rack-mux

require 'rack/mux'

use Rack::Mux
run lambda {|env| [200, {'Content-Type' => 'text/plain'}, [env['X-Mux-Uri']]] }

% rackup config.ru -p 9292

>> require 'rack/client'
=> true
>> uri = Rack::Client.new("http://localhost:9292/").get("/").body
=> 'http://0.0.0.0:4000'
>> Rack::Client.new(uri).get('/').status
=> 200