QNetworkInterface: add MTU

[ChangeLog][QtNetwork][QNetworkInterface] Added maxTransmissionUnit().

Change-Id: Iaf4157b7efa2416d898cfffd14d96b2970d6af87
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/network/kernel/qnetworkinterface.cpp

@@ -688,6 +688,29 @@ int QNetworkInterface::index() const
     return d ? d->index : 0;
 }
 
+/*!
+    \since 5.11
+
+    Returns the maximum transmission unit on this interface, if known, or 0
+    otherwise.
+
+    The maximum transmission unit is the largest packet that may be sent on
+    this interface without incurring link-level fragmentation. Applications may
+    use this value to calculate the size of the payload that will fit an
+    unfragmented UDP datagram. Remember to subtract the sizes of headers used
+    in your communication over the interface, e.g. TCP (20 bytes) or UDP (12),
+    IPv4 (20) or IPv6 (40, absent some form of header compression), when
+    computing how big a payload you can transmit. Also note that the MTU along
+    the full path (the Path MTU) to the destination may be smaller than the
+    interface's MTU.
+
+    \sa QUdpSocket
+*/
+int QNetworkInterface::maxTransmissionUnit() const
+{
+    return d ? d->mtu : 0;
+}
+
 /*!
     Returns the name of this network interface. On Unix systems, this
     is a string containing the type of the interface and optionally a

src/network/kernel/qnetworkinterface.h

@@ -153,6 +153,7 @@ public:
     bool isValid() const;
 
     int index() const;
+    int maxTransmissionUnit() const;
     QString name() const;
     QString humanReadableName() const;
     InterfaceFlags flags() const;

src/network/kernel/qnetworkinterface_linux.cpp

@@ -293,6 +293,11 @@ static QList<QNetworkInterfacePrivate *> getInterfaces(int sock, char *buf)
                 iface->name = QString::fromLatin1(payloadPtr, payloadLen - 1);
                 break;
 
+            case IFLA_MTU:
+                Q_ASSERT(payloadLen == sizeof(int));
+                iface->mtu = *reinterpret_cast<int *>(payloadPtr);
+                break;
+
             case IFLA_OPERSTATE:    // operational state
                 if (*payloadPtr != IF_OPER_UNKNOWN) {
                     // override the flag

src/network/kernel/qnetworkinterface_p.h

@@ -88,6 +88,7 @@ public:
     { }
 
     int index;                  // interface index, if know
+    int mtu = 0;
     QNetworkInterface::InterfaceFlags flags;
     QNetworkInterface::InterfaceType type = QNetworkInterface::Unknown;
 

src/network/kernel/qnetworkinterface_unix.cpp

@@ -139,6 +139,15 @@ QString QNetworkInterfaceManager::interfaceNameFromIndex(uint index)
     return QString::number(uint(index));
 }
 
+static int getMtu(int socket, struct ifreq *req)
+{
+#ifdef SIOCGIFMTU
+    if (qt_safe_ioctl(socket, SIOCGIFMTU, req) == 0)
+        return req->ifr_mtu;
+#endif
+    return 0;
+}
+
 #ifdef QT_NO_GETIFADDRS
 // getifaddrs not available
 
@@ -278,6 +287,7 @@ static QList<QNetworkInterfacePrivate *> interfaceListing()
         if (qt_safe_ioctl(socket, SIOCGIFFLAGS, &req) >= 0) {
             iface->flags = convertFlags(req.ifr_flags);
         }
+        iface->mtu = getMtu(socket, &req);
 
 #ifdef SIOCGIFHWADDR
         // Get the HW address
@@ -458,9 +468,16 @@ static QNetworkInterface::InterfaceType probeIfType(int socket, int iftype, stru
 static QList<QNetworkInterfacePrivate *> createInterfaces(ifaddrs *rawList)
 {
     QList<QNetworkInterfacePrivate *> interfaces;
-    struct ifmediareq mediareq;
+    union {
+        struct ifmediareq mediareq;
+        struct ifreq req;
+    };
     int socket = -1;
 
+    // ensure both structs start with the name field, of size IFNAMESIZ
+    Q_STATIC_ASSERT(sizeof(mediareq.ifm_name) == sizeof(req.ifr_name));
+    Q_ASSERT(&mediareq.ifm_name == &req.ifr_name);
+
     // on NetBSD we use AF_LINK and sockaddr_dl
     // scan the list for that family
     for (ifaddrs *ptr = rawList; ptr; ptr = ptr->ifa_next)
@@ -476,6 +493,7 @@ static QList<QNetworkInterfacePrivate *> createInterfaces(ifaddrs *rawList)
 
             strlcpy(mediareq.ifm_name, ptr->ifa_name, sizeof(mediareq.ifm_name));
             iface->type = probeIfType(openSocket(socket), sdl->sdl_type, &mediareq);
+            iface->mtu = getMtu(socket, &req);
         }
 
     if (socket != -1)

src/network/kernel/qnetworkinterface_win.cpp

@@ -151,6 +151,7 @@ static QList<QNetworkInterfacePrivate *> interfaceListing()
         else if (ptr->IfIndex != 0)
             iface->index = ptr->IfIndex;
 
+        iface->mtu = ptr->Mtu;
         iface->flags = QNetworkInterface::CanBroadcast;
         if (ptr->OperStatus == IfOperStatusUp)
             iface->flags |= QNetworkInterface::IsUp | QNetworkInterface::IsRunning;

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

@@ -140,6 +140,7 @@ void tst_QNetworkInterface::dump()
         qDebug() << "    flags:     " << qPrintable(flags);
         qDebug() << "    type:      " << i.type();
         qDebug() << "    hw address:" << qPrintable(i.hardwareAddress());
+        qDebug() << "    MTU:       " << i.maxTransmissionUnit();
 
         int count = 0;
         foreach (const QNetworkAddressEntry &e, i.addressEntries()) {