The precise set of allowed user and group names on Linux systems is weakly defined. Depending on the distribution a different set of requirements and restrictions on the syntax of user/group names are enforced — on some distributions the accepted syntax is even configurable by the administrator. In the interest of interoperability systemd enforces different rules when processing users/group defined by other subsystems and when defining users/groups itself, following the principle of “Be conservative in what you send, be liberal in what you accept”. Also in the interest of interoperability systemd will enforce the same rules everywhere and not make them configurable or distribution dependent. The precise rules are described below.
Generally, the same rules apply for user as for group names.
On POSIX the set of valid user names is defined as lower and upper case ASCII letters, digits, period, underscore, and hyphen, with the restriction that hyphen is not allowed as first character of the user name. Interestingly no size limit is declared, i.e. in neither direction, meaning that strictly speaking, according to POSIX, both the empty string is a valid user name as well as a string of gigabytes in length.
Debian/Ubuntu based systems enforce the regular expression ^[a-z][-a-z0-9]*$, i.e. only lower case ASCII letters, digits and hyphens. As first character only lowercase ASCII letters are allowed. This regular expression is configurable by the administrator at runtime though. This rule enforces a minimum length of one character but no maximum length.
Upstream shadow-utils enforces the regular expression ^[a-z_][a-z0-9_-]*[$]$, i.e. is similar to the Debian/Ubuntu rule, but allows underscores and hyphens, but the latter not as first character. Also, an optional trailing dollar character is permitted.
Fedora/Red Hat based systems enforce the regular expression of ^[a-zA-Z0-9_.][a-zA-Z0-9_.-]{0,30}[a-zA-Z0-9_.$-]?$, i.e. a size limit of 32 characters, with upper and lower case letters, digits, underscores, hyphens and periods. No hyphen as first character though, and the last character may be a dollar character. On top of that, . and .. are not allowed as user/group names.
sssd is known to generate user names with embedded @ and white-space characters, as well as non-ASCII (i.e. UTF-8) user/group names.
winbindd is known to generate user/group names with embedded \ and white-space characters, as well as non-ASCII (i.e. UTF-8) user/group names.
Other operating systems enforce different rules; in this documentation we'll focus on Linux systems only however, hence those are out of scope. That said, software like Samba is frequently deployed on Linux for providing compatibility with Windows systems; on such systems it might be wise to stick to user/group names also valid according to Windows rules.
Distilled from the above, below are the rules systemd enforces on user/group names. An additional, common rule between both modes listed below is that empty strings are not valid user/group names.
Philosophically, the strict mode described below enforces an allow list of what‘s allowed and prohibits everything else, while the relaxed mode described below implements a deny list of what’s not allowed and permits everything else.
Strict user/group name syntax is enforced whenever a systemd component is used to register a user or group in the system, for example a system user/group using systemd-sysusers.service or a regular user with systemd-homed.service.
In strict mode, only uppercase and lowercase characters are allowed, as well as digits, underscores and hyphens. The first character may not be a digit or hyphen. A size limit is enforced: the minimum of sysconf(_SC_LOGIN_NAME_MAX) (typically 256 on Linux; rationale: this is how POSIX suggests to detect the limit), UT_NAMESIZE-1 (typically 31 on Linux; rationale: names longer than this cannot correctly appear in utmp/wtmp and create ambiguity with login accounting) and NAME_MAX (255 on Linux; rationale: user names typically appear in directory names, i.e. the home directory), thus MIN(256, 31, 255) = 31.
Note that these rules are both more strict and more relaxed than all of the rules enforced by other systems listed above. A user/group name conforming to systemd's strict rules will not necessarily pass a test by the rules enforced by these other subsystems.
Written as regular expression the above is: ^[a-zA-Z_][a-zA-Z0-9_-]{0,30}$
Relaxed user/group name syntax is enforced whenever a systemd component accepts and makes use of user/group names registered by other (non-systemd) components of the system, for example in systemd-logind.service.
Relaxed syntax is also enforced by the User= setting in service unit files, i.e. for system services used for running services. Since these users may be registered by a variety of tools relaxed mode is used, but since the primary purpose of these users is to run a system service and thus a job for systemd a warning is shown if the specified user name does not qualify by the strict rules above.
No embedded NUL bytes (rationale: handling in C must be possible and straightforward)
No names consisting fully of digits (rationale: avoid confusion with numeric UID/GID specifications)
Similar, no names consisting of an initial hyphen and otherwise entirely made up of digits (rationale: avoid confusion with negative, numeric UID/GID specifications, e.g. -1)
No strings that do not qualify as valid UTF-8 (rationale: we want to be able to embed these strings in JSON, with permits only valid UTF-8 in its strings; user names using other character sets, such as JIS/Shift-JIS will cause validation errors)
No control characters (i.e. characters in ASCII range 1…31; rationale: they tend to have special meaning when output on a terminal in other contexts, moreover the newline character — as a specific control character — is used as record separator in /etc/passwd, and hence it's crucial to avoid ambiguities here)
No colon characters (rationale: it is used as field separator in /etc/passwd)
The two strings . and .. are not permitted, as these have special meaning in file system paths, and user names are frequently included in file system paths, in particular for the purpose of home directories.
Similar, no slashes, as these have special meaning in file system paths
No leading or trailing white-space is permitted; and hence no user/group names consisting of white-space only either (rationale: this typically indicates parsing errors, and creates confusion since not visible on screen)
Note that these relaxed rules are implied by the strict rules above, i.e. all user/group names accepted by the strict rules are also accepted by the relaxed rules, but not vice versa.
Note that this relaxed mode does not refuse a couple of very questionable syntaxes. For example, it permits a leading or embedded period. A leading period is problematic because the matching home directory would typically be hidden from the user‘s/administrator’s view. An embedded period is problematic since it creates ambiguity in traditional chown syntax (which is still accepted today) that uses it to separate user and group names in the command's parameter: without consulting the user/group databases it is not possible to determine if a chown invocation would change just the owning user or both the owning user and group. It also allows embedding @ (which is confusing to MTAs).
Combining all rules listed above, user/group names that shall be considered valid in all systemd contexts and on all Linux systems should match the following regular expression (at least according to our understanding):
^[a-z][a-z0-9-]{0,30}$