I'm going to have an Ubuntu 11.10 box in a customer's data center running a custom webapp. The customer will not have ssh access to the box, but will need authentication and authorization to access the webapp. The customer needs to have the option of either pointing the webapp at something that we've installed locally on the machine, or to use an Active Directory server that they have. I plan on using a standard "users belong to groups; groups have sets of permissions; the webapp requires certain permissions to respond" auth setup.
What software stack can I install locally that will allow an easy switch to and from an Active Directory server, while keeping the configuration as simple as possible (both for me and the end customer)?
I would like to use as much off-the-shelf software for this as possible; I do not want to be in the business of keeping user passwords secure. I could see handling the user/group/permission relationships myself if there is not a good out-of-the-box solution (but that seems highly unlikely).
I will accept answers in the form of links to "here is what you need" pages, but not "here is what Kerberos does" unless that page also tells me if it's required for my use case (essentially, I know that AD can speak Kerberos, but I can't tell if I need it to, or if I can just use LDAP, or...).