[horde] passwd again

[Frank Lienhard]

I installed passwd according to official doku via pear.

But I get:
No backend configured for this host
when trying to change passwords as user.

I understand there is some backend to configure, but I lack the
knowledge where to define and more important what backend (most post
talk about ldap, which seams wrong in my setup, since I don't use any
ldap server. So what is the default way of how horde organizes user/passwd)?