diff -up man-pages-3.40/man5/passwd.5.orig man-pages-3.40/man5/passwd.5
--- man-pages-3.40/man5/passwd.5.orig	2012-04-27 00:58:39.000000000 +0200
+++ man-pages-3.40/man5/passwd.5	2012-04-27 14:03:10.113106253 +0200
@@ -50,6 +50,15 @@ and the encrypted passwords are in
 .IR /etc/shadow ,
 which is readable by the superuser only.
 .PP
+If the encrypted password, whether in /etc/passwd or in /etc/shadow, is
+an empty string, login is allowed without even asking for a password.
+Note that this functionality may be intentionally disabled in applications,
+or configurable (for example using the "nullok" or "nonull" arguments to
+pam_unix.so).
+.PP
+If the encrypted password in /etc/passwd is "*NP*" (without the quotes),
+the shadow record should be obtained from a NIS+ server.
+.PP
 Regardless of whether shadow passwords are used, many system administrators
 use an asterisk (*) in the encrypted password field to make sure
 that this user can not authenticate him- or herself using a