systemd/UIDS-GIDS.md

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 32bit 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 16bit 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.) Because Fedora is a bit confused the nobody user is called nfsnobody there (and they have a different nobody user at UID 99). I hope this will be corrected eventually though. (Also, some distributions call the nobody group nogroup. I wish they didn't.)

3. 4294967295, aka "32bit (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 "16bit (uid_t) -1" → Before Linux kernel 2.4 uid_t used to be 16bit, 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.

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 initial RAM disk).

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. 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 16bit 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.

2. 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 16bit subrange of this range and assign it to the container. The range is picked so that the upper 16bit of the 32bit UIDs are constant for all users of the container, while the lower 16bit directly encode the 65536 UIDs assigned to the container. This mode of allocation means that the upper 16bit of any UID assigned to a container are kind of a "container ID", while the lower 16bit 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 16bit boundary. Moreover it's below the 31bit 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 nss-mymachines glibc NSS module 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.)

Note that systemd does not make any of these values runtime-configurable. All these boundaries are chosen during build time. That said, the system UID/GID boundary is traditionally configured in /etc/login.defs, though systemd won't look there during runtime.

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 16bit 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 16bit 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 16bits all set to zero. That way, the upper 16bits become a container ID of some kind, while the lower 16bits directly encode the internal container UID. This is the way systemd-nspawn allocates UID ranges (see above). Following this allocation logic ensures best compability 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 16bit, and insert the upper 16bit 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
   

4. 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.

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…61183	Unused
61184…65519	Dynamic service users	systemd	nss-systemd
65520…65533	Unused
65534	nobody user	Linux	/etc/passwd + nss-systemd
65535	16bit (uid_t) -1	Linux
65536…524287	Unused
524288…1879048191	Container UID ranges	systemd	nss-mymachines
1879048192…4294967294	Unused
4294967295	32bit (uid_t) -1	Linux

Note that "Unused" in the table above doesn't meant 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.