You may want to take a look at the WebSock & WebSockAdapter libraries, along with the new upgrade_adapter/3 function added in Plug 1.14. This is the stack that Phoenix is based on as of 1.7, and comprises a fully open standard WebSocket support in both Bandit and Cowboy web servers.
For what you’re looking to do, you’d want to grab the relevant parts of the Plug.Conn request as part of your HTTP handler for your WebSocket upgrade, and then pass those to your WebSock implementation as the second element of the arg tuple to upgrade_adapter/3. Your WebSock implementation will then see those values as the value to its init/1 call.
Howto docs are a little bit sparse at the moment, but some of the test servers in the Bandit test suite (such as this one) should probably be pretty instructive.
