mux.js

module.exports = function () {

  var server = createMuxDemux()

  var methods = {
          protocols: function (cb) {
            cb(getProtocols())
          }

        , open: function (protocol, cb) {
            if (speaks(protocol)) {
              var read = createStream(protocol)
              var mux = server.createStream(protocol)

              var path = '/home/magnus/client.txt'
              var write = fs.createWriteStream(path)

              read.pipe(mux)
              mux.pipe(write)

              mux.on('data', function (data) {
                // this never happens
                console.error('M <- ' + data.toString())
              })

              cb(null)
            } else {
              cb({ message: 'Invalid protocol', protocol: protocol })
            }
          }
      }

  var d = dnode(methods)
  d.pipe(server.createStream('dnode')).pipe(d)

  return server
}

The server variable returned from the function is returned back to an ordinary tcp connection