fix(server): use RCode=ServFail instead of HTTP 500 for internal errors

RFC 8484 Section 4.2.1:
> A successful HTTP response with a 2xx status code (see
> Section 6.3 of RFC7231) is used for any valid DNS response,
> regardless of the DNS response code.  For example, a successful 2xx
> HTTP status code is used even with a DNS message whose DNS response
> code indicates failure, such as SERVFAIL or NXDOMAIN.
https://www.rfc-editor.org/rfc/rfc8484#section-4.2.1

server/server.go

@@ -615,8 +615,12 @@ func (s *Server) OnRequest(
 	}
 }
 
-func (s *Server) resolve(ctx context.Context, request *model.Request) (*model.Response, error) {
-	var response *model.Response
+func (s *Server) resolve(ctx context.Context, request *model.Request) (response *model.Response, rerr error) {
+	defer func() {
+		if val := recover(); val != nil {
+			rerr = fmt.Errorf("panic occurred: %v", val)
+		}
+	}()
 
 	contextUpstreamTimeoutMultiplier := 100
 	timeoutDuration := time.Duration(contextUpstreamTimeoutMultiplier) * s.cfg.Upstreams.Timeout.ToDuration()

server/server_endpoints.go

@@ -134,7 +134,6 @@ func (s *Server) dohPostRequestHandler(rw http.ResponseWriter, req *http.Request
 
 func (s *Server) processDohMessage(rawMsg []byte, rw http.ResponseWriter, req *http.Request) {
 	msg := new(dns.Msg)
-
 	if err := msg.Unpack(rawMsg); err != nil {
 		logger().Error("can't deserialize message: ", err)
 		http.Error(rw, err.Error(), http.StatusBadRequest)
@@ -142,6 +141,25 @@ func (s *Server) processDohMessage(rawMsg []byte, rw http.ResponseWriter, req *h
 		return
 	}
 
+	rw.Header().Set("content-type", dnsContentType)
+
+	writeErr := func(err error) {
+		log.Log().Error(err)
+
+		msg := new(dns.Msg)
+		msg.SetRcode(msg, dns.RcodeServerFailure)
+
+		buff, err := msg.Pack()
+		if err != nil {
+			return
+		}
+
+		// https://www.rfc-editor.org/rfc/rfc8484#section-4.2.1
+		rw.WriteHeader(http.StatusOK)
+
+		_, _ = rw.Write(buff)
+	}
+
 	clientID := chi.URLParam(req, "clientID")
 	if clientID == "" {
 		clientID = extractClientIDFromHost(req.Host)
@@ -151,22 +169,20 @@ func (s *Server) processDohMessage(rawMsg []byte, rw http.ResponseWriter, req *h
 
 	resResponse, err := s.resolve(ctx, r)
 	if err != nil {
-		logAndResponseWithError(err, "unable to process query: ", rw)
+		writeErr(fmt.Errorf("unable to process query: %w", err))
 
 		return
 	}
 
 	b, err := resResponse.Res.Pack()
 	if err != nil {
-		logAndResponseWithError(err, "can't serialize message: ", rw)
+		writeErr(fmt.Errorf("can't serialize message: %w", err))
 
 		return
 	}
 
-	rw.Header().Set("content-type", dnsContentType)
-
 	_, err = rw.Write(b)
-	logAndResponseWithError(err, "can't write response: ", rw)
+	log.Log().Error(fmt.Errorf("can't write response: %w", err))
 }
 
 func (s *Server) Query(

server/server_test.go

@@ -529,8 +529,8 @@ var _ = Describe("Running DNS server", func() {
 					Expect(resp).Should(HaveHTTPStatus(http.StatusUnsupportedMediaType))
 				})
 			})
-			When("Internal error occurs", func() {
-				It("should return 'Internal server error'", func() {
+			When("DNS error occurs", func() {
+				It("should return 'ServFail'", func() {
 					msg = util.NewMsgWithQuestion("error.", A)
 					rawDNSMessage, err := msg.Pack()
 					Expect(err).Should(Succeed())
@@ -540,7 +540,41 @@ var _ = Describe("Running DNS server", func() {
 					Expect(err).Should(Succeed())
 					DeferCleanup(resp.Body.Close)
 
-					Expect(resp).Should(HaveHTTPStatus(http.StatusInternalServerError))
+					Expect(resp).Should(HaveHTTPStatus(http.StatusOK))
+
+					body, err := io.ReadAll(resp.Body)
+					Expect(err).Should(Succeed())
+
+					msg := new(dns.Msg)
+					Expect(msg.Unpack(body)).Should(Succeed())
+					Expect(msg.Rcode).Should(Equal(dns.RcodeServerFailure))
+				})
+			})
+			When("Internal error occurs", func() {
+				BeforeEach(func() {
+					bak := sut.queryResolver
+					sut.queryResolver = nil // trigger a panic
+					DeferCleanup(func() { sut.queryResolver = bak })
+				})
+
+				It("should return 'ServFail'", func() {
+					msg = util.NewMsgWithQuestion("error.", A)
+					rawDNSMessage, err := msg.Pack()
+					Expect(err).Should(Succeed())
+
+					resp, err = http.Post(queryURL,
+						"application/dns-message", bytes.NewReader(rawDNSMessage))
+					Expect(err).Should(Succeed())
+					DeferCleanup(resp.Body.Close)
+
+					Expect(resp).Should(HaveHTTPStatus(http.StatusOK))
+
+					body, err := io.ReadAll(resp.Body)
+					Expect(err).Should(Succeed())
+
+					msg := new(dns.Msg)
+					Expect(msg.Unpack(body)).Should(Succeed())
+					Expect(msg.Rcode).Should(Equal(dns.RcodeServerFailure))
 				})
 			})
 		})