VirtualBox port forwarding issue

I'm having a strange issue with port forwarding. I've used it many times and it seems to work without issue for me (Ubuntu guest, Ubuntu host).

I'm now dealing with a problem where a specific NodeJS-based server will curl in the guest but not the host, despite port forwarding set on that port. I'm running another server on the same box (with a different port on both guest and host) which is working normally, so I know it's not a system problem with virtualbox, etc.

I've found numerous port forwarding questions and lots of solutions, but none of them have worked for me. I've checked that my firewall is not a problem. I've tried running both tcp and udp. I've tried changing the port from 3000 to 8123 which I know I have used successfully in the past. I've tried messing with almost every possible configuration of 0.0.0.0 and 127.0.0.1.

Basically, I'm out of ideas and looking for other things to try. What else could cause this type of issue? Are there any other debugging strategies to try?

via David