Since a setuid root program is systematically run under the super-user identity, it is very important to ensure it is secure and reliable. Indeed, a user who would manage to subvert it to call a command of their choice could then impersonate the root user and have all rights on the system.
A directory is handled differently. Read access gives the right to consult the list of its entries (files and directories), write access allows creating or deleting files, and execute access allows crossing through it (especially to go there with the cd command). Being able to cross through a directory without being able to read it gives permission to access the entries therein that are known by name, but not to find them if you do not know that they exist or under what name.
SECURITY setgid directory and sticky bit
The setgid bit also applies to directories. Any newly-created item in such directories is automaticallly assigned the owner group of the parent directory, instead of inheriting the creator's main group as usual. This setup avoids the user to have to change its main group (with the newgrp command) when working in a file tree shared between several users of the same dedicated group.
The “sticky” bit (symbolized by the letter “t”) is a permission that is only useful in directories. It is especially used for temporary directories where everybody have write access (such as /tmp/): it restricts deletion of files so that only their owner (or the owner of the parent directory) can do it. Lacking this, everyone could delete other users' files in /tmp/.
Three commands control the permissions associated with a file:
chown user file changes the owner of the file;
chgrp group file alters the owner group;
chmod rights file changes the permissions for the file.
There are two ways of presenting rights. Among them, the symbolic representation is probably the easiest to understand and remember. It involves the letter symbols mentioned above. You can define rights for each category of users (u/g/o), by setting them explicitly (with =), by adding (+), or subtracting (-). Thus the u=rwx,g+rw,o-r formula gives the owner read, write, and execute rights, adds read and write rights for the owner group, and removes read rights for other users. Rights not altered by the addition or subtraction in such a command remain unmodified. The letter a, for “all”, covers all three categories of users, so that a=rx grants all three categories the same rights (read and execute, but not write).
The (octal) numeric representation associates each right with a value: 4 for read, 2 for write, and 1 for execute. We associate each combination of rights with the sum of the figures. Each value is then assigned to different categories of users by putting them end to end in the usual order (owner, group, others).
For instance, the chmod 754 file command will set the following rights: read, write and execute for the owner (since 7 = 4 + 2 + 1); read and execute for the group (since 5 = 4 + 1); read-only for others. The 0 means no rights; thus chmod 600 file allows for read/write rights for the owner, and no rights for anyone else. The most frequent right combinations are 755 for executable files and directories, and 644 for data files.
To represent special rights, you can prefix a fourth digit to this number according to the same principle, where the setuid, setgid and sticky bits are 4, 2 and 1, respectively. chmod 4754 will associate the setuid bit with the previously described rights.
Note that the use of octal notation only allows to set all the rights at once on a file; you can not use it to simply add a new right, such as read access for the group owner, since you must take into account the existing rights and compute the new corresponding numerical value.
TIP Recursive operation
Sometimes we have to change rights for an entire file tree. All the commands above have a -R option to operate recursively in sub-directories.
The distinction between directories and files sometimes causes problems with recursive operations. That's why the “X” letter has been introduced in the symbolic representation of rights. It represents a right to execute which applies only to directories (and not to files lacking this right). Thus, chmod -R a+X directory will only add execute rights for all categoties of users (a) for all of the sub-directories and files for which at least one category of user (even if their sole owner) already has execute rights.
TIP Changing the user and group
Frequently you want to change the group of a file at the same time that you change the owner. The chown command has a special syntax for that: chown user:group
GOING FURTHER umask
When an application creates a file, it assigns indicative permissions, knowing that the system automatically removes certain rights, given by the command umask. Enter umask in a shell; you will see a mask such as 0022. This is simply an octal representation of the rights to be systematically removed (in this case, the write right for the group and other users).
If you give it a new octal value, the umask command modifies the mask. Used in a shell initialization file (for example, ~/.bash_profile), it will effectively change the default mask for your work sessions.
9.4. Administration Interfaces
Using a graphical interface for administration is interesting in various circumstances. An administrator does not necessarily know all the configuration details for all their services, and doesn't always have the time to go seeking out the documentation on the matter. A graphical interface for administration can thus accelerate the deployment of a new service. It can also simplify the setup of services which are hard to configure.
Such an interface is only an aid, and not an end in itself. In all cases, the administrator must master its behavior in order to understand and work around any potential problem.
Since no interface is perfect, you may be tempted to try several solutions. This is to be avoided as much as possible, since different tools are sometimes incompatible in their work methods. Even if they all target to be very flexible and try to adopt the configuration file as a single reference, they are not always able to integrate external changes.
9.4.1. Administrating On a Web Interface: webmin
This is, without a doubt, one of the most successful administration interfaces. It is a modular system managed through a web browser, covering a wide array of areas and tools. Furthermore, it is internationalized and available in many languages.
Sadly, webmin is no longer part of Debian since Etch. Its Debian maintainer — Jaldhar H. Vyas — removed the packages he created because he no longer had the time required to maintain them at an acceptable quality level. Nobody has officially taken over, so Squeeze does not have the webmin package.