I have a bunch of Windows 2016 servers on site at a client installation. The client has a single outgoing fiber connection and through some IPSec magic we can RDP into the server when we need to - no problem.

The client now wants us to hook up a secondary way in for emergencies, in case the first internet connection goes down. The servers are used to control some serious real world equipment, so retaining control is a good idea.

Since the location where the machines are installed is extremely remote, running another copper or fiber cable is not an option.

I got a Teltonika RUTx09 4G gateway for backup. I plugged it into the servers and was able, with port-forwarding, to connect to the RDP on all servers. I then had to remove the default gateway for the Teltonika, as the servers are communicating with some other DCOM equipment and those would no longer route.

This killed the RDP connectivity via the Teltonika. If I put the default gateway back in the config on the server on the interface connected to the Teltonika, I can connect to RDP via the Teltonika. If I remove it, I cannot. I can ping back and forth, I can visit the Teltonika web interface from the server - all connectivity appears to work, except for RDP. If I remove the default gateway, the NIC also states "unidentified network" instead of "Network 2" - but that might be unrelated.

I really cannot have the default gateway configured on the interface connected to the Teltonika and I really see no reason why RDP should not work when I can clearly communicate between server and router via any other means.

Does anyone have any insight on this?

Thanks in advance!


This is an asymmetric routing problem: when a packet comes to the server through the Teltonika gateway and the server sends back an answer through the same Teltonika gateway, this works correctly; but if the server uses another default gateway, when a packet comes in through the Teltonika the server sends its answer through the other gateway (and its different Internet connection), and this just cannot work.

If you need to set up two Internet connections for your network, you need to handle them using a router (or a firewall) which can manage multiple connections and switch between them when needed; in this scenario, that device should be the default gateway of your network. There are several devices which can do this, even non-high-end ones.

  • I understand the basics at play here, but then why can I ping back and forth? The secondary network is a completely different IP range and I have no desire to send traffic out through it. I just want to connect from the 4G router to RDP within the same subnet without having to set this device as the default router on Windows. Strange thing is; apart from ping I also tested a webserver and I CAN access the webserver when the default gateway is not set. This must be something specific to RDP. – Gerard Oct 22 at 7:03

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