em-proxy-http.rb

require 'em-proxy'

host = "0.0.0.0"
port = 9889
puts "listening on #{host}:#{port}..."

Sessions = {}

Proxy.start(:host => host, :port => port) do |conn|

  conn.on_connect do|data,b|
    puts [:on_connect, data, b].inspect
  end

  # modify / process request stream
  conn.on_data do |data|
    if data.match(/^[GET|POST]/)
      raw_headers = data.split("\n").map{|h| h.strip }
      req_method = raw_headers.shift
      headers = {}

      raw_headers.each do|kv|
        next if kv.strip == ''
        key   = kv.gsub(/:.*$/,'').strip.downcase
        value = kv.gsub(/.*:/,'').strip
        headers[key] = value
      end
      session_token = "#{headers['host']}_#{Time.now.to_i}_#{req_method}"
      session_port = headers['host'].split(':').last.to_i || 80
      session_port = 80 if session_port == 0
      puts "using port: #{session_port.inspect}"
      conn.server session_token, :host => headers['host'], :port => session_port

      #puts [:on_data, data].inspect
      puts "New session: #{session_token} (#{req_method})"
    end

    data
  end

  # modify / process response stream
  conn.on_response do |backend, resp|
    puts [:on_response, backend, resp].inspect
    resp
  end

  # termination logic
  conn.on_finish do |backend, name|
    puts [:on_finish, name].inspect

    # terminate connection (in duplex mode, you can terminate when prod is done)
    # unbind if backend == :srv
  end
end

Revised to change the conn server identifier by request name...

Having some problems with this approach - connections are definitely getting mixed... so the wrong response is being sent to some requests...