Official Red Hat Linux Administrators Guide

If you are in a multi-user environment and not using a networked authentication scheme such as Kerberos, consider using Shadow Utilities (also known as shadow passwords) for the enhanced protection offered for your system’s authentication files. During the installation of Red Hat Linux, shadow password protection for your system is enabled by default, as are MD5 passwords (an alternative and arguably more secure method of encrypting passwords for storage on your system).

The use of shadow passwords offers a few distinct advantages over the previous standard of storing passwords on UNIX and Linux systems, including:

• Improved system security by moving the encrypted passwords (normally found in /etc/passwd) to /etc/shadow, which is readable only by root

• Information concerning password aging (how long it has been since a password was last changed)

• Control over how long a password can remain unchanged before the user is required to change it

• The ability to use the /etc/login.defs file to enforce a security policy, especially concerning password aging

The shadow-utils package contains a number of utilities that support:

• Conversion from normal to shadow passwords and back (pwconv, pwunconv)

• Verification of the password, group, and associated shadow files (pwck, grpck)

• Industry-standard methods of adding, deleting, and modifying user accounts (useradd, usermod, and userdel)

• Industry-standard methods of adding, deleting, and modifying user groups (groupadd, groupmod, and groupdel)

• Industry-standard method of administering the /etc/group file using gpasswd

There are some additional points of interest concerning these utilities:

• The utilities will work properly whether shadowing is enabled or not.

• The utilities have been slightly modified to support Red Hat’s user private group scheme. For a description of the modifications, see the useradd man page. For more information on user private groups, see the “User Private Groups” section of this chapter.

• The adduser script has been replaced with a symbolic link to /usr/sbin/useradd.

• The tools in the shadow-utils package are not Kerberos-, NIS-, Hesiod-, or LDAP-enabled. New users will be local only. For more information on Kerberos and LDAP, see Chapters 17 and 23.