6.6. Linux Password & Shadow File Formats

Traditional Unix systems keep user account information, including one-way encrypted passwords, in a text file called ``/etc/passwd''. As this file is used by many tools (such as ``ls'') to display file ownerships, etc. by matching user id #'s with the user's names, the file needs to be world-readable. Consequentally, this can be somewhat of a security risk.

Another method of storing account information, one that I always use, is with the shadow password format. As with the traditional method, this method stores account information in the /etc/passwd file in a compatible format. However, the password is stored as a single "x" character (ie. not actually stored in this file). A second file, called ``/etc/shadow'', contains encrypted password as well as other information such as account or password expiration values, etc. The /etc/shadow file is readable only by the root account and is therefore less of a security risk.

While some other Linux distributions forces you to install the Shadow Password Suite in order to use the shadow format, Red Hat makes it simple. To switch between the two formats, type (as root):

  /usr/sbin/pwconv	To convert to the shadow format
  /usr/sbin/pwunconv	To convert back to the traditional format

With shadow passwords, the ``/etc/passwd'' file contains account information, and looks like this:

smithj:x:561:561:Joe Smith:/home/smithj:/bin/bash

Each field in a passwd entry is separated with ":" colon characters, and are as follows:

Perhaps you do not wish to provide shell accounts for your users. You could create a script file called ``/bin/sorrysh'', for example, that would display some kind of error message and log the user off, and then set this script as their default shell.

Note: Note: If the account needs to provide "FTP" transfers to update web pages, etc. then the shell account will need to be set to ``/bin/bash'' -- and then special permissions will need to be set up in the user's home directory to prevent shell logins. See Section 7.1 for details on this.

The ``/etc/shadow'' file contains password and account expiration information for users, and looks like this:


As with the passwd file, each field in the shadow file is also separated with ":" colon characters, and are as follows: