forward traffic from port X to computer B with c# “UDP punch hole into firewall”

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Your new work router has probably got UPnP disabled, hence your null reference.

Without this your server cannot be made visible to inbound traffic as the router doesn’t know where to send the inbound packets. In this case the router acts as a firewall blocking the incoming traffic to your server.

The basic ways around this are:

1) open up UPnP

This enables your application to instruct the router how to forward inbound traffic back to your server.

2) set up a port forwarding

As above by manually configuring the router.

3) make your work server the client

Routers work by allowing outbound connections to initiate the connection. It remembers the return address, rewrites the externally visible IP, and provides an unused port for external traffic to talk back on (NAT). This allows outbound requests to establish communication with the outside and bypass the firewall. If your home IP is fixed you could setup a client at work that tries to call home on a schedule (until you start the server and can establish the connection).

4) use P2P (mediation server)

I’m not sure where you would begin with this, but the principle is this. It usually works on a single UDP port. A server that is not behind NAT is used for establishing connections. The clients send their IP to the server in a UDP packet, and the router rewrites the UDP header with the router return address. The server takes this data and sends it to other peers. With everyone now knowing each others return address, they can send TCP traffic directly to each other and the server steps out of the way.

There’s some really good article here regarding the basics of NAT, explained in simple terms. And a good article here which explains how P2P leverages NAT to bypass firewalls.

Hope this gives you some ideas.

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