I trying to implement a very basic auth system that will grant internet through a non-password router (TP Link TL-WR841ND) on a form post to a URL. The TP Link has openWRT installed. 

I have searched around and have evaluated a few options such as chilli, coova-chilli, wifidog, but as far as I can understand they do require radius on an external server to perform auth which I would like to avoid since it's more complicated than what I am willing to take on.

I was wondering if it is possible to achieve this using iptables or traffic rules,

The desired flow:


Users connect to non-password wifi
Users try to access any url 
Users get redirected to the router www/ where the html form live
Users post form to url [myauthservice.com] (only permitted ip)
Response is received from url [json, xml] 
Router allow users to browse freely over the internet for its session


Any ideas, suggestions are welcome!