Rework the listening IP/interface selection code

Closes #11561

src/base/utils/net.cpp

@@ -28,6 +28,7 @@
 
 #include "net.h"
 
+#include <QNetworkInterface>
 #include <QSslCertificate>
 #include <QSslKey>
 #include <QString>
@@ -91,6 +92,37 @@ namespace Utils
             return subnet.first.toString() + '/' + QString::number(subnet.second);
         }
 
+        QHostAddress canonicalIPv6Addr(const QHostAddress &addr)
+        {
+            // Link-local IPv6 textual address always contains a scope id (or zone index)
+            // The scope id is appended to the IPv6 address using the '%' character
+            // The scope id can be either a interface name or an interface number
+            // Examples:
+            // fe80::1%ethernet_17
+            // fe80::1%13
+            // The interface number is the mandatory supported way
+            // Unfortunately for us QHostAddress::toString() outputs (at least on Windows)
+            // the interface name, and libtorrent/boost.asio only support an interface number
+            // as scope id. Furthermore, QHostAddress doesn't have any convenient method to
+            // affect this, so we jump through hoops here.
+            if (addr.protocol() != QAbstractSocket::IPv6Protocol)
+                return QHostAddress{addr.toIPv6Address()};
+
+            // QHostAddress::setScopeId(addr.scopeId()); // Even though the docs say that setScopeId
+            // will convert a name to a number, this doesn't happen. Probably a Qt bug.
+            const QString scopeIdTxt = addr.scopeId();
+            if (scopeIdTxt.isEmpty())
+                return addr;
+
+            const int id = QNetworkInterface::interfaceIndexFromName(scopeIdTxt);
+            if (id == 0) // This failure might mean that the scope id was already a number
+                return addr;
+
+            QHostAddress canonical(addr.toIPv6Address());
+            canonical.setScopeId(QString::number(id));
+            return canonical;
+        }
+
         QList<QSslCertificate> loadSSLCertificate(const QByteArray &data)
         {
             const QList<QSslCertificate> certs {QSslCertificate::fromData(data)};