NIS

On Ubunto when setting up NIS following normal procedures will lead to that you can read ypcat passwd, but you still can't log in.

The following is needed additionally:

Last line in /etc/passwd:
+::::::

Last line in  /etc/group:
+:::

Check that  /etc/nsswitch.conf contains the following:
passwd:         compat
group:          compat
shadow:         compat

hosts:          files  nisplus nis dns mdns