[Samba] Custom Authentication Plugin (passdb backend)

[Nick Coons]

I'm looking to create a "passdb backend" plugin so that Samba can 
authenticate with our existing custom authentication system.  Though I 
haven't been able to locate any documentation on this, I believe it's 
possible (without modifying Samba proper) for the following reasons:

- This project exists (though isn't being maintained and doesn't work
   with the current version):
   https://sourceforge.net/projects/pdbsql

- When I set "passdb backend = foo" in smb.conf, my Samba logs show the
   following error:
   "No builtin nor plugin backend for foo found"
   Which indicates to me that it's at least looking for a plugin that it
   can use.

I understand that it's more than just authentication.  It's also a 
variety of other pieces of data that I'll need to store and be able to 
return.

I'm wondering if documentation exists that can lead me in the right 
direction for how to get started on creating this.