Chapter 16. Authentication with PAM

Contents

16.1. Structure of a PAM Configuration File
16.2. The PAM Configuration of sshd
16.3. Configuration of PAM Modules
16.4. For More Information

Abstract

Linux uses PAM (pluggable authentication modules) in the authentication process as a layer that mediates between user and application. PAM modules are available on a systemwide basis, so they can be requested by any application. This chapter describes how the modular authentication mechanism works and how it is configured.

System administrators and programmers often want to restrict access to certain parts of the system or to limit the use of certain functions of an application. Without PAM, applications must be adapted every time a new authentication mechanism, such as LDAP or SAMBA, is introduced. This process, however, is rather time-consuming and error-prone. One way to avoid these drawbacks is to separate applications from the authentication mechanism and delegate authentication to centrally managed modules. Whenever a newly required authentication scheme is needed, it is sufficient to adapt or write a suitable PAM module for use by the program in question.

Every program that relies on the PAM mechanism has its own configuration file in the directory /etc/pam.d/programname. These files define the PAM modules used for authentication. In addition, there are global configuration files for most PAM modules under /etc/security, which define the exact behavior of these modules (examples include pam_env.conf, pam_pwcheck.conf, pam_unix2.conf, and time.conf). Every application that uses a PAM module actually calls a set of PAM functions, which then process the information in the various configuration files and return the result to the calling application.


16.1. Structure of a PAM Configuration File

Each line in a PAM configuration file contains a maximum of four columns:

<Type of module> <Control flag> <Module path> <Options>
  

PAM modules are processed as stacks. Different types of modules have different purposes, for example, one module checks the password, another one verifies the location from which the system is accessed, and yet another one reads user-specific settings. PAM knows about four different types of modules:

auth

The purpose of this type of module is to check the user's authenticity. This is traditionally done by querying a password, but it can also be achieved with the help of a chip card or through biometrics (fingerprints or iris scan).

account

Modules of this type check whether the user has general permission to use the requested service. As an example, such a check should be performed to ensure that no one can log in under the username of an expired account.

password

The purpose of this type of module is to enable the change of an authentication token. In most cases, this is a password.

session

Modules of this type are responsible for managing and configuring user sessions. They are started before and after authentication to register login attempts in system logs and configure the user's specific environment (mail accounts, home directory, system limits, etc.).

The second column contains control flags to influence the behavior of the modules started:

required

A module with this flag must be successfully processed before the authentication may proceed. After the failure of a module with the required flag, all other modules with the same flag are processed before the user receives a message about the failure of the authentication attempt.

requisite

Modules having this flag must also be processed successfully, in much the same way as a module with the required flag. However, in case of failure a module with this flag gives immediate feedback to the user and no further modules are processed. In case of success, other modules are subsequently processed, just like any modules with the required flag. The requisite flag can be used as a basic filter checking for the existence of certain conditions that are essential for a correct authentication.

sufficient

After a module with this flag has been successfully processed, the calling application receives an immediate message about the success and no further modules are processed, provided there was no preceding failure of a module with the required flag. The failure of a module with the sufficient flag has no direct consequences, in the sense that any subsequent modules are processed in their respective order.

optional

The failure or success of a module with this flag does not have any direct consequences. This can be useful for modules that are only intended to display a message (for example, to tell the user that mail has arrived) without taking any further action.

include

If this flag is given, the file specified as argument is inserted at this place.

The module path does not need to be specified explicitly, as long as the module is located in the default directory /lib/security (for all 64-bit platforms supported by SUSE Linux, the directory is /lib64/security). The fourth column may contain an option for the given module, such as debug (enables debugging) or nullok (allows the use of empty passwords).