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| -rw-r--r-- | docs/USER_NAMES.md | 91 |
1 files changed, 42 insertions, 49 deletions
diff --git a/docs/USER_NAMES.md b/docs/USER_NAMES.md index 74c24b5..fe0ca7f 100644 --- a/docs/USER_NAMES.md +++ b/docs/USER_NAMES.md @@ -7,49 +7,45 @@ SPDX-License-Identifier: LGPL-2.1-or-later # User/Group Name Syntax -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 +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 +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. +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. ## Other Systems -* On POSIX the set of [valid user - names](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_437) - is defined as [lower and upper case ASCII letters, digits, period, - underscore, and - hyphen](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282), - 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 +* On POSIX the set of + [valid user names](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_437) + is defined as + [lower and upper case ASCII letters, digits, period, underscore, and hyphen](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282), + 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]*$`, i.e. only lower case ASCII letters, digits and - hyphens. As first character only lowercase ASCII letters are allowed. This - regular expression is configurable by the administrator at runtime - though. This rule enforces a minimum length of one character but no maximum - length. +* Debian/Ubuntu based systems enforce the regular expression `^[a-z][-a-z0-9]*$`, 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. + `^[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. + 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. @@ -58,16 +54,15 @@ Generally, the same rules apply for user as for group names. 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 +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. ## Rules systemd enforces -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. +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 @@ -83,18 +78,17 @@ or a regular user with [`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.html). 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)` +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. +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 +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}$` @@ -107,8 +101,8 @@ components of the system, for example in [`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.html). 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 +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. @@ -150,16 +144,15 @@ 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 +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). +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). ## Common Core |