Does anyone know of any addons to nginx that would let it do content checks to determine if an upstream server is alive or not, i.e. check a certain page for a given string?

We are hoping to replace a lot of old ldirectord load balancers but this is a feature we really need to have. All too often a web server (cough cough IIS) will be responding to connections but giving errors instead of showing valid content (cough cough ASP.NET), but by doing a content check the load balancer will take the broken server out of the farm.

Thanks!