User Administration

Administering users and groups in IBM AIX is an essential part of an administrator's duties. You can manage users and groups in many different ways: You can maintain them through the AIX System Management Interface Tool (SMIT), manually editing the configuration files, or by using command-line commands. This article discusses the configuration files behind users and groups on AIX, as well as command-line tools designed to help manage the users and groups easily.

Learn the files first, then the commands

Before going into the commands to create, modify, and maintain users and groups in AIX, it is important that you know what is happening behind the scenes. For example, you should understand the files and what they mean.

Look at the some of the files that affect the user itself:

• /etc/passwd

• /etc/security/.profile

• /etc/security/limits

• /etc/security/passwd

• /etc/security/user

• /usr/lib/security/mkuser.default

/etc/passwd

The file /etc/passwd contains the basics of a user and is probably the best-known file to UNIX® and Linux® users for user administration.Listing 1 provides an example of an /etc/passwd file.

Listing 1. Example /etc/passwd file

As you can see, the file is colon (:) delimited, and each entry contains seven fields in the following format (with spaces added before and after delimiter to ease reading):

IN06411C: ! : 6411 : 666 : IN/I/06411C//Brijesh.Gogia: /home/IN06411C : /bin/ksh

Here's the line-by-line breakdown:

• • Username. This is the login/user name associated with the account.

  • Password Flag. This field varies slightly in different flavors of UNIX and Linux. In AIX, the second field can contain one of two characters, either ! or *. If the ! is displayed, a password has been set for the user. If no password has been set, *appears. The passwords themselves are stored in /etc/security/passwd.

  • UID. The User Identifier (UID) is a numeric identifier to a user.

  • GID. The Group Identifier (GID) is similar to the UID but is associated with groups. The GIDs are defined in /etc/group.

  • GECOS. The General Electric Comprehensive Operating System (GECOS) information is stored in the fifth field. The user's name, phone numbers, and other generic personal information are stored here.

  • Home. This is the user's home directory.

  • Shell/Command. Typically, the seventh and final field contains the shell that is started at the user's login. Administrators can also change this field to execute other commands instead of shells to limit access (for example, /bin/false).

/etc/security/.profile

The file /etc/security/.profile can really save you some valuable time and ease frustration. When you create a user using themkuser command, the script /usr/lib/security/mkuser.sys is executed. This script creates the user's directory, sets the correct permissions, and "creates" the user's .profile. The mkuser.sys script actually copies the /etc/security/.profile file into the user's new home directory.

If you are building a new system, or maybe a new division of 100 people needs accounts on a system, make sure you make your changes to the /etc/security/.profile file before creating all the users' accounts. If you create the accounts and then realize that you need to make a simple change in a variable or modify another setting, you're going to have to manually make the change to everyone's profile. It could be scripted out easily, but life would have been much simpler if you would have just changed the /etc/security/.profile.

 Example /etc/security/.profile file

/etc/security/limits

The /etc/security/limits file contains all the ulimits, or users' system resource limitations. Table 1 defines the fields in the /etc/security/limits file and their use.

Table 1. Fields in /etc/security/limits

What's the difference between soft and hard limits? A soft limit is a value that a user or application can change on the fly up to the maximum (the hard limit). The hard limit is just that -- the maximum value a parameter can be set to. If setting the parameter to a numeric value is too difficult (for example, if a developer has no idea how much memory his or her program is going to take or the number of files it will need to open), you can set the parameter to -1, which translates to unlimited.

It isn't imperative that you set individual ulimits for each and every user, however. The /etc/security/limits file contains a section called default that defines a template of standard values for each user unless that user has set custom values. If the defaultsection doesn't exist, IBM kindly set predetermined limits just in case.

The IBM default values are:

Listing 3 provides an example of a /etc/security/limits file.

Listing 3. Example /etc/security/limits file

Considering that user "pac" is a junior UNIX administrator, his fsize soft value was reduced from the default section's 4,194,303 to 131,072; however, you were nice enough to allow the user to increase his value to 262,144, if needed. Also, pac is notorious for finding ways to break his own programs. As a result, you've increased his core ulimit to 262,144.

/etc/security/passwd

The /etc/security/passwd file contains the AIX user's password information. The file contains three fields per user:

• • password. Encrypted password.

    • Note: If this field contains only an asterisk (*), the account is locked until a password has been set.

  • lastupdate. Number of seconds since epoch when the password was last updated.

  • flags. Restrictions to changing the user's password. You can set three different flags:

    • • ADMIN. If set, only the root user can change the user's password.

      • ADMCHG. If set, the user is prompted to change his or her password on the next login/su.

      • NOCHECK. If set, any additional restrictions in /etc/security/user are ignored.

Listing 4 provides an example of a /etc/security/password file.

Listing 4. Example /etc/security/password file

In this example, user "amdc" has a password that was set on Tue. Jun. 2 15:46:46 EDT 2009. The next time the user logs in orsus to amdc, he or she will be prompted to change the password.

If you're wondering how I converted the seconds from epoch to something a little more readable, I wrote a Perl script. Here's the gist of that script:

/etc/security/user

Now you're getting into the meat of AIX user administration. The /etc/security/user file contains the most important settings, outside of the basics in /etc/passwd, for a user. Table 2 shows some of the parameters.

Table 2. Parameters in the /etc/security/user file

For a full listing of all parameters, look in your AIX system under /etc/security/user, or see AIX Information Center. Like /etc/security/limits, a default section sets all the fields if not specified by the individual account.