Linux Tactic

Maximizing System Security with Linux-PAM Configuration

Introduction to Linux-PAM

If you are a regular user of Linux, you might have used Pluggable Authentication Modules (PAM) at some point without knowing it. PAM is a system that allows applications and users to be authenticated against a range of authentication services, including passwords, biometrics, smart cards, and even fingerprint scanners.

In this article, we will provide an in-depth explanation of Linux-PAM, discuss its importance in terms of system security, and explain its interface in Ubuntu 20.04. Whether you’re a beginner or an experienced user of Linux, this article will help you understand the unique features of PAM configuration and how to use them.

Definition and Background of Linux-PAM

Linux-PAM, also known as Unix-PAM, is a framework for authentication services on Linux-based operating systems. It has been widely used since the mid-1990s to provide a flexible and secure authentication mechanism for Linux applications.

It was designed to provide a single, easy-to-use interface to host multiple authentication modules so that administrators could choose the appropriate authentication solutions for their needs. The primary purpose of Linux-PAM is to authenticate users by verifying their identity, usually by using a combination of credentials (e.g., user name, password, security token, etc.).

This authentication process is essential for keeping a system or server safe and secure from unauthorized access, hacks, or attacks.

Importance of Understanding PAM Configuration Files

To understand Linux-PAM, it is crucial to understand PAM configuration files. These files are used by the system administrator to configure the authentication services of PAM.

The configuration files list all of the modules that are used to authenticate users, what order they are applied in, and what options are associated with each module.

System security depends on PAM configuration files, which can be the main point of entry for attackers.

Thus, understanding PAM configuration files is essential for Linux administrators to keep their systems secure. Interfaces of Linux-PAM in Ubuntu 20.04

There are four management groups that handle PAM authentication tasks in Ubuntu 20.04: Account, Authentication, Password, and Session.

1. Account Module

The account module is responsible for determining if the account seems legitimate and active.

It verifies the account’s availability, expiration, grace period, and access privileges, among other things. It works by examining account databases such as /etc/passwd and /etc/shadow to determine the validity of the account.

2. Authentication Module

The authentication module is the heart of the PAM system.

It verifies the password or other credentials entered by the user. It performs several checks, such as checking for weak passwords, expired passwords, and incorrect password attempts.

It can also use one or more of the available authentication services, such as local passwords, LDAP, or Kerberos, to authenticate the user. 3.

Password Module

The password module enforces password policy on user accounts. It checks the password’s strength, expiration, and reuse history.

One useful feature of the password module is support for password changing to promote proactive password management. 4.

Session Module

The session module handles the details of session management, which is useful for logging user activity. For example, a session could be defined as when a user logs in and logs out.

Session management is crucial for tracking user activity, auditing events, and logging out of idle or inactive users.

Importance of Session Module

The session module is essential for monitoring system usage. It covers authentication activity and other actions performed during the user’s session, such as opening files, closing files, and launching applications.

An attacker could perform an unauthorized action during a user’s session if the session was poorly managed. The session module monitors those instances to prevent malicious or unauthorized activities.


In conclusion, Linux-PAM is an essential framework required for authentication services on Linux-based operating systems. It provides applications and users the ability to be authenticated against a range of authentication services, promoting a secure authentication mechanism for Linux applications.

Understanding PAM configuration files is of utmost importance in keeping a system or server from unauthorized access, hacks, or attacks. While the Account, Authentication, Password, and Session modules are essential, the session module is crucial in monitoring the system’s usage, which prevents unauthorized or malicious activity.

How to Validate a Linux-PAM Program in Ubuntu 20.04

As Linux-PAM serves as an essential framework for authentication services in Linux-based operating systems, it is important that applications or services should be “PAM-aware,” meaning that they support authentication using PAM. When you update or install a new program, it is important to verify that it is PAM-aware and implementing PAM usage in the right way.

Using the “ldd” Command to Validate PAM Usage

The “ldd” command shows a shared library’s dependencies. One way to validate PAM usage is by checking if the PAM library is being used by the program using the “ldd” command.

The following example shows how to use “ldd” to verify if the PAM library is being used by the program named “myprogram”:


$ ldd /usr/bin/myprogram | grep libpam => /lib/x86_64-linux-gnu/ (0x00007f7399929000)


The output shows that the “myprogram” program uses the PAM library and the path from where it loads the library. If the output does not show the PAM library, it means that the program is not using PAM or didn’t configure it correctly.

Checking Services That Implement PAM

It is important to check if services that depend on PAM implement it correctly. For instance, the sshd server is dependent on PAM for user authentication.

To verify that sshd server is using PAM, run the following command:


$ sudo grep “UsePAM” /etc/ssh/sshd_config

UsePAM yes


The output shows that “UsePAM” is set to “yes,” which means that sshd server is correctly using PAM. Another command that you can use to verify PAM usage is by checking if the OpenSSH server package is installed.

You can do this by running the following command:


$ dpkg -l | grep openssh-server


If the OpenSSH server package is installed, it means that the PAM library is included, and PAM usage is enabled. How to Configure a Linux-PAM Program in Ubuntu 20.04

Configure control flags in PAM modules to determine how PAM behaves when an authentication process fails or succeeds.

Control flags include “required,” “requisite,” “sufficient,” and “optional.”

Writing the Main Configuration Using Service, Type, Control-Flag, Module, and Module-Argument

The PAM modules are the building blocks for constructing authentication mechanisms. The main configuration file stores the configuration for different services (e.g., sshd, sudo, login, etc.) using a naming convention or service name.

Each service name contains “types” of interaction for authentication, which is defined with module name and module arguments. Lastly, control flags complete the configuration.

Here is a basic syntax format in defining a PAM module configuration:



For example, if you want to define the configuration for the “sshd” service to limit the number of failed authentication attempts, add the following line in the “/etc/pam.d/sshd” file:


auth required deny=5 onerr=fail unlock_time=1800


The “auth,” “required,” and “” represent the type, control flag, and the module name, respectively, while the “deny=5 onerr=fail unlock_time=1800” are the module arguments. Restricting Access Using PAM Module

Restricting access can be done using the PAM module.

This module allows you to define a file containing a list of restricted users, which can be used by the authentication system to prevent access by those users. Here is how to configure PAM to use the module in “/etc/pam.d/sshd” for the sshd service.


auth requisite deny=denyusers.txt


This configuration requires that the file indicated by “deny=denyusers.txt” (which should be present in the same directory where the PAM configuration files are located) lists the user accounts that should not be allowed to access the system. The list would be stored as a simple text file with each line containing one username or UID.


By validating and configuring PAM programs, users can ensure that authentication-related services are implemented correctly and provide a safe and secure environment. We have provided examples of how to check if a program or service is PAM-aware, the use of control flags in PAM modules, and a basic syntax for writing main configurations.

We also show how to restrict access using the PAM module. By following these steps, a user can protect their system and data from unauthorized access.


Linux-PAM offers a powerful and complex authentication framework that can support a variety of authentication methods, including password-based authentication, biometric authentication, and two-factor authentication. Its dynamic authentication architecture makes it highly customizable and enhances its performance, making it an attractive choice for system administrators and developers.

Power and Complexity of Linux-PAM

The true power of the PAM architecture lies in its ability to dynamically authenticate users without relying on any particular authentication method. This approach provides a great deal of flexibility in how the authentication process is carried out and allows for the use of different authentication mechanisms as required.

This complexity, though, comes with a cost. PAM modules require a high level of expertise in terms of configuring, testing, and deploying them.

Unless properly implemented, PAM can lead to bugs, slow loading times, and other performance issues.

Advantages of PAM over Traditional Authentication Mechanisms

One of the main advantages of PAM is that it offers a user-friendly authentication mechanism. Password and username based authentication is a commonly used method that is easy to understand for users.

Users do not need to learn or remember complex authentication processes, which can be the case in other authentication mechanisms. Another advantage of PAM is that it provides a dependable authentication mechanism.

PAM has been tested extensively, and many of its modules have been developed and deployed in critical production environments. This extensiveness ensures that the authentication processes are dependable and accurate.

One more advantage of PAM is its flexibility in terms of authentication methods. PAM supports a wide range of authentication methods, including passwords, smart cards, biometrics, fingerprints, and tokens, among others.

Organizations can select and implement the most appropriate method that best fits their specific security requirements. Lastly, PAM can also help organizations achieve compliance with regulatory requirements.

Most regulatory agencies stipulate that critical systems must have robust security measures in place, including strong authentication. Organizations can implement PAM to ensure that their authentication process meets regulatory requirements.

Final Thoughts

Linux-PAM is a powerful and complex authentication framework that offers a range of authentication methods and flexibility in terms of implementation. By understanding its configuration files, verifying and configuring PAM programs, and implementing its modules securely, organizations can ensure a safe and secure authentication process.

PAM is a dependable authentication mechanism that provides an effective and reliable solution to the organization’s security needs. In conclusion, Linux-PAM is a crucial framework for authentication services in Linux-based operating systems.

Understanding PAM configuration files is essential to ensure system security. Validating PAM programs using tools like “ldd” and checking services that implement PAM, such as the sshd server, help verify proper PAM usage.

Configuring PAM programs involves utilizing control flags and writing the main configuration with service, type, control-flag, module, and module-argument. PAM offers advantages like user-friendliness, dependability, and flexibility compared to traditional authentication mechanisms.

Overall, Linux-PAM is a powerful tool that deserves attention for enhancing system security and ensuring reliable authentication processes.

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