14.4. Sample PAM Configuration Files

Below is a sample PAM application configuration file:

#%PAM-1.0
auth      required  /lib/security/pam_securetty.so
auth      required  /lib/security/pam_unix.so shadow nullok
auth      required  /lib/security/pam_nologin.so
account   required  /lib/security/pam_unix.so
password  required  /lib/security/pam_cracklib.so retry=3
password  required  /lib/security/pam_unix.so shadow nullok use_authtok
session   required  /lib/security/pam_unix.so

The first line is a comment as denoted by the hash mark (#) at the beginning of the line.

Lines two through four stack three modules for login authentication.

auth      required  /lib/security/pam_securetty.so

This module makes sure that if the user is trying to log in as root, the tty on which the user is logging in is listed in the /etc/securetty file, if that file exists.

auth      required  /lib/security/pam_unix.so shadow nullok

This module prompts the user for a password and then checks the password using the information stored in /etc/passwd and, if it exists, /etc/shadow. The pam_unix.so module automatically detects and uses shadow passwords to authenticate users. Please refer to the Section 6.5 Shadow Passwords for more information on shadow passwords.

The argument nullok instructs the pam_unix.so module to allow a blank password.

auth      required  /lib/security/pam_nologin.so

This is the final authentication step. It results to see if the file /etc/nologin exists. If nologin does exist and the user is not root, authentication fails.

	Note
	In this example, all three auth modules are checked, even if the first auth module fails. This prevents the user from knowing at what stage their authentication failed. Such knowledge in the hands of an attacker could allow them to more easily deduce how to crack the system.

account   required  /lib/security/pam_unix.so

This module performs any necessary account verification. For example, if shadow passwords have been enabled, the account component of the pam_unix.so module will check to see if the account has expired or if the user has not changed the password within the grace period allowed.

password  required  /lib/security/pam_cracklib.so retry=3

If a password has expired, the password component of the pam_cracklib.so module prompts for a new password. It then tests the newly created password to see whether it can easily be determined by a dictionary-based password cracking program. If it fails this test the first time, it gives the user two more chances to create a strong password, as specified in the retry=3 argument.

password  required  /lib/security/pam_unix.so shadow nullok use_authtok

This line specifies that if the program changes the user's password, it should use the password component of the pam_unix.so module to do so. This only happens if the auth portion of the pam_unix.so module has determined that the password needs to be changed.

The argument shadow tells the module to create shadow passwords when updating a user's password.

The argument nullok instructs the module to allow the user to change their password from a blank password, otherwise a null password is treated as an account lock.

The final argument on this line, use_authtok, provides a good example of the importance of order when stacking PAM modules. This argument tells the module not to prompt the user for a new password. Instead, it accepts any password that was recorded by a previous password module. In this way all, new passwords must pass the pam_cracklib.so test for secure passwords before being accepted.

session required /lib/security/pam_unix.so

The final line specifies that the session component of the pam_unix.so module will manage the session. This module logs the username and the service type to /var/log/messages at the the beginning and end of each session. It can be supplemented by stacking it with other session modules for more functionality.

The next sample configuration file illustrates auth module stacking for the rlogin program.

#%PAM-1.0
auth      required    /lib/security/pam_nologin.so
auth      required    /lib/security/pam_securetty.so
auth      required    /lib/security/pam_env.so
auth      sufficient  /lib/security/pam_rhosts_auth.so
auth      required    /lib/security/pam_stack.so service=system-auth

First, pam_nologin.so checks to see if /etc/nologin exists. If it does, no one can log in except for root.

auth      required    /lib/security/pam_securetty.so

The pam_securetty.so module prevents the root user from logging in on insecure terminals. This effectively disallows all root rlogin attempts due to the application's limited security safeguards.

	Tip
	To log in remotely as the root user, use OpenSSH instead. For more information on the SSH protocol, see Chapter 18 SSH Protocol.

auth      required    /lib/security/pam_env.so

This line loads the pam_env.so module, which sets the environmental variables specified in /etc/security/pam_env.conf.

auth      sufficient  /lib/security/pam_rhosts_auth.so

The pam_rhosts_auth.so module authenticates the user using .rhosts in the user's home directory. If this succeeds, PAM immediately considers the authentication to have succeeded. If pam_rhosts_auth.so fails to authenticate the user, the authentication attempt is ignored.

auth      required    /lib/security/pam_stack.so service=system-auth

If the pam_rhosts_auth.so module fails to successfully authenticate the user, the pam_stack.so module performs normal password authentication.

The argument service=system-auth indicates that the user must now pass through the PAM configuration for system authentication as found in /etc/pam.d/system-auth.

	Tip
	To prevent PAM from prompting for a password when the securetty result fails, change the pam_securetty.so module from required to requisite.