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User Private Groups

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Unix introduced a simple model of file permissions in the 70's which has proven to be quite effective and easy to understand. In recent times, there was an attempt to extend it using ACLs, that are used in Windows  but very few organization have adopted this approach on Unix servers due to the complexity. Existence of two models also confuse administrators and represent a serious security risk. 

In the original Unix model, each file has three access categories of user: User (u), Group (g) and Other (o). Group is essentially a role and primary group is a primary role. Any user can be a member of any number of groups.

There is also auxiliary concept of  system groups which is similar to the concept of privileged ports. For example, all groups with GID below 100 are considered to be system groups.  System groups has special properties, and designed mainly for partitioning of permission space. For example in most users which have system group as primary group have no legitimate shell (/bin/noshell or /bin/false is used instead; the former logs in access attempts), so nobody can login as such a user.

The most severe limitation of this model is that a file can only be a member of one group. That can be partially rectified by usage of "metagroups" -- groups that are just and aggregations of existing groups, but that solution  requires additional efforts and discipline (in this case /etc/group need to be automatically generated from some template with macros).   If we assume that the number of group allowed is large (approximately the same as the number of files/directories)  metagroup approach is as powerful as ACL model and is much simpler.  It requires relatively simple modification of the /etc/group file.

One interesting step toward this model is the concept of  User Private Groups (UPG) introduced in Red Hat around year 2000( they are present in Red Hat Linux 7.0). But later Red Hat abandoned the idea and there is no UPG behavior of useradd function in RHEL 6.5 and later

UPG scheme makes UNIX groups  easier to use. It does not add or change anything in the standard UNIX way of handling groups. This is simply a new convention for handling classic Unix groups Whenever you create a new user, by default, he or she has a unique group. The idea is very simple and effective:  Each user has their own primary group, of which only they are a member.

It is a better idea then setting umask = 022. Of couse umask is 022 also prevents other users and other members of a user's primary group from modifying a user's files. but you can't distinguish between human and non-human account. If every "human" user has their own private group "group protection" is not needed. At the same time for applications this allow to create directories access to which is share between the members of primary group. The umask is typically set in /etc/profile.

A complementary capability that we should mention here is setgid bit on Directories. Since each user's home directory is owned by the user and their private group, it is recommended to set the setgid bit on the home directory. That is especially useful for system administrators who are often work as root and ftp files back and forth from their PC.  It is also useful for application directories where is important that the whole subtree belongs to the same group.

You can imitate USG-behaviors of useradd typical for versions of RHEL before 6.5 on SUSE or Solaris using shell function or Perl scripts. In the most simplified form it can look like

function uadd {
    groupadd -g %1 %2
    useradd -u %1 -g %1 -m %2
}

The rest of parameters can be set separately using usermod or more complex function can be used.User Private Group Rationale

UPG is used in Red Hat Linux since version 5 or even earlier. Currently it is not used on any other Linux distribution. The following is the rationale for the scheme.

At this point, by making the default umask 002 and giving everyone a private default group, you can easily set up groups that users can take advantage of without doing any magic. Just create the group, add the users, and do the above chown and chmod on the group's directories.


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