Suggestion please...

[Beast <beast@setuid.com>]

Hello,

We have four sites connected using leased line. We want to build directory
server for user authentication.
What is the best design for this which support these follwing requirement:
- user in all site can lookup each other.
- user in site A can also login and auth form site B and vice versa.
- when link is down, user in one site can still "see" all users in all sites.
- when link is down, user in every site can still login and modify their
own password (in their own site of course).

any suggestion using other tools (ie. rsync) are welcome.

Tks & regards,

B.