Sunday Oct 24, 2010

Managing users with UAT

In the various Unix/Linux flavours, each user is assigned a numeric UID (Unique IDentifier) that is fundamental for granting privileges and granting access a user to the various system resources.


Even though every distribution still keeps the original command line tools to manage the users (useradd/del/mod, etc.) various tools have been developed to ease the burden of system administration, but I found some restrictions with the UAT (User Administration Tool) that is a component of the GNOME desktop of the Ubuntu distribution.


If you intend to manage the users with this tool, be aware that by default settings it 'masks' all the users whose UID is smaller than 1000 and bigger than 60000; so if you assign such UIDs to your users and restart the UAT, they simply vanish in the haze: you're not able to manage them anymore with this tool... unless... you change the shadow password suite configuration file: /etc/logins.defs. In this file, you can find the following definitions:


#
# Min/max values for automatic uid selection in useradd
#
UID_MIN                  1000
UID_MAX                 60000
# System accounts
#SYS_UID_MIN              100
#SYS_UID_MAX              999

#
# Min/max values for automatic gid selection in groupadd
#
GID_MIN                  1000
GID_MAX                 60000
# System accounts
#SYS_GID_MIN              100
#SYS_GID_MAX              999


That prevent you to create and manage users and groups outside the interval 1000-60000. Once you change these values to a more reasonable number according to your needs, restart the UAT... et voila' your users and groups are back in the tool.



P.S.: For the full story and historical reasons of UIDs, please consult the related UID Wiki page

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Marco Milo

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