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docs/UID-GIDS: mention that ranges are actually configurable

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It silly for our docs to say that they aren't when we added support for this a
few years ago.

Also, drop some mentions of "runtime". This implied that those values can be
changed almost at will, but actually, they can only be meaningfully changed
_before_ the allocations are made.

(cherry picked from commit 7e4f30c3d0)
(cherry picked from commit 00ed84c34b)
(cherry picked from commit 8992968463)
This commit is contained in:
Zbigniew Jędrzejewski-Szmek 2024-01-09 12:10:18 +01:00 committed by Luca Boccassi
parent 1b1434fa42
commit 27913c1aee
1 changed files with 11 additions and 11 deletions
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22
docs/UIDS-GIDS.md
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@ -21,7 +21,7 @@ validity for GIDs too.
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
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
@ -57,20 +57,20 @@ Distributions generally split the available UID range in two:
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.
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. And it's also usually
user-configurable, too.
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
networking available — a requirement that is not made for regular users. This
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).