docs/USER_NAMES.md

   1 ---
   2 title: User/Group Name Syntax
   3 category: Users, Groups and Home Directories
   4 layout: default
   5 ---
   6
   7 # User/Group Name Syntax
   8
   9 The precise set of allowed user and group names on Linux systems is weakly
  10 defined. Depending on the distribution a different set of requirements and
  11 restrictions on the syntax of user/group names are enforced — on some
  12 distributions the accepted syntax is even configurable by the administrator. In
  13 the interest of interoperability systemd enforces different rules when
  14 processing users/group defined by other subsystems and when defining users/groups
  15 itself, following the principle of "Be conservative in what you send, be
  16 liberal in what you accept". Also in the interest of interoperability systemd
  17 will enforce the same rules everywhere and not make them configurable or
  18 distribution dependent. The precise rules are described below.
  19
  20 Generally, the same rules apply for user as for group names.
  21
  22 ## Other Systems
  23
  24 * On POSIX the set of [valid user
  25   names](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_437)
  26   is defined as [lower and upper case ASCII letters, digits, period,
  27   underscore, and
  28   hyphen](https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap03.html#tag_03_282),
  29   with the restriction that hyphen is not allowed as first character of the
  30   user name. Interestingly no size limit is declared, i.e. in neither
  31   direction, meaning that strictly speaking according to POSIX both the empty
  32   string is a valid user name as well as a string of gigabytes in length.
  33
  34 * Debian/Ubuntu based systems enforce the regular expression
  35   `^[a-z][-a-z0-9]*$`, i.e. only lower case ASCII letters, digits and
  36   hyphens. As first character only lowercase ASCII letters are allowed. This
  37   regular expression is configurable by the administrator at runtime
  38   though. This rule enforces a minimum length of one character but no maximum
  39   length.
  40
  41 * Upstream shadow-utils enforces the regular expression
  42   `^[a-z_][a-z0-9_-]*[$]$`, i.e. is similar to the Debian/Ubuntu rule, but
  43   allows underscores and hyphens, but the latter not as first character. Also,
  44   an optional trailing dollar character is permitted.
  45
  46 * Fedora/Red Hat based systems enforce the regular expression of
  47   `^[a-zA-Z0-9_.][a-zA-Z0-9_.-]{0,30}[a-zA-Z0-9_.$-]?$`, i.e. a size limit of
  48   32 characters, with upper and lower case letters, digits, underscores,
  49   hyphens and periods. No hyphen as first character though, and the last
  50   character may be a dollar character. On top of that, `.` and `..` are not
  51   allowed as user/group names.
  52
  53 * sssd is known to generate user names with embedded `@` and white-space
  54   characters, as well as non-ASCII (i.e. UTF-8) user/group names.
  55
  56 * winbindd is known to generate user/group names with embedded `\` and
  57   white-space characters, as well as non-ASCII (i.e. UTF-8) user/group names.
  58
  59 Other operating systems enforce different rules; in this documentation we'll
  60 focus on Linux systems only however, hence those are out of scope. That said,
  61 software like Samba is frequently deployed on Linux for providing compatibility
  62 with Windows systems; on such systems it might be wise to stick to user/group
  63 names also valid according to Windows rules.
  64
  65 ## Rules systemd enforces
  66
  67 Distilled from the above, below are the rules systemd enforces on user/group
  68 names. An additional, common rule between both modes listed below is that empty
  69 strings are not valid user/group names.
  70
  71 Philosophically, the strict mode described below enforces an allow list of
  72 what's allowed and prohibits everything else, while the relaxed mode described
  73 below implements a deny list of what's not allowed and permits everything else.
  74
  75 ### Strict mode
  76
  77 Strict user/group name syntax is enforced whenever a systemd component is used
  78 to register a user or group in the system, for example a system user/group
  79 using
  80 [`systemd-sysusers.service`](https://www.freedesktop.org/software/systemd/man/systemd-sysusers.html)
  81 or a regular user with
  82 [`systemd-homed.service`](https://www.freedesktop.org/software/systemd/man/systemd-homed.html).
  83
  84 In strict mode, only uppercase and lowercase characters are allowed, as well as
  85 digits, underscores and hyphens. The first character may not be a digit or
  86 hyphen. A size limit is enforced: the minimum of `sysconf(_SC_LOGIN_NAME_MAX)`
  87 (typically 256 on Linux; rationale: this is how POSIX suggests to detect the
  88 limit), `UT_NAMESIZE-1` (typically 31 on Linux; rationale: names longer than
  89 this cannot correctly appear in `utmp`/`wtmp` and create ambiguity with login
  90 accounting) and `FILENAME_MAX` (4096 on Linux; rationale: user names typically
  91 appear in directory names, i.e. the home directory), thus MIN(256, 31, 4096) =
  92 31.
  93
  94 Note that these rules are both more strict and more relaxed than all of the
  95 rules enforced by other systems listed above. A user/group name conforming to
  96 systemd's strict rules will not necessarily pass a test by the rules enforced
  97 by these other subsystems.
  98
  99 Written as regular expression the above is: `^[a-zA-Z_][a-zA-Z0-9_-]{0,30}$`
 100
 101 ### Relaxed mode
 102
 103 Relaxed user/group name syntax is enforced whenever a systemd component accepts
 104 and makes use of user/group names registered by other (non-systemd)
 105 components of the system, for example in
 106 [`systemd-logind.service`](https://www.freedesktop.org/software/systemd/man/systemd-logind.html).
 107
 108 Relaxed syntax is also enforced by the `User=` setting in service unit files,
 109 i.e. for system services used for running services. Since these users may be
 110 registered by a variety of tools relaxed mode is used, but since the primary
 111 purpose of these users is to run a system service and thus a job for systemd a
 112 warning is shown if the specified user name does not qualify by the strict
 113 rules above.
 114
 115 * No embedded NUL bytes (rationale: handling in C must be possible and
 116   straight-forward)
 117
 118 * No names consisting fully of digits (rationale: avoid confusion with numeric
 119   UID/GID specifications)
 120
 121 * Similar, no names consisting of an initial hyphen and otherwise entirely made
 122   up of digits (rationale: avoid confusion with negative, numeric UID/GID
 123   specifications, e.g. `-1`)
 124
 125 * No strings that do not qualify as valid UTF-8 (rationale: we want to be able
 126   to embed these strings in JSON, with permits only valid UTF-8 in its strings;
 127   user names using other character sets, such as JIS/Shift-JIS will cause
 128   validation errors)
 129
 130 * No control characters (i.e. characters in ASCII range 1…31; rationale: they
 131   tend to have special meaning when output on a terminal in other contexts,
 132   moreover the newline character — as a specific control character — is used as
 133   record separator in `/etc/passwd`, and hence it's crucial to avoid
 134   ambiguities here)
 135
 136 * No colon characters (rationale: it is used as field separator in `/etc/passwd`)
 137
 138 * The two strings `.` and `..` are not permitted, as these have special meaning
 139   in file system paths, and user names are frequently included in file system
 140   paths, in particular for the purpose of home directories.
 141
 142 * Similar, no slashes, as these have special meaning in file system paths
 143
 144 * No leading or trailing white-space is permitted; and hence no user/group names
 145   consisting of white-space only either (rationale: this typically indicates
 146   parsing errors, and creates confusion since not visible on screen)
 147
 148 Note that these relaxed rules are implied by the strict rules above, i.e. all
 149 user/group names accepted by the strict rules are also accepted by the relaxed
 150 rules, but not vice versa.
 151
 152 Note that this relaxed mode does not refuse a couple of very questionable
 153 syntaxes. For example it permits a leading or embedded period. A leading period
 154 is problematic because the matching home directory would typically be hidden
 155 from the user's/administrator's view. An embedded period is problematic since
 156 it creates ambiguity in traditional `chown` syntax (which is still accepted
 157 today) that uses it to separate user and group names in the command's
 158 parameter: without consulting the user/group databases it is not possible to
 159 determine if a `chown` invocation would change just the owning user or both the
 160 owning user and group. It also allows embedding `@` (which is confusing to
 161 MTAs).
 162
 163 ## Common Core
 164
 165 Combining all rules listed above, user/group names that shall be considered
 166 valid in all systemd contexts and on all Linux systems should match the
 167 following regular expression (at least according to our understanding):
 168
 169 `^[a-z][a-z0-9-]{0,30}$`