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Introduction to Unix Security

With the discussion of DOT files should come a few notes about security. When the ls -l command is given, (the long listing) the security settings, so to speak, are visible for each file. For example, giving the ls -l command in a user's home directory might give output similar to the following:

% ls -l
total 347
-rwxr--r--   1 author   users        2540 Jan  5 20:30 appenda1.tex.gz
-rwxr--r--   1 author   users       67043 Jan  2 20:53 bourne.tex
-rwxr--r--   1 author   users       86011 Feb 23 19:33 cshell.tex
-rwxr--r--   1 author   users       82476 Feb 23 18:46 cshell.tex~
-rwxr--r--   1 author   users       11142 Jan  3 12:58 introduction.tex
drwx------   2 author   users        1024 Jan 12 21:31 mail
drwxr-xr-x   2 author   users        1024 Feb 12 21:04 scripts
drwxr-xr-x   2 author   users        1024 Jan 14 18:50 bin

The string on the left side of each listing gives the security attributes as well as the file type attribute. The above example shows that there are three directories (denoted by the d in the first place of the attribute string) and five regular files (denoted by dash's -'s). The remaining nine characters in the string represent the security attributes (or more correctly, permissions) for each file and directory. The directory settings will be left for discussion in a more appropriate text, as they are not as straight forward as the regular file permission settings. Each permission string contains nine characters which are actually three sets of three related settings which are broken down as follows:

- - -   - - -  - - -
r w x   r w x  r w x

The three sets are the USER set, which corresponds to the user who owns the file, the GROUP set, which corresponds to group ownership of the file, and the OTHER set which corresponds to any other user on the system. Displaying the files in a directory with the long listing command ls -l (ls -lg on SunOS) the user and group owners will be displayed. In the above example, the user author owns all of the files and the files are all owned by the users group. Each of the three groups has a read (r), write (w), and execute (x) setting. If a dash (-) is resent in the string it means that permissions for that action are not given. In the example all of the files are readable by everyone, but only the owner, author, can write or execute (which means nothing in this case) the files. To learn about changing the owner or group ownership of a file (or directory) the Unix commands chown and chgrp can be used. More importantly, to change the permissions, the chmod command can be used. This command takes the form:

% chmod [options] filename

For the particular option the man pages can be examined. For example adding write permission to all users in the group users for the TeX files would look like this:

% chmod g+w *.tex

This would not necessarily be a good idea as now any user in the users group could alter or even delete the TeX files. The point of this book is clearly not Unix security, but the point of these permissions becomes of great importance with DOT files used by the shell. If the permissions were set such that any user could write to the .login or .cshrc file, they could alter it in any way they wanted. If this user wanted to cause damage (say delete all of the files in this user's directory), all they would have to do is add the line

rm -rf *

While most users are clearly not that mallicious, some are. Another point to keep in mind is that if group ownership is given to a file, group members can also be given write access to a DOT file. There is no situation where another person needs to have write access to another user's DOT files and thus dot files should have permission settings similar to the following:

-rwxr--r--   1 jblow    users       86011 Feb 23 19:33 .cshrc
-rwxr--r--   1 jblow    users       86011 Feb 23 19:33 .login
-rwxr--r--   1 jblow    users       86011 Feb 23 19:33 .logout

This will prevent anything bad from happening (at least due to unsafe permission settings).

next up previous contents
Next: The TC Shell Up: Special Files Previous: Special Files

Douglas M Gingrich
Mon Apr 27 15:25:49 MDT 1998