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2 title: systemd-homed and JSON User/Group Record Support in Desktop Environments
3 category: Users, Groups and Home Directories
4 layout: default
5 SPDX-License-Identifier: LGPL-2.1-or-later
6 ---
8 # `systemd-homed` and JSON User/Group Record Support in Desktop Environments
10 Starting with version 245, systemd supports a new subsystem
11 [`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.service.html)
12 for managing regular ("human") users and their home directories.
13 Along with it a new concept `userdb` got merged that brings rich, extensible JSON user/group
14 records, extending the classic UNIX/glibc NSS `struct passwd`/`struct group` structures.
15 Both additions are added in a fully backwards compatible way, accessible through `getpwnam()`/`getgrnam()`/… (i.e. libc NSS) and PAM as
16 usual, meaning that for basic support no changes in the upper layers of the
17 stack (in particular desktop environments, such as GNOME or KDE) have to be made.
18 However, for better support a number of changes to desktop environments are recommended.
19 A few areas where that applies are discussed below.
21 Before reading on, please read up on the basic concepts, specifically:
23 * [Home Directories](/HOME_DIRECTORY)
24 * [JSON User Records](/USER_RECORD)
25 * [JSON Group Records](/GROUP_RECORD)
26 * [User/Group Record Lookup API via Varlink](/USER_GROUP_API)
28 ## Support for Suspending Home Directory Access during System Suspend
30 One key feature of `systemd-homed` managed encrypted home directories is the
31 ability that access to them can be suspended automatically during system sleep,
32 removing any cryptographic key material from memory while doing so.
33 This is important in a world where most laptop users seldom shut down their computers
34 but most of the time just suspend them instead.
35 Previously, the encryption keys for the home directories remained in memory during system suspend, so that
36 sufficiently equipped attackers could read them from there and gain full access to the device.
37 By removing the key material from memory before suspend, and re-requesting it on resume this attack vector can be closed down effectively.
39 Supporting this mechanism requires support in the desktop environment, since
40 the encryption keys (i.e. the user's login password) need to be reacquired on
41 system resume, from a lock screen or similar.
42 This lock screen must run in system context, and cannot run in the user's own context, since otherwise it
43 might end up accessing the home directory of the user even though access to it
44 is temporarily suspended and thus will hang if attempted.
46 It is suggested that desktop environments that implement lock screens run them
47 from system context, for example by switching back to the display manager, and
48 only revert back to the session after re-authentication via this system lock
49 screen (re-authentication in this case refers to passing the user's login
50 credentials to the usual PAM authentication hooks).
51 Or in other words, when going into system suspend it is recommended that GNOME Shell switches back to
52 the GNOME Display Manager login screen which now should double as screen lock,
53 and only switches back to the shell's UI after the user re-authenticated there.
55 Note that this change in behavior is a good idea in any case, and does not
56 create any dependencies on `systemd-homed` or systemd-specific APIs.
57 It's simply a change of behavior regarding use of existing APIs, not a suggested hook-up to any new APIs.
59 A display manager which supports this kind of out-of-context screen lock
60 operation needs to inform systemd-homed about this so that systemd-homed knows
61 that it is safe to suspend the user's home directory on suspend.
62 This is done via the `suspend=` argument to the
63 [`pam_systemd_home`](https://www.freedesktop.org/software/systemd/man/pam_systemd_home.html)
64 PAM module.
65 A display manager should hence change its PAM stack configurationto set this parameter to on.
66 `systemd-homed` will not suspend home directories if there's at least one active session of the user that does not support
67 suspending, as communicated via this parameter.
69 ## User Management UIs
71 The rich user/group records `userdb` and `systemd-homed` support carry various
72 fields of relevance to UIs that manage the local user database or parts thereof.
73 In particular, most of the metadata `accounts-daemon` (also see below)
74 supports is directly available in these JSON records.
75 Hence it makes sense for any user management UI to expose them directly.
77 `systemd-homed` exposes APIs to add, remove and make changes to local users via
78 D-Bus, with full [polkit](https://www.freedesktop.org/software/polkit/docs/latest/) hook-up.
79 On the command line this is exposed via the `homectl` command. A graphical UI that exposes similar functionality would be
80 very useful, exposing the various new account settings, and in particular
81 providing a stream-lined UI for enrolling new-style authentication tokens such
82 as PKCS#11/YubiKey-style devices.
83 (Ideally, if the user plugs in an uninitialized YubiKey during operation it might be nice if the Desktop would
84 automatically ask if a key pair shall be written to it and the local account be
85 bound to it, `systemd-homed` provides enough YubiKey/PKCS#11 support to make
86 this a reality today; except that it will not take care of token
87 initialization).
89 A strong point of `systemd-homed` is per-user resource management.
90 In particular disk space assignments are something that most likely should be
91 exposed in a user management UI. Various metadata fields are supplied allowing
92 exposure of disk space assignment "slider" UI.
93 Note however that the file system back-ends of `systemd-homed.service` have different feature sets.
94 Specifically, only btrfs has online file system shrinking support, ext4 only offline file
95 system shrinking support, and xfs no shrinking support at all (all three file
96 systems support online file system growing however).
97 This means if the LUKS back-end is used, disk space assignment cannot be instant for logged in users, unless btrfs is used.
99 Note that only `systemd-homed` provides an API for modifying/creating/deleting users.
100 The generic `userdb` subsystem (which might have other back-ends, besides
101 `systemd-homed`, for example LDAP or Windows) exclusively provides a read-only interface.
102 (This is unlikely to change, as the other back-ends might have very
103 different concepts of adding or modifying users, i.e. might not even have any local concept for that at all).
104 This means any user management UI that intends to change (and not just view) user accounts should talk directly to
105 `systemd-homed` to make use of its features; there's no abstraction available
106 to support other back-ends under the same API.
108 Unfortunately there's currently no documentation for the `systemd-homed` D-Bus API.
109 Consider using the `homectl` sources as guidelines for implementing a user management UI.
110 The JSON user/records are well documented however, see above,
111 and the D-Bus API provides limited introspection.
113 ## Relationship to `accounts-daemon`
115 For a long time `accounts-daemon` has been included in Linux distributions
116 providing richer user accounts.
117 The functionality of this daemon overlaps in many areas with the functionality of `systemd-homed` or `userdb`, but there are
118 systematic differences, which means that `systemd-homed` cannot replace
119 `accounts-daemon` fully.
120 Most importantly: `accounts-daemon` provides "side-car" metadata for *any* type of user account, while `systemd-homed` only
121 provides additional metadata for the users it defines itself.
122 In other words: `accounts-daemon` will augment foreign accounts; `systemd-homed` cannot be used
123 to augment users defined elsewhere, for example in LDAP or as classic `/etc/passwd` records.
125 This probably means that for the time being, a user management UI (or other UI)
126 that wants to support rich user records with compatibility with the status quo
127 ante should probably talk to both `systemd-homed` and `accounts-daemon` at the
128 same time, and ignore `accounts-daemon`'s records if `systemd-homed` defines them.
129 While I (Lennart) personally believe in the long run `systemd-homed` is
130 the way to go for rich user records, any UI that wants to manage and support
131 rich records for classic records has to support `accounts-daemon` in parallel
132 for the time being.
134 In the short term, it might make sense to also expose the `userdb` provided
135 records via `accounts-daemon`, so that clients of the latter can consume them
136 without changes. However, I think in the long run `accounts-daemon` should
137 probably be removed from the general stack, hence this sounds like a temporary
138 solution only.
140 In case you wonder, there's no automatic mechanism for converting existing
141 users registered in `/etc/passwd` or LDAP to users managed by `systemd-homed`.
142 There's documentation for doing this manually though, see
143 [Converting Existing Users to systemd-homed managed Users](/CONVERTING_TO_HOMED).
145 ## Future Additions
147 JSON user/group records are extensible, hence we can easily add any additional fields desktop environments require.
148 For example, pattern-based authentication is likely very useful on touch-based devices,
149 and the user records should hence learn them natively.
150 Fields for other authentication mechanisms, such as fingerprint authentication should be provided as well, eventually.
152 It is planned to extend the `userdb` Varlink API to support look-ups by partial
153 user name and real name (GECOS) data, so that log-in screens can optionally
154 implement simple complete-as-you-type login screens.
156 It is planned to extend the `systemd-homed` D-Bus API to instantly inform clients
157 about hardware associated with a specific user being plugged in, to which login
158 screens can listen in order to initiate authentication.
159 Specifically, any YubiKey-like security token plugged in that is associated with a local user
160 record should initiate authentication for that user, making typing in of the
161 username unnecessary.