Client: Add SRV-based virtual hosting support

When connecting to host names without port (e.g. example.org), the
client will now perform an SRV lookup to _jamulus._udp.example.org.
If this lookup returns exactly one result, the result is used to select
the actual target address/port.

If the lookup does not return a usable result, the regular connect logic
kicks in (A lookup, default port).

Related: https://github.com/orgs/jamulussoftware/discussions/1772

src/client.cpp

@@ -477,7 +477,7 @@ void CClient::StartDelayTimer()
 bool CClient::SetServerAddr ( QString strNAddr )
 {
     CHostAddress HostAddress;
-    if ( NetworkUtil().ParseNetworkAddress ( strNAddr, HostAddress, bEnableIPv6 ) )
+    if ( NetworkUtil().ParseNetworkAddressWithSrvDiscovery ( strNAddr, HostAddress, bEnableIPv6 ) )
     {
         // apply address to the channel
         Channel.SetAddress ( HostAddress );

src/util.cpp

@@ -914,6 +914,72 @@ bool NetworkUtil::ParseNetworkAddressName ( QString strAddress, QHostAddress& In
     return false;
 }
 
+bool NetworkUtil::ParseNetworkAddressSrv ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 )
+{
+    // init requested host address with invalid address first
+    HostAddress = CHostAddress();
+
+    QRegularExpression plainHostRegex ( "^([^\\[:0-9.][^:]*)$" );
+    if ( plainHostRegex.match ( strAddress ).capturedStart() != 0 )
+    {
+        // not a plain hostname? then don't attempt SRV lookup and fail
+        // immediately.
+        return false;
+    }
+
+    QDnsLookup* dns = new QDnsLookup();
+    dns->setType ( QDnsLookup::SRV );
+    dns->setName ( QString ( "_jamulus._udp.%1" ).arg ( strAddress ) );
+    dns->lookup();
+    // QDnsLookup::lookup() works asynchronously. Therefore, wait for
+    // it to complete here by resuming the main loop here.
+    // This is not nice and blocks the UI, but is similar to what
+    // the regular resolve function does as well.
+    QTime dieTime = QTime::currentTime().addMSecs ( DNS_SRV_RESOLVE_TIMEOUT_MS );
+    while ( QTime::currentTime() < dieTime && !dns->isFinished() )
+    {
+        QCoreApplication::processEvents ( QEventLoop::ExcludeUserInputEvents, 100 );
+    }
+    QList<QDnsServiceRecord> records = dns->serviceRecords();
+    dns->deleteLater();
+    if ( records.length() != 1 )
+    {
+        return false;
+    }
+    QDnsServiceRecord record = records.first();
+    if ( record.target() == "." || record.target() == "" )
+    {
+        // RFC2782 says that "." indicates that the service is not available.
+        // (Qt strips the trailing dot, which is why we check for empty string
+        // as well)
+        // Therefore, end processing here (= return true), but pass back an
+        // invalid HostAddress to let the connect logic fail properly.
+        HostAddress = CHostAddress ( QHostAddress ( "." ), 0 );
+        return true;
+    }
+    qDebug() << qUtf8Printable (
+        QString ( "resolved %1 to a single SRV record: %2:%3" ).arg ( strAddress ).arg ( record.target() ).arg ( record.port() ) );
+
+    QHostAddress InetAddr;
+    if ( ParseNetworkAddressName ( record.target(), InetAddr, bEnableIPv6 ) )
+    {
+        HostAddress = CHostAddress ( InetAddr, record.port() );
+        return true;
+    }
+    return false;
+}
+
+bool NetworkUtil::ParseNetworkAddressWithSrvDiscovery ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 )
+{
+    // Try SRV-based discovery first:
+    if ( ParseNetworkAddressSrv ( strAddress, HostAddress, bEnableIPv6 ) )
+    {
+        return true;
+    }
+    // Try regular connect:
+    return ParseNetworkAddress ( strAddress, HostAddress, bEnableIPv6 );
+}
+
 bool NetworkUtil::ParseNetworkAddress ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 )
 {
     QHostAddress InetAddr;

src/util.h

@@ -42,6 +42,7 @@
 #include <QCoreApplication>
 #include <QUdpSocket>
 #include <QHostAddress>
+#include <QDnsLookup>
 #include <QHostInfo>
 #include <QFile>
 #include <QDirIterator>
@@ -79,8 +80,9 @@ class CClient; // forward declaration of CClient
 #endif
 
 /* Definitions ****************************************************************/
-#define METER_FLY_BACK  2
-#define INVALID_MIDI_CH -1 // invalid MIDI channel definition
+#define METER_FLY_BACK             2
+#define INVALID_MIDI_CH            -1 // invalid MIDI channel definition
+#define DNS_SRV_RESOLVE_TIMEOUT_MS 500
 
 /* Global functions ***********************************************************/
 // converting float to short
@@ -1039,6 +1041,8 @@ class NetworkUtil
 {
 public:
     static bool ParseNetworkAddressName ( QString strAddress, QHostAddress& InetAddr, bool bEnableIPv6 );
+    static bool ParseNetworkAddressSrv ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 );
+    static bool ParseNetworkAddressWithSrvDiscovery ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 );
     static bool ParseNetworkAddress ( QString strAddress, CHostAddress& HostAddress, bool bEnableIPv6 );
 
     static QString      FixAddress ( const QString& strAddress );