poky/meta/recipes-extended/pam/libpam/pam.d/common-auth

/etc/pam.d/common-auth - authentication settings common to all services

This file is included from other service-specific PAM config files,

and should contain a list of the authentication modules that define

the central authentication scheme for use on the system

(e.g., /etc/shadow, LDAP, Kerberos, etc.). The default is to use the

traditional Unix authentication mechanisms.

here are the per-package modules (the "Primary" block)

auth [success=1 default=ignore] pam_unix.so nullok_secure

here's the fallback if no module succeeds

auth requisite pam_deny.so

prime the stack with a positive return value if there isn't one already;

this avoids us returning an error just because nothing sets a success code

since the modules above will each just jump around

auth required pam_permit.so

and here are more per-package modules (the "Additional" block)