10.3.3 User Private Groups

 

Red Hat Linux uses a user private group (UPG) scheme, which makes UNIX groups much easier to use. The UPG scheme does not add or change anything in the standard UNIX way of handling groups. It simply offers a new convention for handling groups. Whenever you create a new user, by default, he or she has a unique group. The scheme works as follows:

User Private Group

Each user has it's own primary group, to which only it is a member.

umask = 002

The traditional UNIX umask is 022, which prevents other users and other members of a user's primary group from modifying a user's files. Since every user has their own private group in the UPG scheme, this ``group protection'' is not needed. A umask of 002 will prevent users from modifying other users' private files. The umask is set in /etc/profile.

SGID bit on Directories

If you set the SGID bit on a directory (with chmod g+s directory), files created in that directory will have their group set to the directory's group.

Most computing sites like to create a group for each major project and assign people to the groups they need to be in. Managing files traditionally has been difficult, though, because when someone creates a file it is owned by the primary group he or she belongs to. When a single person works on multiple projects, it becomes hard to make the files owned by the group that is associated with that project. In the UPG scheme, groups are automatically assigned to files on a project-by-project basis, which makes managing group projects very simple.

Let's say you have a big project called devel, with many people editing the devel files in a devel directory. Make a group called devel, chgrp the devel directory to devel, and add the all the devel users to the devel group. Now, all the devel users will be able to edit the devel files and create new files in the devel directory, and these files will always retain their devel group. Thus, they will always be edit-able by other devel users.

If you have multiple projects like devel, and users who are working on multiple projects, these users will never have to change their umask or group when they move from project to project. The SGID bit on each project's main directory ``selects'' the proper group.

Since each user's HOME directory is owned by the user and their private group, it is safe to set the SGID bit on the HOME directory. However, by default, files are created with the primary group of the user, so the SGID bit would be redundant.

 
• User Private Group Rationale