Add service discovery API endpoint (#31089)

relates to #31057 

adds an endpoint to expose the fleet handled service discovery endpoint.

> NOTE: test will be done in a follow up PR

- [x] Changes file added for user-visible changes in `changes/`,
`orbit/changes/` or `ee/fleetd-chrome/changes`.
See [Changes
files](https://github.com/fleetdm/fleet/blob/main/docs/Contributing/guides/committing-changes.md#changes-files)
for more information.

cmd/fleet/serve.go

@@ -1206,6 +1206,7 @@ the way that the Fleet server works.
 					mdmCheckinAndCommandService,
 					ddmService,
 					commander,
+					appCfg.ServerSettings.ServerURL,
 				); err != nil {
 					initFatal(err, "setup mdm apple services")
 				}

server/mdm/apple/apple_mdm.go

@@ -36,6 +36,8 @@ const (
 	SCEPPath = "/mdm/apple/scep"
 	// MDMPath is Fleet's HTTP path for the core MDM service.
 	MDMPath = "/mdm/apple/mdm"
+	// MDMServiceDiscoveryPath is Fleet's HTTP path for the MDM service discovery service.
+	ServiceDiscoveryPath = "/mdm/apple/service_discovery"
 
 	// EnrollPath is the HTTP path that serves the mobile profile to devices when enrolling.
 	EnrollPath = "/api/mdm/apple/enroll"

server/service/handler.go

@@ -1130,6 +1130,7 @@ func RegisterAppleMDMProtocolServices(
 	checkinAndCommandService nanomdm_service.CheckinAndCommandService,
 	ddmService nanomdm_service.DeclarativeManagement,
 	profileService nanomdm_service.ProfileService,
+	serverURLPrefix string,
 ) error {
 	if err := registerSCEP(mux, scepConfig, scepStorage, mdmStorage, logger); err != nil {
 		return fmt.Errorf("scep: %w", err)
@@ -1137,6 +1138,30 @@ func RegisterAppleMDMProtocolServices(
 	if err := registerMDM(mux, mdmStorage, checkinAndCommandService, ddmService, profileService, logger); err != nil {
 		return fmt.Errorf("mdm: %w", err)
 	}
+	if err := registerMDMServiceDiscovery(mux, logger, serverURLPrefix); err != nil {
+		return fmt.Errorf("service discovery: %w", err)
+	}
+	return nil
+}
+
+func registerMDMServiceDiscovery(
+	mux *http.ServeMux,
+	logger kitlog.Logger,
+	serverURLPrefix string,
+) error {
+	serviceDiscoveryLogger := kitlog.With(logger, "component", "mdm-apple-service-discovery")
+	fullMDMEnrollmentURL := fmt.Sprintf("%s%s", serverURLPrefix, apple_mdm.AccountDrivenEnrollPath)
+	serviceDiscoveryHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		serviceDiscoveryLogger.Log("msg", "serving MDM service discovery response", "url", fullMDMEnrollmentURL)
+		w.Header().Set("Content-Type", "application/json")
+		w.WriteHeader(http.StatusOK)
+		_, err := fmt.Fprintf(w, `{"Servers":[{"Version": "mdm-byod", "BaseURL": "%s"}]}`, fullMDMEnrollmentURL)
+		if err != nil {
+			serviceDiscoveryLogger.Log("err", "error writing service discovery response", "err", err)
+			http.Error(w, "Internal Server Error", http.StatusInternalServerError)
+		}
+	})
+	mux.Handle(apple_mdm.ServiceDiscoveryPath, serviceDiscoveryHandler)
 	return nil
 }
 

server/service/testing_utils.go

@@ -429,6 +429,7 @@ func RunServerForTestsWithServiceWithDS(t *testing.T, ctx context.Context, ds fl
 					logger: logger,
 				},
 				commander,
+				"https://test-url.com",
 			)
 			require.NoError(t, err)
 		}