identifiers are commonly derived from the machine - ID, for example, IPv6 addresses or transient MAC addresses. + as many network-visible identifiers are commonly derived from the machine ID, + for example, IPv6 addresses or transient MAC addresses. -2. Remove the `/var/lib/systemd/random-seed` file (see +2. Remove the `/var/lib/systemd/random-seed` file(see [`systemd-random-seed(8)`](https://www.freedesktop.org/software/systemd/man/systemd-random-seed.service.html)), - which is used to seed the kernel's random pool on boot. If this file is - shipped pre-initialized, every instance will seed its random pool with the + which is used to seed the kernel's random pool on boot. + If this file is shipped pre-initialized, every instance will seed its random pool with the same random data that is included in the image, and thus possibly generate - random data that is more similar to other instances booted off the same - image than advisable. + random data that is more similar to other instances booted off the same image than advisable. 3. Remove the `/loader/random-seed` file (see [`systemd-boot(7)`](https://www.freedesktop.org/software/systemd/man/systemd-boot.html)) - from the UEFI System Partition (ESP), in case the `systemd-boot` boot loader - is used in the image. + from the UEFI System Partition (ESP), in case the `systemd-boot` boot loader is used in the image. 4. It might also make sense to remove [`/etc/hostname`](https://www.freedesktop.org/software/systemd/man/hostname.html) @@ -69,24 +68,25 @@ The logic used to generate [Boot Loader Specification Type #1](https://uapi-group.org/specifications/specs/boot_loader_specification/#type-1-boot-loader-specification-entries) entries by default uses the machine ID as stored in `/etc/machine-id` for -naming boot menu entries and the directories in the ESP to place kernel images -in. This is done in order to allow multiple installations of the same OS on the +naming boot menu entries and the directories in the ESP to place kernel images in. +This is done in order to allow multiple installations of the same OS on the same system without conflicts. However, this is problematic if the machine ID shall be generated automatically on first boot: if the ID is not known before the first boot it cannot be used to name the most basic resources required for the boot process to complete. Thus, for images that shall acquire their identity on first boot only, it is -required to use a different identifier for naming boot menu entries. To allow -this the `kernel-install` logic knows the generalized *entry* *token* concept, +required to use a different identifier for naming boot menu entries. +To allow this the `kernel-install` logic knows the generalized *entry* *token* concept, which can be a freely chosen string to use for identifying the boot menu -resources of the OS. If not configured explicitly it defaults to the machine -ID. The file `/etc/kernel/entry-token` may be used to configure this string -explicitly. Thus, golden image builders should write a suitable identifier into +resources of the OS. +If not configured explicitly it defaults to the machineID. +The file `/etc/kernel/entry-token` may be used to configure this string explicitly. +Thus, golden image builders should write a suitable identifier into this file, for example, the `IMAGE_ID=` or `ID=` field from [`/etc/os-release`](https://www.freedesktop.org/software/systemd/man/os-release.html) -(also see below). It is recommended to do this before the `kernel-install` -functionality is invoked (i.e. before the package manager is used to install +(also see below). +It is recommended to do this before the `kernel-install` functionality is invoked (i.e. before the package manager is used to install packages into the OS tree being prepared), so that the selected string is automatically used for all entries to be generated. @@ -94,16 +94,16 @@ automatically used for all entries to be generated. `systemd` is designed to be able to come up safely and robustly if the `/var/` file system or even the entire root file system (with exception of `/usr/`, -i.e. the vendor OS resources) is empty (i.e. "unpopulated"). With this in mind -it's relatively easy to build images that only ship a `/usr/` tree, and +i.e. the vendor OS resources) is empty (i.e. "unpopulated"). +With this in mind it's relatively easy to build images that only ship a `/usr/` tree, and otherwise carry no other data, populating the rest of the directory hierarchy on first boot as needed. Specifically, the following mechanisms are in place: 1. The `switch-root` logic in systemd, that is used to switch from the initrd - phase to the host will create the basic OS hierarchy skeleton if missing. It - will create a couple of directories strictly necessary to boot up + phase to the host will create the basic OS hierarchy skeleton if missing. + It will create a couple of directories strictly necessary to boot up successfully, plus essential symlinks (such as those necessary for the dynamic loader `ld.so` to function). @@ -136,14 +136,18 @@ Specifically, the following mechanisms are in place: remains resolvable, even without `/etc/hosts` around. With these mechanisms the hierarchies below `/var/` and `/etc/` can be safely -and robustly populated on first boot, so that the OS can safely boot up. Note -that some auxiliary package are not prepared to operate correctly if their +and robustly populated on first boot, so that the OS can safely boot up. +Note that some auxiliary package are not prepared to operate correctly if their configuration data in `/etc/` or their state directories in `/var/` are -missing. This can typically be addressed via `systemd-tmpfiles` lines that -ensure the missing files and directories are created if missing. In particular, -configuration files that are necessary for operation can be automatically +missing. + +This can typically be addressed via `systemd-tmpfiles` lines that +ensure the missing files and directories are created if missing. +In particular, configuration files that are necessary for operation can be automatically copied or symlinked from the `/usr/share/factory/etc/` tree via the `C` or `L` -line types. That said, we recommend that all packages safely fall back to +line types. + +That said, we recommend that all packages safely fall back to internal defaults if their configuration is missing, making such additional steps unnecessary. @@ -156,17 +160,17 @@ manual work might be required to make this scenario work. Typically, if an image is `dd`-ed onto a target disk it will be minimal: i.e. only consist of necessary vendor data, and lack "payload" data, that shall -be individual to the system, and dependent on host parameters. On first boot, -the OS should take possession of the backing storage as necessary, dynamically +be individual to the system, and dependent on host parameters. +On first boot, the OS should take possession of the backing storage as necessary, dynamically using available space. Specifically: 1. Additional partitions should be created, that make no sense to ship - pre-built in the image. For example, `/tmp/` or `/home/` partitions, or even - `/var/` or the root file system (see above). + pre-built in the image. + For example, `/tmp/` or `/home/` partitions, or even `/var/` or the root file system (see above). 2. Additional partitions should be created that shall function as A/B - secondaries for partitions shipped in the original image. In other words: if - the `/usr/` file system shall be updated in an A/B fashion it typically + secondaries for partitions shipped in the original image. + In other words: if the `/usr/` file system shall be updated in an A/B fashion it typically makes sense to ship the original A file system in the deployed image, but create the B partition on first boot. @@ -191,10 +195,10 @@ it, then format it. 1. The [`systemd-repart(8)`](https://www.freedesktop.org/software/systemd/man/systemd-repart.service.html) component may manipulate GPT partition tables automatically on boot, growing - partitions or adding in partitions taking the backing storage size into - account. It can also encrypt partitions automatically it creates (even bind - to TPM2, automatically) and populate partitions from various sources. It - does this all in a robust fashion so that aborted invocations will not leave + partitions or adding in partitions taking the backing storage size into account. + It can also encrypt partitions automatically it creates (even bind + to TPM2, automatically) and populate partitions from various sources. + It does this all in a robust fashion so that aborted invocations will not leave incompletely set up partitions around. 2. The @@ -215,8 +219,8 @@ it, then format it. While a lot of work has gone into ensuring `systemd` systems can safely boot with unpopulated `/etc/` trees, it sometimes is desirable to set a couple of -basic settings *after* `dd`-ing the image to disk, but *before* first boot. For -this the tool +basic settings *after* `dd`-ing the image to disk, but *before* first boot. +For this the tool [`systemd-firstboot(1)`](https://www.freedesktop.org/software/systemd/man/systemd-firstboot.html) can be useful, with its `--image=` switch. It may be used to set very basic settings, such as the root password or hostname on an OS disk image or @@ -225,36 +229,33 @@ installed block device. ## Distinguishing First Boot For various purposes it's useful to be able to distinguish the first boot-up of -the system from later boot-ups (for example, to set up TPM hardware -specifically, or register a system somewhere). `systemd` provides mechanisms to -implement that. Specifically, the `ConditionFirstBoot=` and `AssertFirstBoot=` -settings may be used to conditionalize units to only run on first boot. See -[`systemd.unit(5)`](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#ConditionFirstBoot=) +the system from later boot-ups (for example, to set up TPM hardware specifically, or register a system somewhere). +`systemd` provides mechanisms to implement that. +Specifically, the `ConditionFirstBoot=` and `AssertFirstBoot=` settings may be used to conditionalize units to only run on first boot. +See [`systemd.unit(5)`](https://www.freedesktop.org/software/systemd/man/systemd.unit.html#ConditionFirstBoot=) for details. A special target unit `first-boot-complete.target` may be used as milestone to -safely handle first boots where the system is powered off too early: if the -first boot process is aborted before this target is reached, the following boot -process will be considered a first boot, too. Once the target is reached, -subsequent boots will not be considered first boots anymore, even if the boot -process is aborted immediately after. Thus, services that must complete fully -before a system shall be considered fully past the first boot should be ordered -before this target unit. +safely handle first boots where the system is powered off too early: +if the first boot process is aborted before this target is reached, the following boot +process will be considered a first boot, too. +Once the target is reached, subsequent boots will not be considered first boots anymore, even if the boot +process is aborted immediately after. +Thus, services that must complete fully before a system shall be considered fully past the first boot should be ordered before this target unit. Whether a system will come up in first boot state or not is derived from the -initialization status of `/etc/machine-id`: if the file already carries a valid -ID the system is already past the first boot. If it is not initialized yet it -is still considered in the first boot state. For details see -[`machine-id(5)`](https://www.freedesktop.org/software/systemd/man/machine-id.html). +initialization status of `/etc/machine-id`: +if the file already carries a valid ID the system is already past the first boot. +If it is not initialized yet it is still considered in the first boot state. +For details see [`machine-id(5)`](https://www.freedesktop.org/software/systemd/man/machine-id.html). ## Image Metadata Typically, when operating with golden disk images it is useful to be able to -identify them and their version. For this the two fields `IMAGE_ID=` and -`IMAGE_VERSION=` have been defined in -[`os-release(5)`](https://www.freedesktop.org/software/systemd/man/os-release.html). These -fields may be accessed from unit files and similar via the `%M` and `%A` -specifiers. +identify them and their version. +For this the two fields `IMAGE_ID=` and `IMAGE_VERSION=` have been defined in +[`os-release(5)`](https://www.freedesktop.org/software/systemd/man/os-release.html). +These fields may be accessed from unit files and similar via the `%M` and `%A` specifiers. Depending on how the images are put together it might make sense to leave the OS distribution's `os-release` file as is in `/usr/lib/os-release` but to |