QAbstractSocket: Add socketOption for the Path MTU

This allow retrieving the value of the known PMTU for the current
socket. This works on Linux (IPv6 and IPv4) and FreeBSD (IPv6 only) --
the other OSes don't have the necessary API.

Note: do we need add IP_MTU_DISCOVER?

Change-Id: I6e9274c1e7444ad48c81fffd14dcaf97a18ce335
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/network/socket/qabstractsocket.cpp

@@ -387,6 +387,11 @@
     (see \l{QAbstractSocket::}{setReadBufferSize()}).
     This enum value has been introduced in Qt 5.3.
 
+    \value PathMtuSocketOption Retrieves the Path Maximum Transmission Unit
+    (PMTU) value currently known by the IP stack, if any. Some IP stacks also
+    allow setting the MTU for transmission.
+    This enum value was introduced in Qt 5.11.
+
     Possible values for \e{TypeOfServiceOption} are:
 
     \table
@@ -2023,6 +2028,10 @@ void QAbstractSocket::setSocketOption(QAbstractSocket::SocketOption option, cons
         case ReceiveBufferSizeSocketOption:
             d_func()->socketEngine->setOption(QAbstractSocketEngine::ReceiveBufferSocketOption, value.toInt());
             break;
+
+        case PathMtuSocketOption:
+            d_func()->socketEngine->setOption(QAbstractSocketEngine::PathMtuInformation, value.toInt());
+            break;
     }
 }
 
@@ -2065,6 +2074,10 @@ QVariant QAbstractSocket::socketOption(QAbstractSocket::SocketOption option)
         case ReceiveBufferSizeSocketOption:
                 ret = d_func()->socketEngine->option(QAbstractSocketEngine::ReceiveBufferSocketOption);
                 break;
+
+        case PathMtuSocketOption:
+                ret = d_func()->socketEngine->option(QAbstractSocketEngine::PathMtuInformation);
+                break;
     }
     if (ret == -1)
         return QVariant();

src/network/socket/qabstractsocket.h

@@ -120,7 +120,8 @@ public:
         MulticastLoopbackOption, // IP_MULTICAST_LOOPBACK
         TypeOfServiceOption, //IP_TOS
         SendBufferSizeSocketOption,    //SO_SNDBUF
-        ReceiveBufferSizeSocketOption  //SO_RCVBUF
+        ReceiveBufferSizeSocketOption,  //SO_RCVBUF
+        PathMtuSocketOption // IP_MTU
     };
     Q_ENUM(SocketOption)
     enum BindFlag {

src/network/socket/qabstractsocketengine_p.h

@@ -105,7 +105,8 @@ public:
         TypeOfServiceOption,
         ReceivePacketInformation,
         ReceiveHopLimit,
-        MaxStreamsSocketOption
+        MaxStreamsSocketOption,
+        PathMtuInformation
     };
 
     enum PacketHeaderOption {

src/network/socket/qnativesocketengine_unix.cpp

@@ -223,6 +223,20 @@ static void convertToLevelAndOption(QNativeSocketEngine::SocketOption opt,
 #ifdef IP_RECVTTL               // IP_RECVTTL is a non-standard extension supported on some OS
             level = IPPROTO_IP;
             n = IP_RECVTTL;
+#endif
+        }
+        break;
+
+    case QNativeSocketEngine::PathMtuInformation:
+        if (socketProtocol == QAbstractSocket::IPv6Protocol || socketProtocol == QAbstractSocket::AnyIPProtocol) {
+#ifdef IPV6_MTU
+            level = IPPROTO_IPV6;
+            n = IPV6_MTU;
+#endif
+        } else {
+#ifdef IP_MTU
+            level = IPPROTO_IP;
+            n = IP_MTU;
 #endif
         }
         break;
@@ -331,6 +345,20 @@ int QNativeSocketEnginePrivate::option(QNativeSocketEngine::SocketOption opt) co
         return -1;
     }
 
+    case QNativeSocketEngine::PathMtuInformation:
+#if defined(IPV6_PATHMTU) && !defined(IPV6_MTU)
+        // Prefer IPV6_MTU (handled by convertToLevelAndOption), if available
+        // (Linux); fall back to IPV6_PATHMTU otherwise (FreeBSD):
+        if (socketProtocol == QAbstractSocket::IPv6Protocol) {
+            ip6_mtuinfo mtuinfo;
+            QT_SOCKOPTLEN_T len = sizeof(mtuinfo);
+            if (::getsockopt(socketDescriptor, IPPROTO_IPV6, IPV6_PATHMTU, &mtuinfo, &len) == 0)
+                return int(mtuinfo.ip6m_mtu);
+            return -1;
+        }
+#endif
+        break;
+
     default:
         break;
     }
@@ -420,6 +448,8 @@ bool QNativeSocketEnginePrivate::setOption(QNativeSocketEngine::SocketOption opt
     }
 #endif
 
+    if (n == -1)
+        return false;
     return ::setsockopt(socketDescriptor, level, n, (char *) &v, sizeof(v)) == 0;
 }
 

src/network/socket/qnativesocketengine_win.cpp

@@ -209,7 +209,7 @@ static inline void qt_socket_getPortAndAddress(SOCKET socketDescriptor, const qt
 static void convertToLevelAndOption(QNativeSocketEngine::SocketOption opt,
                                     QAbstractSocket::NetworkLayerProtocol socketProtocol, int &level, int &n)
 {
-    n = 0;
+    n = -1;
     level = SOL_SOCKET; // default
 
     switch (opt) {
@@ -281,6 +281,9 @@ static void convertToLevelAndOption(QNativeSocketEngine::SocketOption opt,
             n = IP_HOPLIMIT;
         }
         break;
+
+    case QAbstractSocketEngine::PathMtuInformation:
+        break;          // not supported on Windows
     }
 }
 
@@ -471,9 +474,11 @@ int QNativeSocketEnginePrivate::option(QNativeSocketEngine::SocketOption opt) co
     QT_SOCKOPTLEN_T len = sizeof(v);
 
     convertToLevelAndOption(opt, socketProtocol, level, n);
-    if (getsockopt(socketDescriptor, level, n, (char *) &v, &len) == 0)
+    if (n != -1) {
-        return v;
+        if (getsockopt(socketDescriptor, level, n, (char *) &v, &len) == 0)
-    WS_ERROR_DEBUG(WSAGetLastError());
+            return v;
+        WS_ERROR_DEBUG(WSAGetLastError());
+    }
     return -1;
 }
 
@@ -514,6 +519,8 @@ bool QNativeSocketEnginePrivate::setOption(QNativeSocketEngine::SocketOption opt
 
     int n, level;
     convertToLevelAndOption(opt, socketProtocol, level, n);
+    if (n == -1)
+        return false;
     if (::setsockopt(socketDescriptor, level, n, (char*)&v, sizeof(v)) != 0) {
         WS_ERROR_DEBUG(WSAGetLastError());
         return false;

src/network/socket/qnativesocketengine_winrt.cpp

@@ -1514,6 +1514,7 @@ int QNativeSocketEnginePrivate::option(QAbstractSocketEngine::SocketOption opt)
     case QAbstractSocketEngine::MulticastLoopbackOption:
     case QAbstractSocketEngine::TypeOfServiceOption:
     case QAbstractSocketEngine::MaxStreamsSocketOption:
+    case QAbstractSocketEngine::PathMtuInformation:
     default:
         return -1;
     }
@@ -1573,6 +1574,7 @@ bool QNativeSocketEnginePrivate::setOption(QAbstractSocketEngine::SocketOption o
     case QAbstractSocketEngine::MulticastLoopbackOption:
     case QAbstractSocketEngine::TypeOfServiceOption:
     case QAbstractSocketEngine::MaxStreamsSocketOption:
+    case QAbstractSocketEngine::PathMtuInformation:
     default:
         return false;
     }

tests/auto/network/kernel/qnetworkinterface/BLACKLIST

@@ -1,2 +0,0 @@
-[localAddress]
-linux

tests/auto/network/kernel/qnetworkinterface/tst_qnetworkinterface.cpp

@@ -40,6 +40,8 @@
 #include "../../../network-settings.h"
 #include "emulationdetector.h"
 
+Q_DECLARE_METATYPE(QHostAddress)
+
 class tst_QNetworkInterface : public QObject
 {
     Q_OBJECT
@@ -57,6 +59,7 @@ private slots:
     void consistencyCheck();
     void loopbackIPv4();
     void loopbackIPv6();
+    void localAddress_data();
     void localAddress();
     void interfaceFromXXX_data();
     void interfaceFromXXX();
@@ -210,18 +213,69 @@ void tst_QNetworkInterface::loopbackIPv6()
     QList<QHostAddress> all = QNetworkInterface::allAddresses();
     QVERIFY(all.contains(QHostAddress(QHostAddress::LocalHostIPv6)));
 }
+void tst_QNetworkInterface::localAddress_data()
+{
+    QTest::addColumn<QHostAddress>("target");
+
+    QTest::newRow("localhost-ipv4") << QHostAddress(QHostAddress::LocalHost);
+    if (isIPv6Working())
+        QTest::newRow("localhost-ipv6") << QHostAddress(QHostAddress::LocalHostIPv6);
+
+    QTest::newRow("test-server") << QtNetworkSettings::serverIP();
+
+    // Since we don't actually transmit anything, we can list any IPv4 address
+    // and it should work. But we're using a linklocal address so that this
+    // test can pass even machines that failed to reach a DHCP server.
+    QTest::newRow("linklocal-ipv4") << QHostAddress("169.254.0.1");
+
+    if (isIPv6Working()) {
+        // On the other hand, we can't list just any IPv6 here. It's very
+        // likely that this machine has not received a route via ICMPv6-RA or
+        // DHCPv6, so it won't have a global route. On some OSes, IPv6 may be
+        // enabled per interface, so we need to know which ones work.
+        const QList<QHostAddress> addrs = QNetworkInterface::allAddresses();
+        for (const QHostAddress &addr : addrs) {
+            QString scope = addr.scopeId();
+            if (scope.isEmpty())
+                continue;
+            QTest::addRow("linklocal-ipv6-%s", qPrintable(scope))
+                    << QHostAddress("fe80::1234%" + scope);
+        }
+    }
+}
 
 void tst_QNetworkInterface::localAddress()
 {
+    QFETCH(QHostAddress, target);
     QUdpSocket socket;
-    socket.connectToHost(QtNetworkSettings::serverName(), 80);
+    socket.connectToHost(target, 80);
     QVERIFY(socket.waitForConnected(5000));
 
     QHostAddress local = socket.localAddress();
 
-    // test that we can find the address that QUdpSocket reported
+    // find the interface that contains the address QUdpSocket reported
-    QList<QHostAddress> all = QNetworkInterface::allAddresses();
+    QList<QNetworkInterface> ifaces = QNetworkInterface::allInterfaces();
-    QVERIFY(all.contains(local));
+    const QNetworkInterface *outgoingIface = nullptr;
+    for (const QNetworkInterface &iface : ifaces) {
+        QList<QNetworkAddressEntry> addrs = iface.addressEntries();
+        for (const QNetworkAddressEntry &entry : addrs) {
+            if (entry.ip() == local) {
+                outgoingIface = &iface;
+                break;
+            }
+        }
+        if (outgoingIface)
+            break;
+    }
+    QVERIFY(outgoingIface);
+
+    // we get QVariant() if the QNativeSocketEngine doesn't know how to get the PMTU
+    int pmtu = socket.socketOption(QAbstractSocket::PathMtuSocketOption).toInt();
+    qDebug() << "Connected to" << target.toString() << "via interface" << outgoingIface->name()
+             << "pmtu" << pmtu;
+
+    // check that the Path MTU is less than or equal the interface's MTU
+    QVERIFY(pmtu <= outgoingIface->maxTransmissionUnit());
 }
 
 void tst_QNetworkInterface::interfaceFromXXX_data()