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:

1. 0 → The root super-user.

2. 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 on systemd. If you do change it you void your warranty.) (Also, some distributions call the nobody group nogroup. I wish they didn’t.)

3. 4294967295, aka “32-bit (uid_t) -1” → This UID is not a valid user ID, as setresuid(), 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.

4. 65535, aka “16-bit (uid_t) -1” → Before Linux kernel 2.4 uid_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. 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.

2. 1000…65533 and 65536…4294967294 → Everything else, i.e. regular (human) users.

Some older systems placed the boundary at 499/500, or even 99/100, and some distributions allow the boundary between system and regular users to be changed via local configuration. In systemd, the boundary is configurable during compilation time and is also queried from /etc/login.defs at runtime, if the -Dcompat-mutable-uid-boundaries=true compile-time setting is used. We strongly discourage downstreams from changing 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. This range can also be configured using /etc/login.defs.

Note that systemd requires that system users and groups are resolvable without network — 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:

1. 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.

2. 61184…65519 → UIDs for dynamic users are allocated from this range (see the DynamicUser= documentation in systemd.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 by adduser 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). The nss-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.

3. 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’s devpts file system) erroneously considers UIDs signed integers, and hence can’t deal with values above 2^31. The systemd-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 a 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=system_uid_max systemd
999
$ pkg-config --variable=dynamic_uid_min systemd
61184
$ pkg-config --variable=dynamic_uid_max systemd
65519
$ pkg-config --variable=container_uid_base_min systemd
524288
$ pkg-config --variable=container_uid_base_max 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:

1. 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 the nobody 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.

2. 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 nor systemd’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.

3. 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 with systemd-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 makes chown()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

1. 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 a lckpwdf() + 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.

2. 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 by systemd-homed cannot be owned by UIDs/GIDs outside of these four ranges (attempts to chown() files to UIDs outside of these ranges will fail). Thus, if container trees are to be placed within a home directory managed by systemd-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 whether systemd-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

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 theseranges.

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.