Be more strict about MTU; OS X's bridge interface can pave over it wi…

…th 1500 otherwise, wreaking havoc.

HoRNDIS.cpp

@@ -235,13 +235,23 @@ bool HoRNDIS::openInterfaces() {
 /* We need our own createInterface (overriding the one in IOEthernetController) because we need our own subclass of IOEthernetInterface.  Why's that, you say?  Well, we need that because that's the only way to set a different default MTU.  Sigh... */
 
 bool HoRNDISInterface::init(IONetworkController * controller, int mtu) {
+	maxmtu = mtu;
 	if (IOEthernetInterface::init(controller) == false)
 		return false;
 	LOG(V_NOTE, "starting up with MTU %d", mtu);
 	setMaxTransferUnit(mtu);
 	return true;
 }
 
+bool HoRNDISInterface::setMaxTransferUnit(UInt32 mtu) {
+	if (mtu > maxmtu) {
+		LOG(V_NOTE, "Excuse me, but I said you could have an MTU of %u, and you just tried to set an MTU of %d.  Good try, buddy.", maxmtu, mtu);
+		return false;
+	}
+	IOEthernetInterface::setMaxTransferUnit(mtu);
+	return true;
+}
+
 /* Overrides IOEthernetController::createInterface */
 IONetworkInterface *HoRNDIS::createInterface() {
 	HoRNDISInterface * netif = new HoRNDISInterface;

HoRNDIS.h

@@ -307,6 +307,8 @@ class HoRNDISUSBInterface : public HoRNDIS {
 
 class HoRNDISInterface : public IOEthernetInterface {
 	OSDeclareDefaultStructors(HoRNDISInterface);
+	int maxmtu;
 public:
 	virtual bool init(IONetworkController * controller, int mtu);
+	virtual bool setMaxTransferUnit(UInt32 mtu);
 };