code: func httpTransport(df dialerFunc) *http.Transport {
  	return &http.Transport{
  		DialContext: df,
  		// We are talking to a client that does not yet have a certificate,
  		// so we accept a temporary, invalid one.
  		TLSClientConfig: &tls.Config{InsecureSkipVerify: true},
  		// We don't expect to make multiple requests to a client, so close
  		// connection immediately.
  		DisableKeepAlives: true,
  		// We don't want idle connections, but 0 means "unlimited," so we pick 1.
  		MaxIdleConns:        1,
  		IdleConnTimeout:     time.Second,
  		TLSHandshakeTimeout: 10 * time.Second,
  	}
  }
docs: // httpTransport constructs a HTTP Transport with settings appropriate for
  // HTTP-01 validation. The provided dialerFunc is used as the Transport's
  // DialContext handler.