I'm looking for a list of characters that are not allowable (or that cause problems) for passwords if any under a standard Debian GNU/Linuxinstall (using md5).
AFAIK, there aren't any. You may run into limitations in particular programs, but there shouldn't be any limits on the input to the hash function whose output is stored in the shadow file.[0] Mike Stone 0. With the obvious exception that C strings don't like null bytes. So try to avoid hitting the null key on your keyboard. :)