UDP port should be a constant

Instead of always doing so on MacOS, do it only when we get a
DatagramTooLargeError. On MacOS, the size is limited only for
broadcast but not for unicast.

CMakeLists.txt

@@ -65,7 +65,7 @@ else()
     find_package(Qca-qt${QT_MAJOR_VERSION} ${QCA_MIN_VERSION} REQUIRED)
     set(Qca_LIBRARY qca-qt${QT_MAJOR_VERSION})
 
-    set(KF5_REQUIRED_COMPONENTS I18n ConfigWidgets DBusAddons IconThemes Notifications KIO KCMUtils Service Solid Kirigami2 People WindowSystem GuiAddons)
+    set(KF5_REQUIRED_COMPONENTS I18n ConfigWidgets DBusAddons IconThemes Notifications KIO KCMUtils Service Solid Kirigami2 People WindowSystem GuiAddons DNSSD)
     set(KF5_OPTIONAL_COMPONENTS DocTools)
 
     set_package_properties(KF5Kirigami2 PROPERTIES

core/CMakeLists.txt

@@ -64,6 +64,7 @@ PRIVATE
     Qt${QT_MAJOR_VERSION}::DBus
     KF5::I18n
     KF5::ConfigCore
+    KF5::DNSSD
 )
 
 if(${KF5KIO_FOUND})

core/backends/lan/CMakeLists.txt

@@ -8,6 +8,7 @@ set(backends_kdeconnect_SRCS
     backends/lan/compositeuploadjob.cpp
     backends/lan/uploadjob.cpp
     backends/lan/socketlinereader.cpp
+    backends/lan/mdnsdiscovery.cpp
 
     PARENT_SCOPE
 )

core/backends/lan/lanlinkprovider.cpp

@@ -37,14 +37,13 @@
 static const int MAX_UNPAIRED_CONNECTIONS = 42;
 static const int MAX_REMEMBERED_IDENTITY_PACKETS = 42;
 
-LanLinkProvider::LanLinkProvider(bool testMode, quint16 udpBroadcastPort, quint16 udpListenPort)
+LanLinkProvider::LanLinkProvider(bool testMode)
     : m_server(new Server(this))
     , m_udpSocket(this)
     , m_tcpPort(0)
-    , m_udpBroadcastPort(udpBroadcastPort)
-    , m_udpListenPort(udpListenPort)
     , m_testMode(testMode)
     , m_combineBroadcastsTimer(this)
+    , m_mdnsDiscovery(this)
 {
     m_combineBroadcastsTimer.setInterval(0); // increase this if waiting a single event-loop iteration is not enough
     m_combineBroadcastsTimer.setSingleShot(true);
@@ -82,12 +81,12 @@ void LanLinkProvider::onStart()
 {
     const QHostAddress bindAddress = m_testMode ? QHostAddress::LocalHost : QHostAddress::Any;
 
-    bool success = m_udpSocket.bind(bindAddress, m_udpListenPort, QUdpSocket::ShareAddress);
+    bool success = m_udpSocket.bind(bindAddress, UDP_PORT, QUdpSocket::ShareAddress);
     if (!success) {
         QAbstractSocket::SocketError sockErr = m_udpSocket.error();
         // Refer to https://doc.qt.io/qt-5/qabstractsocket.html#SocketError-enum to decode socket error number
         QString errorMessage = QString::fromLatin1(QMetaEnum::fromType<QAbstractSocket::SocketError>().valueToKey(sockErr));
-        qCritical(KDECONNECT_CORE) << QLatin1String("Failed to bind UDP socket on port") << m_udpListenPort << QLatin1String("with error") << errorMessage;
+        qCritical(KDECONNECT_CORE) << QLatin1String("Failed to bind UDP socket on port") << UDP_PORT << QLatin1String("with error") << errorMessage;
     }
     Q_ASSERT(success);
 
@@ -101,12 +100,17 @@ void LanLinkProvider::onStart()
         }
     }
 
-    onNetworkChange();
+    broadcastUdpIdentityPacket();
+    m_mdnsDiscovery.startAnnouncing();
+    m_mdnsDiscovery.startDiscovering();
+
     qCDebug(KDECONNECT_CORE) << "LanLinkProvider started";
 }
 
 void LanLinkProvider::onStop()
 {
+    m_mdnsDiscovery.stopAnnouncing();
+    m_mdnsDiscovery.stopDiscovering();
     m_udpSocket.close();
     m_server->close();
     qCDebug(KDECONNECT_CORE) << "LanLinkProvider stopped";
@@ -125,44 +129,26 @@ void LanLinkProvider::onNetworkChange()
 void LanLinkProvider::broadcastToNetwork()
 {
     if (!m_server->isListening()) {
-        // Not started
+        qWarning() << "TCP server not listening, not broadcasting";
         return;
     }
 
     Q_ASSERT(m_tcpPort != 0);
 
+    broadcastUdpIdentityPacket();
+    m_mdnsDiscovery.stopDiscovering();
+    m_mdnsDiscovery.startDiscovering();
+}
+
+void LanLinkProvider::broadcastUdpIdentityPacket()
+{
+    if (qEnvironmentVariableIsSet("KDECONNECT_DISABLE_UDP_BROADCAST")) {
+        qWarning() << "Not broadcasting UDP because KDECONNECT_DISABLE_UDP_BROADCAST is set";
+        return;
+    }
     qCDebug(KDECONNECT_CORE()) << "Broadcasting identity packet";
 
-    QList<QHostAddress> destinations = getBroadcastAddresses();
-
-    NetworkPacket np = KdeConnectConfig::instance().deviceInfo().toIdentityPacket();
-    np.set(QStringLiteral("tcpPort"), m_tcpPort);
-#if defined(Q_OS_MAC) || defined(Q_OS_FREEBSD)
-    // On macOS and FreeBSD, the too large UDP packet (larger than MTU) causes
-    // incomplete transmission.
-    // We remove the capacitilities to reduce the discovery packet to the min
-    // MTU of the interfaces with broadcast feature.
-    int mtu = 1500;
-    for (const QNetworkInterface &iface : QNetworkInterface::allInterfaces()) {
-        if ((iface.flags() & QNetworkInterface::IsUp) && (iface.flags() & QNetworkInterface::IsRunning) && (iface.flags() & QNetworkInterface::CanBroadcast)) {
-            int ifaceMtu = iface.maximumTransmissionUnit();
-            if (ifaceMtu < mtu && ifaceMtu > 0) {
-                mtu = ifaceMtu;
-            }
-        }
-    }
-    QByteArray payload = np.serialize();
-    if (payload.length() > mtu) {
-        // First try to drop the less important outgoing capabilities
-        np.set(QStringLiteral("outgoingCapabilities"), QStringList());
-        payload = np.serialize();
-    }
-    if (payload.length() > mtu) {
-        // If still too large, drop the incoming capabilities
-        np.set(QStringLiteral("incomingCapabilities"), QStringList());
-        payload = np.serialize();
-    }
-#endif
+    QList<QHostAddress> addresses = getBroadcastAddresses();
 
 #if defined(Q_OS_WIN) || defined(Q_OS_FREEBSD)
     // On Windows and FreeBSD we need to broadcast from every local IP address to reach all networks
@@ -175,17 +161,18 @@ void LanLinkProvider::broadcastToNetwork()
                 if (sourceAddress.protocol() == QAbstractSocket::IPv4Protocol && sourceAddress != QHostAddress::LocalHost) {
                     qCDebug(KDECONNECT_CORE()) << "Broadcasting as" << sourceAddress;
                     sendSocket.bind(sourceAddress);
-                    sendBroadcasts(sendSocket, np, destinations);
+                    sendUdpIdentityPacket(sendSocket, addresses);
                     sendSocket.close();
                 }
             }
         }
     }
 #else
-    sendBroadcasts(m_udpSocket, np, destinations);
+    sendUdpIdentityPacket(addresses);
 #endif
 }
 
+
 QList<QHostAddress> LanLinkProvider::getBroadcastAddresses()
 {
     const QStringList customDevices = KdeConnectConfig::instance().customDevices();
@@ -209,12 +196,31 @@ QList<QHostAddress> LanLinkProvider::getBroadcastAddresses()
     return destinations;
 }
 
-void LanLinkProvider::sendBroadcasts(QUdpSocket &socket, const NetworkPacket &np, const QList<QHostAddress> &addresses)
+void LanLinkProvider::sendUdpIdentityPacket(const QList<QHostAddress> &addresses)
 {
-    const QByteArray payload = np.serialize();
+    sendUdpIdentityPacket(m_udpSocket, addresses);
+}
+
+void LanLinkProvider::sendUdpIdentityPacket(QUdpSocket &socket, const QList<QHostAddress> &addresses)
+{
+    DeviceInfo myDeviceInfo = KdeConnectConfig::instance().deviceInfo();
+    NetworkPacket identityPacket = myDeviceInfo.toIdentityPacket();
+    identityPacket.set(QStringLiteral("tcpPort"), m_tcpPort);
+    const QByteArray payload = identityPacket.serialize();
 
     for (auto &address : addresses) {
-        socket.writeDatagram(payload, address, m_udpBroadcastPort);
+        qint64 bytes = socket.writeDatagram(payload, address, UDP_PORT);
+        if (bytes == -1 && socket.error() == QAbstractSocket::DatagramTooLargeError) {
+            // On macOS and FreeBSD, UDP broadcasts larger than MTU get dropped. See:
+            // https://opensource.apple.com/source/xnu/xnu-3789.1.32/bsd/netinet/ip_output.c.auto.html#:~:text=/*%20don%27t%20allow%20broadcast%20messages%20to%20be%20fragmented%20*/
+            // We remove the capabilities to reduce the size of the packet.
+            // This should only happen for broadcasts, so UDP packets sent from MDNS discoveries should still work.
+            qWarning() << "Identity packet to" << address << "got rejected because it was too large. Retrying without including the capabilities";
+            identityPacket.set(QStringLiteral("outgoingCapabilities"), QStringList());
+            identityPacket.set(QStringLiteral("incomingCapabilities"), QStringList());
+            const QByteArray smallPayload = identityPacket.serialize();
+            socket.writeDatagram(smallPayload, address, UDP_PORT);
+        }
     }
 }
 
@@ -295,7 +301,7 @@ void LanLinkProvider::connectError(QAbstractSocket::SocketError socketError)
     qCDebug(KDECONNECT_CORE) << "Fallback (1), try reverse connection (send udp packet)" << socket->errorString();
     NetworkPacket np = KdeConnectConfig::instance().deviceInfo().toIdentityPacket();
     np.set(QStringLiteral("tcpPort"), m_tcpPort);
-    m_udpSocket.writeDatagram(np.serialize(), m_receivedIdentityPackets[socket].sender, m_udpBroadcastPort);
+    m_udpSocket.writeDatagram(np.serialize(), m_receivedIdentityPackets[socket].sender, UDP_PORT);
 
     // The socket we created didn't work, and we didn't manage
     // to create a LanDeviceLink from it, deleting everything.
@@ -350,7 +356,7 @@ void LanLinkProvider::tcpSocketConnected()
         // The socket doesn't seem to work, so we can't create the connection.
 
         qCDebug(KDECONNECT_CORE) << "Fallback (2), try reverse connection (send udp packet)";
-        m_udpSocket.writeDatagram(np2.serialize(), m_receivedIdentityPackets[socket].sender, m_udpBroadcastPort);
+        m_udpSocket.writeDatagram(np2.serialize(), m_receivedIdentityPackets[socket].sender, UDP_PORT);
 
         // Cleanup the network packet now. The socket should be deleted via the disconnected() signal.
         // We don't do this on success, because it is done later in the encrypted() slot.

core/backends/lan/lanlinkprovider.h

@@ -17,6 +17,7 @@
 #include "backends/linkprovider.h"
 #include "kdeconnectcore_export.h"
 #include "landevicelink.h"
+#include "mdnsdiscovery.h"
 #include "server.h"
 
 class KDECONNECTCORE_EXPORT LanLinkProvider : public LinkProvider
@@ -26,10 +27,8 @@ class KDECONNECTCORE_EXPORT LanLinkProvider : public LinkProvider
 public:
     /**
      * @param testMode Some special overrides needed while testing
-     * @param udpBroadcastPort Port which should be used for *sending* identity packets
-     * @param udpListenPort Port which should be used for *receiving* identity packets
      */
-    LanLinkProvider(bool testMode = false, quint16 udpBroadcastPort = UDP_PORT, quint16 udpListenPort = UDP_PORT);
+    LanLinkProvider(bool testMode = false);
     ~LanLinkProvider() override;
 
     QString name() override
@@ -37,6 +36,8 @@ public:
         return QStringLiteral("LanLinkProvider");
     }
 
+    void sendUdpIdentityPacket(const QList<QHostAddress> &addresses);
+
     static void configureSslSocket(QSslSocket *socket, const QString &deviceId, bool isDeviceTrusted);
     static void configureSocket(QSslSocket *socket);
 
@@ -67,15 +68,13 @@ private:
     void onNetworkConfigurationChanged(const QNetworkConfiguration &config);
     void addLink(QSslSocket *socket, const DeviceInfo &deviceInfo);
     QList<QHostAddress> getBroadcastAddresses();
-    void sendBroadcasts(QUdpSocket &socket, const NetworkPacket &np, const QList<QHostAddress> &addresses);
+    void sendUdpIdentityPacket(QUdpSocket &socket, const QList<QHostAddress> &addresses);
+    void broadcastUdpIdentityPacket();
 
     Server *m_server;
     QUdpSocket m_udpSocket;
     quint16 m_tcpPort;
 
-    quint16 m_udpBroadcastPort;
-    quint16 m_udpListenPort;
-
     QMap<QString, LanDeviceLink *> m_links;
 
     struct PendingConnect {
@@ -86,6 +85,8 @@ private:
     QNetworkConfiguration m_lastConfig;
     const bool m_testMode;
     QTimer m_combineBroadcastsTimer;
+
+    MdnsDiscovery m_mdnsDiscovery;
 };
 
 #endif

core/backends/lan/mdnsdiscovery.cpp

@@ -0,0 +1,120 @@
+/**
+ * SPDX-FileCopyrightText: 2023 Albert Vaca <albertvaka@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+ */
+
+#include "mdnsdiscovery.h"
+
+#include "core_debug.h"
+#include "kdeconnectconfig.h"
+#include "lanlinkprovider.h"
+
+#include <KDNSSD/DNSSD/PublicService>
+#include <KDNSSD/DNSSD/RemoteService>
+#include <KDNSSD/DNSSD/ServiceBrowser>
+
+const QString kServiceName = QStringLiteral("_kdeconnect._udp");
+
+MdnsDiscovery::MdnsDiscovery(LanLinkProvider *lanLinkProvider)
+    : lanLinkProvider(lanLinkProvider)
+{
+    switch (KDNSSD::ServiceBrowser::isAvailable()) {
+    case KDNSSD::ServiceBrowser::Stopped:
+        qWarning() << "mDNS or Avahi daemons are not running, mDNS discovery not available";
+        break;
+    case KDNSSD::ServiceBrowser::Working:
+        qCDebug(KDECONNECT_CORE) << "mDNS discovery is available";
+        break;
+    case KDNSSD::ServiceBrowser::Unsupported:
+        qWarning() << "mDNS discovery not available (library built without DNS-SD support)";
+        break;
+    }
+}
+
+MdnsDiscovery::~MdnsDiscovery()
+{
+    stopAnnouncing();
+    stopDiscovering();
+}
+
+void MdnsDiscovery::startAnnouncing()
+{
+    if (m_publisher != nullptr) {
+        qCDebug(KDECONNECT_CORE) << "MDNS already announcing";
+        return;
+    }
+    qCDebug(KDECONNECT_CORE) << "MDNS start announcing";
+
+    KdeConnectConfig &config = KdeConnectConfig::instance();
+
+    m_publisher = new KDNSSD::PublicService(config.deviceId(), kServiceName, LanLinkProvider::UDP_PORT, QStringLiteral("local"));
+    m_publisher->setParent(this);
+
+    // We can't fit the device certificate in this field, so this is not enough info to create a Device and won't be used.
+    QMap<QString, QByteArray> data;
+    data[QStringLiteral("id")] = config.deviceId().toUtf8();
+    data[QStringLiteral("name")] = config.name().toUtf8();
+    data[QStringLiteral("type")] = config.deviceType().toString().toUtf8();
+    data[QStringLiteral("protocol")] = QString::number(NetworkPacket::s_protocolVersion).toUtf8();
+    m_publisher->setTextData(data);
+
+    connect(m_publisher, &KDNSSD::PublicService::published, [](bool successful) {
+        if (successful) {
+            qCDebug(KDECONNECT_CORE) << "MDNS published successfully";
+        } else {
+            qWarning() << "MDNS failed to publish";
+        }
+    });
+
+    m_publisher->publishAsync();
+}
+
+void MdnsDiscovery::stopAnnouncing()
+{
+    if (m_publisher != nullptr) {
+        qCDebug(KDECONNECT_CORE) << "MDNS stop announcing";
+        delete m_publisher;
+        m_publisher = nullptr;
+    }
+}
+
+void MdnsDiscovery::startDiscovering()
+{
+    if (m_serviceBrowser != nullptr) {
+        qCDebug(KDECONNECT_CORE) << "MDNS already discovering";
+        return;
+    }
+    qCDebug(KDECONNECT_CORE) << "MDNS start discovering";
+
+    m_serviceBrowser = new KDNSSD::ServiceBrowser(kServiceName, true);
+
+    connect(m_serviceBrowser, &KDNSSD::ServiceBrowser::serviceAdded, [this](KDNSSD::RemoteService::Ptr service) {
+        if (KdeConnectConfig::instance().deviceId() == service->serviceName()) {
+            qCDebug(KDECONNECT_CORE) << "Discovered myself, ignoring";
+            return;
+        }
+        qCDebug(KDECONNECT_CORE) << "Discovered " << service->serviceName() << " at " << service->hostName();
+        QHostAddress address(service->hostName());
+        lanLinkProvider->sendUdpIdentityPacket(QList<QHostAddress>{address});
+    });
+
+    connect(m_serviceBrowser, &KDNSSD::ServiceBrowser::serviceRemoved, [](KDNSSD::RemoteService::Ptr service) {
+        qCDebug(KDECONNECT_CORE) << "Lost " << service->serviceName();
+    });
+
+    connect(m_serviceBrowser, &KDNSSD::ServiceBrowser::finished, []() {
+        qCDebug(KDECONNECT_CORE) << "Finished discovery";
+    });
+
+    m_serviceBrowser->startBrowse();
+}
+
+void MdnsDiscovery::stopDiscovering()
+{
+    if (m_serviceBrowser != nullptr) {
+        qCDebug(KDECONNECT_CORE) << "MDNS stop discovering";
+        delete m_serviceBrowser;
+        m_serviceBrowser = nullptr;
+    }
+}

core/backends/lan/mdnsdiscovery.h

@@ -0,0 +1,41 @@
+/**
+ * SPDX-FileCopyrightText: 2023 Albert Vaca Cintora <albertvaka@gmail.com>
+ *
+ * SPDX-License-Identifier: GPL-2.0-only OR GPL-3.0-only OR LicenseRef-KDE-Accepted-GPL
+ */
+
+#ifndef KDECONNECT_MDNS_DISCOVERY_H
+#define KDECONNECT_MDNS_DISCOVERY_H
+
+#include <QObject>
+
+#include "kdeconnectcore_export.h"
+
+class LanLinkProvider;
+namespace KDNSSD
+{
+class PublicService;
+class ServiceBrowser;
+};
+
+class KDECONNECTCORE_EXPORT MdnsDiscovery : public QObject
+{
+    Q_OBJECT
+
+public:
+    explicit MdnsDiscovery(LanLinkProvider *parent);
+    ~MdnsDiscovery();
+
+    void startDiscovering();
+    void stopDiscovering();
+
+    void stopAnnouncing();
+    void startAnnouncing();
+
+private:
+    LanLinkProvider *lanLinkProvider = nullptr;
+    KDNSSD::PublicService *m_publisher = nullptr;
+    KDNSSD::ServiceBrowser *m_serviceBrowser = nullptr;
+};
+
+#endif // KDECONNECT_SERVER_H