Fill out port 80 if it's available, otherwise port 3000

control.go

@@ -697,6 +697,7 @@ func handleSafeSearchStatus(w http.ResponseWriter, r *http.Request) {
 type ipport struct {
 	IP      string `json:"ip"`
 	Port    int    `json:"port"`
+	Warning string `json:"warning"`
 }
 
 type firstRunData struct {
@@ -720,8 +721,20 @@ func handleGetDefaultAddresses(w http.ResponseWriter, r *http.Request) {
 	}
 
 	// fill out the fields
-	data.Web.Port = 3000 // TODO: find out if port 80 is available -- if not, fall back to 3000
-	data.DNS.Port = 53   // TODO: find out if port 53 is available -- if not, show a big warning
+
+	// find out if port 80 is available -- if not, fall back to 3000
+	if checkPortAvailable(80) {
+		data.Web.Port = 80
+	} else {
+		data.Web.Port = 3000
+	}
+
+	// find out if port 53 is available -- if not, show a big warning
+	data.DNS.Port = 53
+	if !checkPortAvailable(53) {
+		data.DNS.Warning = "Port 53 is not available for binding -- this will make DNS clients unable to contact AdGuard Home."
+	}
+
 	data.Interfaces = make(map[string]interface{})
 	for _, iface := range ifaces {
 		data.Interfaces[iface.Name] = iface

helpers.go

@@ -258,6 +258,16 @@ func findIPv4IfaceAddr(ifaces []netInterface) string {
 	return ""
 }
 
+// checkPortAvailable is not a cheap test to see if the port is bindable, because it's actually doing the bind momentarily
+func checkPortAvailable(port int) bool {
+	ln, err := net.Listen("tcp", fmt.Sprintf(":%d", port))
+	if err != nil {
+		return false
+	}
+	ln.Close()
+	return true
+}
+
 // ---------------------
 // debug logging helpers
 // ---------------------