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.
Pam
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PAM Configuration Deep Dive -
YubiKey for SSH, GPG, sudo, and FIDO2 A YubiKey is five security tokens in one — FIDO2, PIV, OpenPGP, OATH, and OTP. How to route SSH, GPG signing, sudo, and browser logins through hardware so your credentials live on a key you tap, not a file on disk.