Single sign-on across a mixed fleet: joining Linux hosts to a domain with realmd and SSSD, Kerberos and GSSAPI for SSH, mapping AD users and groups to POSIX, sudo rules sourced from AD, Samba for file shares and as a domain member, and the alternative of FreeIPA with an AD trust. Making one identity work everywhere without running everything on Windows.
Sssd
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Integrating Linux with Active Directory -
PAM Configuration Deep Dive How Linux PAM really decides whether you get a shell — the module stack, control flags, and the auth, account, session, and password phases — so you can read /etc/pam.d, add MFA or SSSD, and debug a silent login rejection.