In some places, "<n> bits" is used when more appropriate.
19 KiB
title | category | layout | SPDX-License-Identifier |
---|---|---|---|
Users, Groups, UIDs and GIDs on systemd Systems | Users, Groups and Home Directories | default | LGPL-2.1-or-later |
Users, Groups, UIDs and GIDs on systemd Systems
Here's a summary of the requirements systemd
(and Linux) make on UID/GID
assignments and their ranges.
Note that while in theory UIDs and GIDs are orthogonal concepts they really aren't IRL. With that in mind, when we discuss UIDs below it should be assumed that whatever we say about UIDs applies to GIDs in mostly the same way, and all the special assignments and ranges for UIDs always have mostly the same validity for GIDs too.
Special Linux UIDs
In theory, the range of the C type uid_t
is 32-bit wide on Linux,
i.e. 0…4294967295. However, four UIDs are special on Linux:
-
0 → The
root
super-user -
65534 → The
nobody
UID, also called the "overflow" UID or similar. It's where various subsystems map unmappable users to, for example file systems only supporting 16-bit UIDs, NFS or user namespacing. (The latter can be changed with a sysctl during runtime, but that's not supported onsystemd
. If you do change it you void your warranty.) Because Fedora is a bit confused thenobody
user is callednfsnobody
there (and they have a differentnobody
user at UID 99). I hope this will be corrected eventually though. (Also, some distributions call thenobody
groupnogroup
. I wish they didn't.) -
4294967295, aka "32-bit
(uid_t) -1
" → This UID is not a valid user ID, assetresuid()
,chown()
and friends treat -1 as a special request to not change the UID of the process/file. This UID is hence not available for assignment to users in the user database. -
65535, aka "16-bit
(uid_t) -1
" → Before Linux kernel 2.4uid_t
used to be 16-bit, and programs compiled for that would hence assume that(uid_t) -1
is 65535. This UID is hence not usable either.
The nss-systemd
glibc NSS module will synthesize user database records for
the UIDs 0 and 65534 if the system user database doesn't list them. This means
that any system where this module is enabled works to some minimal level
without /etc/passwd
.
Special Distribution UID ranges
Distributions generally split the available UID range in two:
-
1…999 → System users. These are users that do not map to actual "human" users, but are used as security identities for system daemons, to implement privilege separation and run system daemons with minimal privileges.
-
1000…65533 and 65536…4294967294 → Everything else, i.e. regular (human) users.
Note that most distributions allow changing the boundary between system and
regular users, even during runtime as user configuration. Moreover, some older
systems placed the boundary at 499/500, or even 99/100. In systemd
, the
boundary is configurable only during compilation time, as this should be a
decision for distribution builders, not for users. Moreover, we strongly
discourage downstreams to change the boundary from the upstream default of
999/1000.
Also note that programs such as adduser
tend to allocate from a subset of the
available regular user range only, usually 1000..60000. And it's also usually
user-configurable, too.
Note that systemd requires that system users and groups are resolvable without networking available — a requirement that is not made for regular users. This means regular users may be stored in remote LDAP or NIS databases, but system users may not (except when there's a consistent local cache kept, that is available during earliest boot, including in the initrd).
Special systemd
GIDs
systemd
defines no special UIDs beyond what Linux already defines (see
above). However, it does define some special group/GID assignments, which are
primarily used for systemd-udevd
's device management. The precise list of the
currently defined groups is found in this sysusers.d
snippet:
basic.conf
It's strongly recommended that downstream distributions include these groups in their default group databases.
Note that the actual GID numbers assigned to these groups do not have to be
constant beyond a specific system. There's one exception however: the tty
group must have the GID 5. That's because it must be encoded in the devpts
mount parameters during earliest boot, at a time where NSS lookups are not
possible. (Note that the actual GID can be changed during systemd
build time,
but downstreams are strongly advised against doing that.)
Special systemd
UID ranges
systemd
defines a number of special UID ranges:
-
60001…60513 → UIDs for home directories managed by
systemd-homed.service(8)
. UIDs from this range are automatically assigned to any home directory discovered, and persisted locally on first login. On different systems the same user might get different UIDs assigned in case of conflict, though it is attempted to make UID assignments stable, by deriving them from a hash of the user name. -
61184…65519 → UIDs for dynamic users are allocated from this range (see the
DynamicUser=
documentation insystemd.exec(5)
). This range has been chosen so that it is below the 16-bit boundary (i.e. below 65535), in order to provide compatibility with container environments that assign a 64K range of UIDs to containers using user namespacing. This range is above the 60000 boundary, so that its allocations are unlikely to be affected byadduser
allocations (see above). And we leave some room upwards for other purposes. (And if you wonder why precisely these numbers: if you write them in hexadecimal, they might make more sense: 0xEF00 and 0xFFEF). Thenss-systemd
module will synthesize user records implicitly for all currently allocated dynamic users from this range. Thus, NSS-based user record resolving works correctly without those users being in/etc/passwd
. -
524288…1879048191 → UID range for
systemd-nspawn
's automatic allocation of per-container UID ranges. When the--private-users=pick
switch is used (or-U
) then it will automatically find a so far unused 16-bit subrange of this range and assign it to the container. The range is picked so that the upper 16-bit of the 32-bit UIDs are constant for all users of the container, while the lower 16-bit directly encode the 65536 UIDs assigned to the container. This mode of allocation means that the upper 16-bit of any UID assigned to a container are kind of a "container ID", while the lower 16-bit directly expose the container's own UID numbers. If you wonder why precisely these numbers, consider them in hexadecimal: 0x00080000…0x6FFFFFFF. This range is above the 16-bit boundary. Moreover it's below the 31-bit boundary, as some broken code (specifically: the kernel'sdevpts
file system) erroneously considers UIDs signed integers, and hence can't deal with values above 2^31. Thesystemd-machined.service
service will synthesize user database records for all UIDs assigned to a running container from this range.
Note for both allocation ranges: when an UID allocation takes place NSS is
checked for collisions first, and a different UID is picked if an entry is
found. Thus, the user database is used as synchronization mechanism to ensure
exclusive ownership of UIDs and UID ranges. To ensure compatibility with other
subsystems allocating from the same ranges it is hence essential that they
ensure that whatever they pick shows up in the user/group databases, either by
providing an NSS module, or by adding entries directly to /etc/passwd
and
/etc/group
. For performance reasons, do note that systemd-nspawn
will only
do an NSS check for the first UID of the range it allocates, not all 65536 of
them. Also note that while the allocation logic is operating, the glibc
lckpwdf()
user database lock is taken, in order to make this logic race-free.
Figuring out the system's UID boundaries
The most important boundaries of the local system may be queried with
pkg-config
:
$ pkg-config --variable=systemuidmax systemd
999
$ pkg-config --variable=dynamicuidmin systemd
61184
$ pkg-config --variable=dynamicuidmax systemd
65519
$ pkg-config --variable=containeruidbasemin systemd
524288
$ pkg-config --variable=containeruidbasemax systemd
1878982656
(Note that the latter encodes the maximum UID base systemd-nspawn
might
pick — given that 64K UIDs are assigned to each container according to this
allocation logic, the maximum UID used for this range is hence
1878982656+65535=1879048191.)
Systemd has compile-time default for these boundaries. Using those defaults is
recommended. It will nevertheless query /etc/login.defs
at runtime, when
compiled with -Dcompat-mutable-uid-boundaries=true
and that file is present.
Support for this is considered only a compatibility feature and should not be
used except when upgrading systems which were created with different defaults.
Considerations for container managers
If you hack on a container manager, and wonder how and how many UIDs best to assign to your containers, here are a few recommendations:
-
Definitely, don't assign less than 65536 UIDs/GIDs. After all the
nobody
user has magic properties, and hence should be available in your container, and given that it's assigned the UID 65534, you should really cover the full 16-bit range in your container. Note that systemd will — as mentioned — synthesize user records for thenobody
user, and assumes its availability in various other parts of its codebase, too, hence assigning fewer users means you lose compatibility with running systemd code inside your container. And most likely other packages make similar restrictions. -
While it's fine to assign more than 65536 UIDs/GIDs to a container, there's most likely not much value in doing so, as Linux distributions won't use the higher ranges by default (as mentioned neither
adduser
norsystemd
's dynamic user concept allocate from above the 16-bit range). Unless you actively care for nested containers, it's hence probably a good idea to allocate exactly 65536 UIDs per container, and neither less nor more. A pretty side-effect is that by doing so, you expose the same number of UIDs per container as Linux 2.2 supported for the whole system, back in the days. -
Consider allocating UID ranges for containers so that the first UID you assign has the lower 16-bits all set to zero. That way, the upper 16-bits become a container ID of some kind, while the lower 16-bits directly encode the internal container UID. This is the way
systemd-nspawn
allocates UID ranges (see above). Following this allocation logic ensures best compatibility withsystemd-nspawn
and all other container managers following the scheme, as it is sufficient then to check NSS for the first UID you pick regarding conflicts, as that's what they do, too. Moreover, it makeschown()
ing container file system trees nicely robust to interruptions: as the external UID encodes the internal UID in a fixed way, it's very easy to adjust the container's base UID without the need to know the original base UID: to change the container base, just mask away the upper 16-bit, and insert the upper 16-bit of the new container base instead. Here are the easy conversions to derive the internal UID, the external UID, and the container base UID from each other:INTERNAL_UID = EXTERNAL_UID & 0x0000FFFF CONTAINER_BASE_UID = EXTERNAL_UID & 0xFFFF0000 EXTERNAL_UID = INTERNAL_UID | CONTAINER_BASE_UID
-
When picking a UID range for containers, make sure to check NSS first, with a simple
getpwuid()
call: if there's already a user record for the first UID you want to pick, then it's already in use: pick a different one. Wrap that call in alckpwdf()
+ulckpwdf()
pair, to make allocation race-free. Provide an NSS module that makes all UIDs you end up taking show up in the user database, and make sure that the NSS module returns up-to-date information before you release the lock, so that other system components can safely use the NSS user database as allocation check, too. Note that if you follow this scheme no changes to/etc/passwd
need to be made, thus minimizing the artifacts the container manager persistently leaves in the system. -
systemd-homed
by default mounts the home directories it manages with UID mapping applied. It will map four UID ranges into that uidmap, and leave everything else unmapped: the range from 0…60000, the user's own UID, the range 60514…65534, and the container range 524288…1879048191. This means files/directories in home directories managed bysystemd-homed
cannot be owned by UIDs/GIDs outside of these four ranges (attempts tochown()
files to UIDs outside of these ranges will fail). Thus, if container trees are to be placed within a home directory managed bysystemd-homed
they should take these ranges into consideration and either place the trees at base UID 0 (and then map them to a higher UID range for use in user namespacing via another level of UID mapped mounts, at runtime) or at a base UID from the container UID range. That said, placing container trees (and in fact any files/directories not owned by the home directory's user) in home directories is generally a questionable idea (regardless of whethersystemd-homed
is used or not), given this typically breaks quota assumptions, makes it impossible for users to properly manage all files in their own home directory due to permission problems, introduces security issues around SETUID and severely restricts compatibility with networked home directories. Typically, it's a much better idea to place container images outside of the home directory, i.e. somewhere below/var/
or similar.
Summary
UID/GID | Purpose | Defined By | Listed in |
---|---|---|---|
0 | root user |
Linux | /etc/passwd + nss-systemd |
1…4 | System users | Distributions | /etc/passwd |
5 | tty group |
systemd |
/etc/passwd |
6…999 | System users | Distributions | /etc/passwd |
1000…60000 | Regular users | Distributions | /etc/passwd + LDAP/NIS/… |
60001…60513 | Human users (homed) | systemd |
nss-systemd |
60514…60577 | Host users mapped into containers | systemd |
systemd-nspawn |
60578…61183 | Unused | ||
61184…65519 | Dynamic service users | systemd |
nss-systemd |
65520…65533 | Unused | ||
65534 | nobody user |
Linux | /etc/passwd + nss-systemd |
65535 | 16-bit (uid_t) -1 |
Linux | |
65536…524287 | Unused | ||
524288…1879048191 | Container UID ranges | systemd |
nss-systemd |
1879048192…2147483647 | Unused | ||
2147483648…4294967294 | HIC SVNT LEONES | ||
4294967295 | 32-bit (uid_t) -1 |
Linux |
Note that "Unused" in the table above doesn't mean that these ranges are
really unused. It just means that these ranges have no well-established
pre-defined purposes between Linux, generic low-level distributions and
systemd
. There might very well be other packages that allocate from these
ranges.
Note that the range 2147483648…4294967294 (i.e. 2^31…2^32-2) should be handled
with care. Various programs (including kernel file systems — see devpts
— or
even kernel syscalls – see setfsuid()
) have trouble with UIDs outside of the
signed 32-bit range, i.e any UIDs equal to or above 2147483648. It is thus
strongly recommended to stay away from this range in order to avoid
complications. This range should be considered reserved for future, special
purposes.
Notes on resolvability of user and group names
User names, UIDs, group names and GIDs don't have to be resolvable using NSS (i.e. getpwuid() and getpwnam() and friends) all the time. However, systemd makes the following requirements:
System users generally have to be resolvable during early boot already. This
means they should not be provided by any networked service (as those usually
become available during late boot only), except if a local cache is kept that
makes them available during early boot too (i.e. before networking is
up). Specifically, system users need to be resolvable at least before
systemd-udevd.service
and systemd-tmpfiles-setup.service
are started, as both
need to resolve system users — but note that there might be more services
requiring full resolvability of system users than just these two.
Regular users do not need to be resolvable during early boot, it is sufficient
if they become resolvable during late boot. Specifically, regular users need to
be resolvable at the point in time the nss-user-lookup.target
unit is
reached. This target unit is generally used as synchronization point between
providers of the user database and consumers of it. Services that require that
the user database is fully available (for example, the login service
systemd-logind.service
) are ordered after it, while services that provide
parts of the user database (for example an LDAP user database client) are
ordered before it. Note that nss-user-lookup.target
is a passive unit: in
order to minimize synchronization points on systems that don't need it the unit
is pulled into the initial transaction only if there's at least one service
that really needs it, and that means only if there's a service providing the
local user database somehow through IPC or suchlike. Or in other words: if you
hack on some networked user database project, then make sure you order your
service Before=nss-user-lookup.target
and that you pull it in with
Wants=nss-user-lookup.target
. However, if you hack on some project that needs
the user database to be up in full, then order your service
After=nss-user-lookup.target
, but do not pull it in via a Wants=
dependency.