resolver_service: Make the service resolution port dynamic by default.

If the service resolution address isn't configured, let it have a random port.

The dispatcher dynamically assigns a port and also registers it as the recipient of resolution requests. It appears that no knowledge of the port is required outside of that.

control/cmd/control/main.go

@@ -200,6 +200,8 @@ func realMain(ctx context.Context) error {
 		return err
 	}
 
+	// FIXME: readability would be improved if we could be consistent with address
+	// representations in NetworkConfig (string or cooked, chose one).
 	nc := infraenv.NetworkConfig{
 		IA: topo.IA(),
 		// Public: (Historical name) The TCP/IP:port address for the control service.

private/app/appnet/infraenv.go

@@ -107,6 +107,14 @@ func (nc *NetworkConfig) QUICStack() (*QUICStack, error) {
 	if nc.QUIC.Address == "" {
 		nc.QUIC.Address = net.JoinHostPort(nc.Public.IP.String(), "0")
 	}
+	if nc.ServiceResolution == nil {
+		srAddr, err := net.ResolveUDPAddr("udp", net.JoinHostPort(nc.Public.IP.String(), "0"))
+		if err != nil {
+			return nil, serrors.WrapStr("parsing service_resolution QUIC address", err)
+		}
+		nc.ServiceResolution = srAddr
+	}
+
 	client, server, err := nc.initQUICSockets()
 	if err != nil {
 		return nil, err
@@ -268,9 +276,11 @@ func (nc *NetworkConfig) initSvcRedirect(quicAddress string) (func(), error) {
 		Dispatcher: packetDispatcher,
 		Metrics:    nc.SCIONNetworkMetrics,
 	}
+
 	conn, err := network.Listen(context.Background(), "udp", nc.ServiceResolution, addr.SvcWildcard)
 	if err != nil {
-		return nil, serrors.WrapStr("listening on SCION", err, "addr", nc.Public)
+		log.Info("Listen failed", "err", err)
+		return nil, serrors.WrapStr("listening on SCION", err, "addr", nc.ServiceResolution)
 	}
 
 	ctx, cancel := context.WithCancel(context.Background())