code: func registerDNSChallengeHandler(ws *webserver.Server, config *serverinit.Config) (*gpgchallenge.Client, error) {
  	camliNetIP := config.CamliNetIP()
  	if camliNetIP == "" {
  		return nil, nil
  	}
  	if ip := net.ParseIP(camliNetIP); ip == nil {
  		return nil, fmt.Errorf("camliNetIP value %q is not a valid IP address", camliNetIP)
  	}
  
  	keyRing, keyId, err := config.KeyRingAndId()
  	if err != nil {
  		return nil, err
  	}
  
  	cl, err := gpgchallenge.NewClient(keyRing, keyId, camliNetIP)
  	if err != nil {
  		return nil, fmt.Errorf("could not init gpgchallenge client: %v", err)
  	}
  	ws.Handle(cl.Handler())
  	return cl, nil
  }
docs: // registerDNSChallengeHandler initializes and returns the
  // gpgchallenge Client if camliNetIP is configured and if so,
  // registers its handler with Perkeep's muxer.
  //
  // If camlistore.net support isn't enabled, it returns (nil, nil).