Server: Use a traffic-less way to get external IP

Previously, the code for finding the current machine's external
IP address created a TCP connection to Cloudflare's DNS. This is unnecessary.
Instead, just creating a socket to a non-private IP suffices to find
that IP. We still use Cloudflare's IP, but we will no longer send any traffic
there as we use an UDP socket without actually sending any data.

Credit for finding this approach goes to @atsampson.
Fixes #633.

Signed-off-by: Christian Hoffmann <mail@hoffmann-christian.info>

src/global.h

@@ -109,10 +109,12 @@ LED bar:      lbr
 #define SOFTWARE_MANUAL_URL              "https://jamulus.io/wiki/Software-Manual"
 
 // determining server internal address uses well-known host and port
-// (You can change the service used here to something like Cloudflare (1.1.1.1), Google DNS (8.8.8.8), or something else reliable)
-#define WELL_KNOWN_HOST                  "1.1.1.1" // CloudFlare
-#define WELL_KNOWN_PORT                  53        // DNS
-#define IP_LOOKUP_TIMEOUT                500       // ms
+// We just need a valid, public Internet IP here. We will not send any
+// traffic there as we will only set up an UDP socket without sending any
+// data.
+#define WELL_KNOWN_HOST                  "1.1.1.1"           // CloudFlare
+#define WELL_KNOWN_PORT                  DEFAULT_PORT_NUMBER
+#define IP_LOOKUP_TIMEOUT                500                 // ms
 
 // system sample rate (the sound card and audio coder works on this sample rate)
 #define SYSTEM_SAMPLE_RATE_HZ            48000 // Hz

src/serverlist.cpp

@@ -53,6 +53,7 @@ CServerListManager::CServerListManager ( const quint16  iNPortNum,
         // User-supplied --serverpublicip
         qhaServerPublicIP = QHostAddress ( strServerPublicIP );
     }
+    qDebug() << "Using" << qhaServerPublicIP.toString() << "as external IP.";
     SlaveCurLocalHostAddress = CHostAddress ( qhaServerPublicIP, iNPortNum );
 
     // prepare the server info information

src/util.cpp

@@ -1024,7 +1024,10 @@ bool NetworkUtil::ParseNetworkAddress ( QString       strAddress,
 
 CHostAddress NetworkUtil::GetLocalAddress()
 {
-    QTcpSocket socket;
+    QUdpSocket socket;
+    // As we are using UDP, the connectToHost() does not generate any traffic at all.
+    // We just require a socket which is pointed towards the Internet in
+    // order to find out the IP of our own external interface:
     socket.connectToHost ( WELL_KNOWN_HOST, WELL_KNOWN_PORT );
 
     if ( socket.waitForConnected ( IP_LOOKUP_TIMEOUT ) )

src/util.h

@@ -25,7 +25,7 @@
 #pragma once
 
 #include <QCoreApplication>
-#include <QTcpSocket>
+#include <QUdpSocket>
 #include <QHostAddress>
 #include <QHostInfo>
 #ifndef HEADLESS