PAM - Pluggable Authentication Modules

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Basic Information

PAM (Pluggable Authentication Modules) acts as a security mechanism that verifies the identity of users attempting to access computer services, controlling their access based on various criteria. It's akin to a digital gatekeeper, ensuring that only authorized users can engage with specific services while potentially limiting their usage to prevent system overloads.

Configuration Files

Solaris and UNIX-based systems typically utilize a central configuration file located at /etc/pam.conf.
Linux systems prefer a directory approach, storing service-specific configurations within /etc/pam.d. For instance, the configuration file for the login service is found at /etc/pam.d/login.

An example of a PAM configuration for the login service might look like this:

auth required /lib/security/pam_securetty.so
auth required /lib/security/pam_nologin.so
auth sufficient /lib/security/pam_ldap.so
auth required /lib/security/pam_unix_auth.so try_first_pass
account sufficient /lib/security/pam_ldap.so
account required /lib/security/pam_unix_acct.so
password required /lib/security/pam_cracklib.so
password required /lib/security/pam_ldap.so
password required /lib/security/pam_pwdb.so use_first_pass
session required /lib/security/pam_unix_session.so

PAM Management Realms

These realms, or management groups, include auth, account, password, and session, each responsible for different aspects of the authentication and session management process:

Auth: Validates user identity, often by prompting for a password.
Account: Handles account verification, checking for conditions like group membership or time-of-day restrictions.
Password: Manages password updates, including complexity checks or dictionary attacks prevention.
Session: Manages actions during the start or end of a service session, such as mounting directories or setting resource limits.

PAM Module Controls

Controls dictate the module's response to success or failure, influencing the overall authentication process. These include:

Required: Failure of a required module results in eventual failure, but only after all subsequent modules are checked.
Requisite: Immediate termination of the process upon failure.
Sufficient: Success bypasses the rest of the same realm's checks unless a subsequent module fails.
Optional: Only causes failure if it's the sole module in the stack.

Example Scenario

In a setup with multiple auth modules, the process follows a strict order. If the pam_securetty module finds the login terminal unauthorized, root logins are blocked, yet all modules are still processed due to its "required" status. The pam_env sets environment variables, potentially aiding in user experience. The pam_ldap and pam_unix modules work together to authenticate the user, with pam_unix attempting to use a previously supplied password, enhancing efficiency and flexibility in authentication methods.

References

https://hotpotato.tistory.com/434