[sclug] Potential in-bound routing failure on BT: any thoughts welcomed

[Neil Brown]

Evening, all

This is more a networking problem than a Linux problem (as far as I can tell...), but it is driving me up the wall. Any thoughts would be welcomed.

I am a BT ADSL domestic / consumer customer, and things have worked fine for the last five or so years. For the last three years, I have had no problems using a DYNDNS service, and connecting back into my home network from outside the network, into various servers run within the house.

On Friday morning, I was connected back through the VPN server in the house. I disconnected and went for coffee, and, on trying to reconnect, it failed ? the server was not responding. As occasionally my DYNDNS account seems to fail to update, and on the possibility that my IP address has been re-issued, I didn't think any more of it.

I got home, and checked the IP address in DYNDNS matched the IP address showing on a "what's my IP" website ? it was.

I rebooted *everything* on the network, releasing my IP and BT handed me a new one. DYNDNS updated correctly, but I could still not make the VPN connection. Using the IP address in place of my DYNDNS URL did not work. From within the network, the VPN server responded correctly.

I looked in the VPN and OpenDirectory logs, and saw no sign of any traffic hitting the servers at all, so it seems not reaching rather than mishandled inside my network.

Same story with accessing the other servers.

(I am pretty confident everything is configured correctly my side, if only because it was working perfectly up until, and indeed on, Friday, and had been for years.)

I noted at this point that my router was reporting that my IP was in the range of 100.xx.xx.xx, whilst the "what's my IP" site was reporting 81.x.x.x ? I had not noticed the difference before, but it might be my oversight.

After a couple of frustrating conversations with BT, trying to work out if there was a problem on their side, I switched out my modem and router for a BT Home Hub. After connecting a test Raspberry Pi running a web server (verified working with the network), and forwarding the port on the HomeHub (and disabling the firewall completely), I still could not access using my WAN IP address. I also note that the HomeHub shows the IP address 100.x.x.x, whilst the "what's my IP" site is still showing a different 81.x.x.x IP address.

I am wondering if BT has switched on CG-NAT or something like that, which might explain both the variance in IP addresses being reported and what seems to me to be an in-bound routing failure but, according to the BT first line team, there is nothing wrong. They are doing their best, and I am not sure this is a common complaint, but it is still a nuisance as I rather like my self-hosted VPN for some protection when using Wi-Fi hotspots when travelling, and to access other services on the network.

Before I try to escalate *again*, can anyone spot anything obvious I am failing to check here? 

Any thoughts very gratefully received.


Best wishes,

Neil