Each PAM-aware application or service has a file in the /etc/pam.d/ directory. Each file in this directory has the same name as the service to which it controls access. For example, the login program defines its service name as login and installs the /etc/pam.d/login PAM configuration file.
Warning
It is highly recommended to configure PAMs using the authconfig tool instead of manually editing the PAM configuration files.
10.2.1.PAM Configuration File Format
Each PAM configuration file contains a group of directives that define the module (the authentication configuration area) and any controls or arguments with it.
The directives all have a simple syntax that identifies the module purpose (interface) and the configuration settings for the module.
module_interfacecontrol_flagmodule_name module_arguments
In a PAM configuration file, the module interface is the first field defined. For example:
authrequiredpam_unix.so
A PAM interface is essentially the type of authentication action which that specific module can perform. Four types of PAM module interface are available, each corresponding to a different aspect of the authentication and authorization process:
auth— This module interface authenticates users. For example, it requests and verifies the validity of a password. Modules with this interface can also set credentials, such as group memberships.account— This module interface verifies that access is allowed. For example, it checks if a user account has expired or if a user is allowed to log in at a particular time of day.password— This module interface is used for changing user passwords.session— This module interface configures and manages user sessions. Modules with this interface can also perform additional tasks that are needed to allow access, like mounting a user's home directory and making the user's mailbox available.
An individual module can provide any or all module interfaces. For instance, pam_unix.so provides all four module interfaces.
The module name, such as pam_unix.so, provides PAM with the name of the library containing the specified module interface. The directory name is omitted because the application is linked to the appropriate version of libpam, which can locate the correct version of the module.
All PAM modules generate a success or failure result when called. Control flags tell PAM what to do with the result. Modules can be listed (stacked) in a particular order, and the control flags determine how important the success or failure of a particular module is to the overall goal of authenticating the user to the service.
There are several simple flags[2], which use only a keyword to set the configuration:
required— The module result must be successful for authentication to continue. If the test fails at this point, the user is not notified until the results of all module tests that reference that interface are complete.requisite— The module result must be successful for authentication to continue. However, if a test fails at this point, the user is notified immediately with a message reflecting the first failedrequiredorrequisitemodule test.sufficient— The module result is ignored if it fails. However, if the result of a module flaggedsufficientis successful and no previous modules flaggedrequiredhave failed, then no other results are required and the user is authenticated to the service.optional— The module result is ignored. A module flagged asoptionalonly becomes necessary for successful authentication when no other modules reference the interface.include— Unlike the other controls, this does not relate to how the module result is handled. This flag pulls in all lines in the configuration file which match the given parameter and appends them as an argument to the module.
Module interface directives can be stacked, or placed upon one another, so that multiple modules are used together for one purpose.
Note
If a module's control flag uses the sufficient or requisite value, then the order in which the modules are listed is important to the authentication process.
Using stacking, the administrator can require specific conditions to exist before the user is allowed to authenticate. For example, the setup utility normally uses several stacked modules, as seen in its PAM configuration file:
[root@MyServer ~]# cat /etc/pam.d/setupauth sufficientpam_rootok.soauth includesystem-authaccount requiredpam_permit.sosession requiredpam_permit.so
auth sufficient pam_rootok.so— This line uses thepam_rootok.somodule to check whether the current user is root, by verifying that their UID is 0. If this test succeeds, no other modules are consulted and the command is executed. If this test fails, the next module is consulted.auth include system-auth— This line includes the content of the/etc/pam.d/system-authmodule and processes this content for authentication.account required pam_permit.so— This line uses thepam_permit.somodule to allow the root user or anyone logged in at the console to reboot the system.session required pam_permit.so— This line is related to the session setup. Usingpam_permit.so, it ensures that thesetuputility does not fail.
PAM uses arguments to pass information to a pluggable module during authentication for some modules.
For example, the pam_pwquality.so module checks how strong a password is and can take several arguments. In the following example, enforce_for_root specifies that even password of the root user must successfully pass the strength check and retry defines that a user will receive three opportunities to enter a strong password.
passwordrequisitepam_pwquality.so enforce_for_root retry=3
Invalid arguments are generally ignored and do not otherwise affect the success or failure of the PAM module. Some modules, however, may fail on invalid arguments. Most modules report errors to the journald service. For information on how to use journald and the related journalctl tool, see the System Administrator's Guide.
Note
The journald service was introduced in RedHat EnterpriseLinux 7.1. In previous versions of RedHat EnterpriseLinux, most modules report errors to the /var/log/secure file.
10.2.2.Annotated PAM Configuration Example
Example10.1, “Simple PAM Configuration” is a sample PAM application configuration file:
Example10.1.Simple PAM Configuration
#%PAM-1.0authrequiredpam_securetty.soauthrequiredpam_unix.so nullokauthrequiredpam_nologin.soaccountrequiredpam_unix.sopasswordrequiredpam_pwquality.so retry=3passwordrequiredpam_unix.so shadow nullok use_authtoksessionrequiredpam_unix.so
The first line is a comment, indicated by the hash mark (
#) at the beginning of the line.Lines two through four stack three modules for login authentication.
auth required pam_securetty.so— This module ensures 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/securettyfile, if that file exists.If the TTY is not listed in the file, any attempt to log in as root fails with a
Login incorrectmessage.auth required pam_unix.so nullok— This module prompts the user for a password and then checks the password using the information stored in/etc/passwdand, if it exists,/etc/shadow.The argument
nullokinstructs thepam_unix.somodule to allow a blank password.auth required pam_nologin.so— This is the final authentication step. It checks whether the/etc/nologinfile exists. If it exists and the user is not root, authentication fails.Note
In this example, all three
authmodules are checked, even if the firstauthmodule 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 pam_unix.so— This module performs any necessary account verification. For example, if shadow passwords have been enabled, the account interface of thepam_unix.somodule checks to see if the account has expired or if the user has not changed the password within the allowed grace period.password required pam_pwquality.so retry=3— If a password has expired, the password component of thepam_pwquality.somodule 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.The argument
retry=3specifies that if the test fails the first time, the user has two more chances to create a strong password.password required pam_unix.so shadow nullok use_authtok— This line specifies that if the program changes the user's password, using thepasswordinterface of thepam_unix.somodule.The argument
shadowinstructs the module to create shadow passwords when updating a user's password.The argument
nullokinstructs 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 instructs 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 thepam_pwquality.sotest for secure passwords before being accepted.
session required pam_unix.so— The final line instructs the session interface of thepam_unix.somodule to manage the session. This module logs the user name and the service type to/var/log/secureat the beginning and end of each session. This module can be supplemented by stacking it with other session modules for additional functionality.
[2] There are many complex control flags that can be set. These are set in attribute=value pairs; a complete list of attributes is available in the pam.d manpage.
