Editing User identifier (section)

=== Reserved ranges ===
The [[Linux Standard Base]] Core Specification specifies that UID values in the range 0 to 99 should be statically allocated by the system, and shall not be created by applications, while UIDs from 100 to 499 should be reserved for dynamic allocation by system administrators and post install scripts.<ref>{{cite web|url=https://refspecs.linuxfoundation.org/LSB_3.0.0/LSB-Core-generic/LSB-Core-generic/uidrange.html |title=9.3. UID Ranges |website=Refspecs.linuxfoundation.org |access-date=2016-09-24}}</ref>

[[Debian Linux]] not only reserves the range 100–999 for dynamically allocated system users and groups, but also centrally and statically allocates users and groups in the range 60000-64999 and further reserves the range 65000–65533.<ref name=debian>{{cite web|url=https://www.debian.org/doc/debian-policy/ch-opersys.html#uid-and-gid-classes|title=Debian Policy Manual – Section 9.2.2: UID and GID classes|website=Debian.org|date=2019-07-18|access-date=2019-07-26}}</ref>

[[Systemd]] defines a number of special UID ranges, including<ref name=systemd>{{cite web|url=https://github.com/systemd/systemd/blob/master/docs/UIDS-GIDS.md|title=Users, groups, UIDs and GIDs on systemd systems|website=[[GitHub]]|access-date=2020-09-26}}</ref>
* 60001-60513: UIDs for home directories managed by systemd-homed
* 61184-65519 (0xef00-0xffef): UIDs for dynamic users

On [[FreeBSD]], porters who need a UID for their package can pick a free one from the range 50 to 999 and then register the static allocation.<ref>{{cite web|url=http://www.freebsd.org/doc/en/books/porters-handbook/ |title=FreeBSD Porter's Handbook |website=Freebsd.org |access-date=2016-09-24}}</ref><ref>{{cite web | url=http://www.freebsd.org/doc/en/books/porters-handbook/users-and-groups.html | title=Chapter 6. Special Considerations }}</ref>

Some POSIX systems allocate UIDs for new users starting from 500 ([[macOS]], [[Red Hat Enterprise Linux]] till version 6), others start at 1000 (Red Hat Enterprise Linux since version 7,<ref>{{cite web|url=https://www.certdepot.net/rhel7-system-changes/ |title=RHEL7 System changes |website=Certdepot.net |date=2016-01-17 |access-date=2017-03-22}}</ref> [[openSUSE]], [[Debian]]<ref name=debian />). On many Linux systems, these ranges are specified  in <code>/etc/login.defs</code>, for <code>useradd</code> and similar tools.

Central UID allocations in enterprise networks (e.g., via [[LDAP]] and [[Network File System|NFS]] servers) may limit themselves to using only UID numbers well above 1000, and outside the range 60000–65535, to avoid potential conflicts with UIDs locally allocated on client computers.  When new users are created locally, the local system is supposed to check for and avoid conflicts with UID's already existing on NSS'<ref>https://systemd.io/UIDS-GIDS/ "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"</ref>

[[OS-level virtualization]] can remap user identifiers, e.g. using [[Linux namespaces]], and therefore need to allocate ranges into which remapped UIDs and GIDs are mapped:
* [[snapd]] maps UIDs and GIDs into the range [https://forum.snapcraft.io/t/system-usernames/13386 524288-589823] (0x80000-0x8ffff)
* [[systemd-nspawn]] automatic allocates of per-container UID ranges uses the range 524288-1879048191 (0x80000-0x6fffffff)<ref name=systemd />
The systemd authors recommend that [[OS-level virtualization]] systems should allocate 65536 (2<sup>16</sup>) UIDs per container, and map them by adding an integer multiple of 2<sup>16</sup>.<ref name=systemd />