Chapter 7. Pluggable Authentication Modules (PAM)

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This line causes the user to be asked for a password and then checks the password using the infor 

mation stored in

/etc/passwd

and, if it exists,

/etc/shadow

. The

pam_unix.so

module automat 

ically detects and utilizes shadow passwords, stored in

/etc/shadow

, to authenticate users. Please

refer to the Section 5.5 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 checks 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 line causes any necessary account verification to be done. 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 their 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 the 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, due to 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

com 

ponent of the

pam_unix.so

module to do so. This will happen only if the

auth

portion of the

pam_unix.so

module has determined that the password needs to be changed   for example, if a

shadow password has expired.

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 how one can stack PAM

modules. This argument tells the module not to prompt the user for a new password. Instead it is

to accept any password that passes through previous password module. 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

if you need more functionality.

The next sample configuration file will illustrate

auth

module stacking for the

rlogin

program   a

program that allows users to log in remotely.