MS Active Directory LDAP – Using Active Directory as a Simple LDAP Server

To do what you're looking for in the easiest manner possible you really want a Windows Domain (as @Joel Coel says in his comment). You don't have to use Windows Server to host the Domain-- Samba has worked fine in that role for years now, and Samba can back-end authentication into LDAP.

Using a Windows Domain for this is preferable to any alternatives because you're leveraginga lot of client code Microsoft wrote and tested, rather than using code that re-invents the wheel. You can move to newer versions of Windows using a Domain, versus being tied to code (like pGina) that doesn't work in newer Windows versions.

Is this possible/feasible?

This is both feasible and common. If you search for something like openldap proxy active directory you'll find a number of useful results.

Is the term "proxy" the good term?

This is absolutely the correct term to use.

Is A Microsoft AD server mandatory or OpenLDAP will do the job fine?

If your clients are only expecting an LDAP server then OpenLDAP will be just fine, particularly if you'll only need read-only access.

I have few knowledge about AD/LDAP, how is the learning curve?

Without knowing your background that's a hard question to answer. I find that LDAP is fundamentally simple, but wrapping your head around access control in OpenLDAP can take a little work.

A few hints where to begin?

If all you need to do is making the AD server available within your local network, then a simple TCP proxy or appropriate iptables rules will be much simpler than a full-blown LDAP proxy. The downside of this is that you would need to perform any access control on the Active Directory side of things.

If you decide to go with OpenLDAP as a proxy:

• Step by Step Installation and Configuration of OpenLDAP as Proxy to Active Directory seems to fit the bill from the title, but it's not very good as a guide.

• The Samba documentation has some notes along these lines.

• This article I wrote a few years ago is more than you want, but does have some configuration examples.