Force remote streaming mode if connecting through a VPN

app/backend/nvcomputer.cpp

@@ -3,6 +3,7 @@
 #include <QUdpSocket>
 #include <QHostInfo>
 #include <QNetworkInterface>
+#include <QNetworkProxy>
 
 #define SER_NAME "hostname"
 #define SER_UUID "uuid"
@@ -237,6 +238,60 @@ bool NvComputer::wake()
     return success;
 }
 
+bool NvComputer::isReachableOverVpn()
+{
+    if (activeAddress.isEmpty()) {
+        return false;
+    }
+
+    QTcpSocket s;
+
+    s.setProxy(QNetworkProxy::NoProxy);
+    s.connectToHost(activeAddress, 47984);
+    if (s.waitForConnected(3000)) {
+        Q_ASSERT(!s.localAddress().isNull());
+
+        for (const QNetworkInterface& nic : QNetworkInterface::allInterfaces()) {
+            // Ensure the interface is up
+            if ((nic.flags() & QNetworkInterface::IsUp) == 0) {
+                continue;
+            }
+
+            for (const QNetworkAddressEntry& addr : nic.addressEntries()) {
+                if (addr.ip() == s.localAddress()) {
+                    qInfo() << "Found matching interface:" << nic.humanReadableName() << nic.type() << nic.flags();
+
+                    if (nic.flags() & QNetworkInterface::IsPointToPoint) {
+                        // Treat point-to-point links as likely VPNs
+                        return true;
+                    }
+
+                    if (nic.type() == QNetworkInterface::Virtual ||
+                            nic.type() == QNetworkInterface::Ppp) {
+                        // Treat PPP and virtual interfaces as likely VPNs
+                        return true;
+                    }
+
+#if QT_VERSION >= QT_VERSION_CHECK(5, 11, 0)
+                    qInfo() << "MTU is" << nic.maximumTransmissionUnit();
+                    if (nic.maximumTransmissionUnit() < 1500) {
+                        // Treat MTUs under 1500 as likely VPNs
+                        return true;
+                    }
+#endif
+
+                    // Didn't meet any of our VPN heuristics
+                    return false;
+                }
+            }
+        }
+    }
+    else {
+        // If we fail to connect, just pretend that it's not a VPN
+        return false;
+    }
+}
+
 QVector<QString> NvComputer::uniqueAddresses()
 {
     QVector<QString> uniqueAddressList;

app/backend/nvcomputer.h

@@ -29,6 +29,9 @@ public:
     bool
     wake();
 
+    bool
+    isReachableOverVpn();
+
     QVector<QString>
     uniqueAddresses();
 

app/streaming/session.cpp

@@ -356,8 +356,6 @@ bool Session::initialize()
     m_StreamConfig.fps = m_Preferences->fps;
     m_StreamConfig.bitrate = m_Preferences->bitrateKbps;
     m_StreamConfig.hevcBitratePercentageMultiplier = 75;
-    m_StreamConfig.streamingRemotely = STREAM_CFG_AUTO;
-    m_StreamConfig.packetSize = 1392;
 
     SDL_LogInfo(SDL_LOG_CATEGORY_APPLICATION,
                 "Video bitrate: %d kbps",
@@ -937,6 +935,19 @@ void Session::exec(int displayOriginX, int displayOriginY)
         hostInfo.serverInfoGfeVersion = siGfeVersion.data();
     }
 
+    // isReachableOverVpn() does network I/O, so we only attempt to check
+    // VPN reachability if we've already contacted the PC successfully
+    if (m_Computer->isReachableOverVpn()) {
+        // It looks like our route to this PC is over a VPN.
+        // Treat it as remote even if the target address is in RFC 1918 address space.
+        m_StreamConfig.streamingRemotely = STREAM_CFG_REMOTE;
+        m_StreamConfig.packetSize = 1024;
+    }
+    else {
+        m_StreamConfig.streamingRemotely = STREAM_CFG_AUTO;
+        m_StreamConfig.packetSize = 1392;
+    }
+
     int err = LiStartConnection(&hostInfo, &m_StreamConfig, &k_ConnCallbacks,
                                 &m_VideoCallbacks,
                                 m_AudioDisabled ? nullptr : &m_AudioCallbacks,