What are the disadvantages of having a dot in a user name?

Solution 1:

POSIX states this about usernames:

[...] To be portable across systems conforming to IEEE Std 1003.1-2001, the value is composed of characters from the portable filename character set. The hyphen should not be used as the first character of a portable user name.

... where the portable filename character set is:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
a b c d e f g h i j k l m n o p q r s t u v w x y z
0 1 2 3 4 5 6 7 8 9 . _ -

Also, the manpage for the /etc/adduser.conf file Manpage icon states:

   VALID NAMES
          adduser and addgroup enforce conformity to IEEE Std 1003.1-2001,
          which  allows  only  the following characters to appear in group
          and user names: letters, digits, underscores, periods, at  signs
          (@) and dashes. The name may not start with a dash. The "$" sign
          is allowed at the end of usernames (to conform to samba).

          An additional  check  can  be  adjusted  via  the  configuration
          parameter NAME_REGEX to enforce a local policy.

However,

Whilst both specifications seem to include the dot, Ubuntu (on my 13.04 at least) seems to disallow it:

⊳ sudo adduser as.df
adduser: Please enter a username matching the regular expression configured
via the NAME_REGEX[_SYSTEM] configuration variable.  Use the `--force-badname'
option to relax this check or reconfigure NAME_REGEX.

The default NAME_REGEX in Ubuntu is (from the /etc/adduser.conf manpage):

^[a-z][-a-z0-9]*$

Starting with a lowercase letter then any number of dashes, lowercase letters or digits. No _, @ or ..

So,

in conclusion a dot . may be used for a Ubuntu username, the NAME_REGEX just has to be changed in /etc/adduser.conf. Seeing as it conforms to POSIX, there shouldn't be any problems with having a . in the username with any POSIX-compliant program.

To enable a dot in usernames

Run this command in a terminal:
```
sudo nano /etc/adduser.conf
```
Locate this line (near the end of the file)
```
#NAME_REGEX="^[a-z][-a-z0-9]*$"
```
and replace it with
```
NAME_REGEX='^[a-z][-.a-z0-9]*$'
```
Note that the - must remain the first character in the bracket expression [...], otherwise it is treated as specifying a range a-z.
Press Ctrl+X, then Y, then Enter.

References:

Why are underscores not allowed in usernames in some distros (Debian for example)
man adduser.conf

Solution 2:

Marc Haber explains a possible downside in Debian bug #604242 (Allow dots in username by default):

Having dots in the user name creates some issues with scipts using chown, which still accepts dots as separator between user name and group name. If chown still accepts dots, there will be scripts using this notation, which will break if a user name contains a dot.

I would recommend keeping the current default (which can be overwritten by local configuration) until chown has stopped accepting dots as separator.

And chown still accepts the dot as separator, although it isn't documented anymore. I agree POSIX compatibility should prevail, and I indeed employ user names containing dots on several systems without any adverse effects.

Solution 3:

Applications that reads usernames might use a regex that assumes your username follows the rules and therefore can't handle your username.

Solution 4:

It seems that there is a reason behind this limitation.

If you try to run systemd service for scripts, it can be starting as root and not as a user. It's caused by systemd not recognize user with dot (domain.com user name for example) as valid user and runs service as root instead. Still this can b fixed already on systemd side, but still has a risk.

Also having dots in the user name creates some issues with scripts using chown, which still accepts dots as separator between user name and group name. If chown still accepts dots, there will be scripts using this notation, which will break if a user name contains a dot.