code: func (s *Server) proxyHTTPChallenge(vhost *SiteConfig, w http.ResponseWriter, r *http.Request) bool {
  	if vhost.Addr.Port != caddytls.HTTPChallengePort {
  		return false
  	}
  	if vhost.TLS != nil && vhost.TLS.Manual {
  		return false
  	}
  	altPort := caddytls.DefaultHTTPAlternatePort
  	if vhost.TLS != nil && vhost.TLS.AltHTTPPort != "" {
  		altPort = vhost.TLS.AltHTTPPort
  	}
  	return caddytls.HTTPChallengeHandler(w, r, vhost.ListenHost, altPort)
  }
docs: // proxyHTTPChallenge solves the ACME HTTP challenge if r is the HTTP
  // request for the challenge. If it is, and if the request has been
  // fulfilled (response written), true is returned; false otherwise.
  // If you don't have a vhost, just call the challenge handler directly.