The PAM routines do the work of password verification and changing. The minimum number of days required between password changes. The maximum number of days the password is valid. The calling program knows nothing of this. The passwd program is not invoked, hence npasswd cannot be inserted in the process.
Checks min to enforce minimal password age.
Does not check max , except to disallow changes under some conditions. Invokes the vendor passwd program to change aging for a password. There can be no password aging without supplimental information. When NIS maps are built, the encrypted password is taken from shadow entries and included in the passwd.byname and passwd.byuid maps. The format of these maps do not include the aging information.