docs: add some documentation about hooking up userdb/homed to desktop environments

Prompted by some inquires from the GNOME camp. let's document this here,
since other DEs might be interested too.

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+---
+title: systemd-homed and JSON User/Group Record Support in Desktop Environments
+category: Interfaces
+layout: default
+---
+
+# `systemd-homed` and JSON User/Group Record Support in Desktop Environments
+
+Starting with version 245, systemd supports a new subsystem
+[`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html)
+for managing regular ("human") users and their home directories. Along with it
+a new concept `userdb` got merged that brings rich, extensible JSON user/group
+records, extending the classic UNIX/glibc NSS `struct passwd`/`struct group`
+structures. Both additions are added in a fully backwards compatible way,
+accessible through `getpwnam()`/`getgrnam()`/… (i.e. libc NSS) and PAM as
+usual, meaning that for basic support no changes in the upper layers of the
+stack (in particular desktop environments, such as GNOME or KDE) have to be
+made. However, for better support a number of changes to desktop environments
+are recommended. A few areas where that applies are discussed below.
+
+Before reading on, please read up on the basic concepts, specifically:
+
+* [Home Directories](https://systemd.io/HOME_DIRECTORY)
+* [JSON User Records](https://systemd.io/USER_RECORD)
+* [JSON Group Records](https://systemd.io/GROUP_RECORD)
+* [User/Group Record Lookup API via Varlink](https://systemd.io/USER_GROUP_API)
+
+## Support for Suspending Home Directory Access during System Suspend
+
+One key feature of `systemd-homed` managed encrypted home directories is the
+ability that access to them can be suspended automatically during system sleep,
+removing any cryptographic key material from memory while doing so. This is
+important in a world where most laptop users seldom shut down their computers
+but most of the time just suspend them instead. Previously, the encryption keys
+for the home directories remained in memory during system suspend, so that
+sufficiently equipped attackers could read them from there and gain full access
+to the device. By removing the key material from memory before suspend, and
+re-requesting it on resume this attack vector can be closed down effectively.
+
+Supporting this mechanism requires support in the desktop environment, since
+the encryption keys (i.e. the user's login password) need to be reacquired on
+system resume, from a lock screen or similar. This lock screen must run in
+system context, and cannot run in the user's own context, since otherwise it
+might end up accessing the home directory of the user even though access to it
+is temporarily suspended and thus will hang if attempted.
+
+It is suggested that desktop environments that implement lock screens run them
+from system context, for example by switching back to the display manager, and
+only revert back to the session after re-authentication via this system lock
+screen (re-authentication in this case refers to passing the user's login
+credentials to the usual PAM authentication hooks). Or in other words, when
+going into system suspend it is recommended that GNOME Shell switches back to
+the GNOME Display Manager login screen which now should double as screen lock,
+and only switches back to the shell's UI after the user re-authenticated there.
+
+Note that this change in behavior is a good idea in any case, and does not
+create any dependencies on `systemd-homed` or systemd-specific APIs. It's
+simply a change of behavior regarding use of existing APIs, not a suggested
+hook-up to a any new API.
+
+A display manager which supports this kind of out-of-context screen lock
+operation needs to inform systemd-homed about this so that systemd-homed knows
+that it is safe to suspend the user's home directory on suspend. This is done
+via the `suspend=` argument to the
+[`pam_systemd_home`](https://www.freedesktop.org/software/systemd/man/pam_systemd_home.html)
+PAM module. A display manager should hence change its PAM stack configuration
+to set this parameter to on. `systemd-homed` will not suspend home directories
+if there's at least one active session of the user that does not support
+suspending, as communicated via this parameter.
+
+## User Management UIs
+
+The rich user/group records `userdb` and `systemd-homed` support carry various
+fields of relevance to UIs that manage the local user database or parts
+thereof. In particular, most of the metadata `accounts-daemon` (also see below)
+supports is directly available in these JSON records. Hence it makes sense for
+any user management UI to expose them directly.
+
+`systemd-homed` exposes APIs to add, remove and make changes to local users via
+D-Bus, with full PolicyKit hook-up. On the command line this is exposed via the
+`homectl` command. A graphical UI that exposes similar functionality would be
+very useful, exposing the various new account settings, and in particular
+providing a stream-lined UI for enrolling new-style authentication tokens such
+as PKCS#11/YubiKey-style devices. (Ideally, if the user plugs in an
+uninitialized YubiKey during operation it might be nice if the Desktop would
+automatically ask if a key pair shall be written to it and the local account be
+bound to it, `systemd-homed` provides enough YubiKey/PKCS#11 support to make
+this a reality today; except that it will not take care of token
+initialization).
+
+A strong point of `systemd-homed` is per-user resource management. In
+particular disk space assignments are something that most likely should be
+exposed in a user management UI. Various metadata fields are supplied allowing
+exposure of disk space assignment "slider" UI. Note however that the file system
+back-ends of `systemd-homed.service` have different feature sets. Specifically,
+only btrfs has online file system shrinking support, ext4 only offline file
+system shrinking support, and xfs no shrinking support at all (all three file
+systems support online file system growing however). This means if the LUKS
+back-end is used, disk space assignment cannot be instant for logged in users,
+unless btrfs is used.
+
+Note that only `systemd-homed` provides an API for modifying/creating/deleting
+users. The generic `userdb` subsystem (which might have other back-ends, besides
+`systemd-homed`, for example LDAP or Windows) exclusively provides a read-only
+interface. (This is unlikely to change, as the other back-ends might have very
+different concepts of adding or modifying users, i.e. might not even have any
+local concept for that at all). This means any user management UI that intends
+to change (and not just view) user accounts should talk directly to
+`systemd-homed` to make use of its features; there's no abstraction available
+to support other back-ends under the same API.
+
+Unfortunately there's currently no documentation for the `systemd-homed` D-Bus
+API. Consider using the `homectl` sources as guidelines for implementing a user
+management UI. The JSON user/records are well documented however, see above,
+and the D-Bus API provides limited introspection.
+
+## Relationship to `accounts-daemon`
+
+For a long time `accounts-daemon` has been included in Linux distributions
+providing richer user accounts. The functionality of this daemon overlaps in
+many areas with the functionality of `systemd-homed` or `userdb`, but there are
+systematic differences, which means that `systemd-homed` cannot replace
+`accounts-daemon` fully. Most importantly: `accounts-daemon` provides
+"side-car" metadata for *any* type of user account, while `systemd-homed` only
+provides additional metadata for the users it defines itself.  In other words:
+`accounts-daemon` will augment foreign accounts; `systemd-homed` cannot be used
+to augment users defined elsewhere, for example in LDAP or as classic
+`/etc/passwd` records.
+
+This probably means that for the time being, a user management UI (or other UI)
+that wants to support rich user records with compatibility with the status quo
+ante should probably talk to both `systemd-homed` and `accounts-daemon` at the
+same time, and ignore `accounts-daemon`'s records if `systemd-homed` defines
+them. While I (Lennart) personally believe in the long run `systemd-homed` is
+the way to go for rich user records, any UI that wants to manage and support
+rich records for classic records has to support `accounts-daemon` in parallel
+for the time being.
+
+In the short term, it might make sense to also expose the `userdb` provided
+records via `accounts-daemon`, so that clients of the latter can consume them
+without changes. However, I think in the long run `accounts-daemon` should
+probably be removed from the general stack, hence this sounds like a temporary
+solution only.
+
+In case you wonder, there's no automatic mechanism for converting existing
+users registered in `/etc/passwd` or LDAP to users managed by
+`systemd-homed`. There's documentation for doing this manually though, see
+[Converting Existing Users to systemd-homed managed
+Users](https://systemd.io/CONVERTING_TO_HOMED).
+
+## Future Additions
+
+JSON user/group records are extensible, hence we can easily add any additional
+fields desktop environments require. For example pattern-based authentication
+is likely very useful on touch-based devices, and the user records should hence
+learn them natively. Fields for other authentication mechanisms, such as
+fingerprint authentication should be provided as well, eventually.
+
+It is planned to extend the `userdb` Varlink API to support look-ups by partial
+user name and real name (GECOS) data, so that log-in screens can optionally
+implement simple complete-as-you-type login screens.
+
+It is planned to extend the `systemd-homed` D-Bus API to instantly inform clients
+about hardware associated with a specific user being plugged in, to which login
+screens can listen in order to initiate authentication. Specifically, any
+YubiKey-like security token plugged in that is associated with a local user
+record should initiate authentication for that user, making typing in of the
+username unnecessary.