Use dns.DefaultMsgSize instead of magic number 4096

doh-server/google.go

@@ -150,7 +150,7 @@ func (s *Server) parseRequestGoogle(w http.ResponseWriter, r *http.Request) *DNS
 	opt := new(dns.OPT)
 	opt.Hdr.Name = "."
 	opt.Hdr.Rrtype = dns.TypeOPT
-	opt.SetUDPSize(4096)
+	opt.SetUDPSize(dns.DefaultMsgSize)
 	opt.SetDo(true)
 	if ednsClientAddress != nil {
 		edns0Subnet := new(dns.EDNS0_SUBNET)

doh-server/ietf.go

@@ -93,7 +93,7 @@ func (s *Server) parseRequestIETF(w http.ResponseWriter, r *http.Request) *DNSRe
 		opt = new(dns.OPT)
 		opt.Hdr.Name = "."
 		opt.Hdr.Rrtype = dns.TypeOPT
-		opt.SetUDPSize(4096)
+		opt.SetUDPSize(dns.DefaultMsgSize)
 		opt.SetDo(false)
 		msg.Extra = append(msg.Extra, opt)
 	}

doh-server/server.go

@@ -58,7 +58,7 @@ func NewServer(conf *config) (s *Server) {
 		conf: conf,
 		udpClient: &dns.Client{
 			Net:     "udp",
-			UDPSize: 4096,
+			UDPSize: dns.DefaultMsgSize,
 			Timeout: time.Duration(conf.Timeout) * time.Second,
 		},
 		tcpClient: &dns.Client{