UNIX user name. This field is the only mandatory field, all others are
optional. Corresponds with the `pw_name` field of of `struct passwd` and the
`sp_namp` field of `struct spwd` (i.e. the shadow user record stored in
-`/etc/shadow`).
+`/etc/shadow`). See [User/Group Name Syntax](https://systemd.io/USER_NAMES) for
+the (relaxed) rules the various systemd components enforce on user/group names.
`realm` → The "realm" a user is defined in. This concept allows distinguishing
users with the same name that originate in different organizations or
<listitem><para>Create a new home directory/user account of the specified name. Use the various
user record property options (as documented above) to control various aspects of the home directory
- and its user accounts.</para></listitem>
+ and its user accounts.</para>
+
+ <para>The specified user name should follow the strict syntax described on <ulink
+ url="https://systemd.io/USER_NAMES">User/Group Name Syntax</ulink>.</para></listitem>
</varlistentry>
<varlistentry>
is set, the default group of the user is used. This setting does not affect commands whose command line is
prefixed with <literal>+</literal>.</para>
- <para>Note that restrictions on the user/group name syntax are enforced: the specified name must consist only
- of the characters a-z, A-Z, 0-9, <literal>_</literal> and <literal>-</literal>, except for the first character
- which must be one of a-z, A-Z or <literal>_</literal> (i.e. numbers and <literal>-</literal> are not permitted
- as first character). The user/group name must have at least one character, and at most 31. These restrictions
- are enforced in order to avoid ambiguities and to ensure user/group names and unit files remain portable among
- Linux systems.</para>
+ <para>Note that this enforces only weak restrictions on the user/group name syntax, but will generate
+ warnings in many cases where user/group names do not adhere to the following rules: the specified
+ name should consist only of the characters a-z, A-Z, 0-9, <literal>_</literal> and
+ <literal>-</literal>, except for the first character which must be one of a-z, A-Z and
+ <literal>_</literal> (i.e. digits and <literal>-</literal> are not permitted as first character). The
+ user/group name must have at least one character, and at most 31. These restrictions are made in
+ order to avoid ambiguities and to ensure user/group names and unit files remain portable among Linux
+ systems. For further details on the names accepted and the names warned about see <ulink
+ url="https://systemd.io/USER_NAMES">User/Group Name Syntax</ulink>.</para>
<para>When used in conjunction with <varname>DynamicUser=</varname> the user/group name specified is
dynamically allocated at the time the service is started, and released at the time the service is
A-Z or <literal>_</literal> (i.e. numbers and <literal>-</literal> are not permitted as first character). The
user/group name must have at least one character, and at most 31.</para>
+ <para>For further details about the syntax of user/group names, see <ulink
+ url="https://systemd.io/USER_NAMES">User/Group Name Syntax</ulink>.</para>
+
<para>It is strongly recommended to pick user and group names that are unlikely to clash with normal users
created by the administrator. A good scheme to guarantee this is by prefixing all system and group names with the
underscore, and avoiding too generic names.</para>