pam.d

TLDR

View login PAM config

$ cat /etc/pam.d/login

View SSH PAM config

$ cat /etc/pam.d/sshd

View sudo PAM config

$ cat /etc/pam.d/sudo

View common authentication

$ cat /etc/pam.d/common-auth

SYNOPSIS

/etc/pam.d/ - PAM configuration directory

/etc/pam.d/ contains per-service PAM configuration files. Each file defines authentication rules for a specific service (login, sshd, sudo, etc.).
This directory-based configuration replaced the monolithic /etc/pam.conf on modern systems.

FILE FORMAT

$ # /etc/pam.d/login
type  control  module  [arguments]

auth      required   pam_securetty.so
auth      required   pam_unix.so
account   required   pam_unix.so
session   required   pam_limits.so

COMMON FILES

$ common-auth     - Shared auth rules
common-account  - Shared account rules
common-password - Password rules
common-session  - Session rules
login           - Console login
sshd            - SSH daemon
sudo            - Sudo command

TYPES

$ auth     - Authentication
account  - Account management
password - Password updates
session  - Session setup/teardown

CAVEATS

Errors can lock users out. Test with spare root session. Use @include for common configs.