In this lesson, you will learn how to bridge your local Linux authentication stack with an Active Directory (AD) domain using realmd.
Understanding the Integration
Integrating Linux with Active Directory means shifting from local /etc/passwd management to a centralized identity provider. While PAM (Pluggable Authentication Modules) handles the how of authentication, realmd simplifies the setup by automatically configuring SSSD (System Security Services Daemon) and the necessary PAM stacks to recognize AD users.
When you join a domain, realmd updates your PAM configuration files (typically found in /etc/pam.d/) to include modules like pam_sss.so. This ensures that when a user attempts to log in, PAM queries SSSD, which in turn contacts the AD domain controller to verify credentials.
Joining the Domain
Before starting, ensure your system has the necessary packages installed (e.g., realmd, sssd, and adcli). You can discover your domain and join it with just a few commands:
# Verify the domain is reachable
realm discover ad.example.com
# Join the domain using an administrator account
realm join ad.example.com -U administrator
Once the join is successful, realmd automatically configures the system to allow AD users to authenticate. It will modify files like /etc/pam.d/system-auth and /etc/pam.d/password-auth, adding lines similar to:
auth sufficient pam_sss.so
account [success=1 default=ignore] pam_sss.so
Why PAM Integration Matters
Without PAM integration, your Linux machine wouldn't know how to handle the Kerberos tickets or security identifiers (SIDs) associated with AD users. By plugging pam_sss.so into the stack, you allow your system to dynamically create home directories for AD users upon their first login (if pam_mkhomedir is also configured) and enforce the organization's global password policies.
If you ever need to debug the handshake between PAM and AD, you can inspect the SSSD logs in /var/log/sssd/ or use journalctl -u sssd to see authentication events in real-time.
Key Takeaway: Using realmd to join an Active Directory domain automates the complex PAM configurations required to treat remote AD identities as local users, ensuring seamless authentication across your infrastructure.