UDP packets not reaching my app on port 3478 (Machines, LHR region)

JavaScript
1

I’m running an app deployed on Fly (Machines, not Nomad) that’s supposed to receive UDP packets on port 3478 (for STUN-style inspection). Everything appears to be configured correctly:

  • fly.toml includes a [[services]] block for protocol = "udp", internal_port = 3478
  • Logs show: ✅ UDP server listening on port 3478
  • App is deployed to the lhr region
  • App binds using Node.js dgram.createSocket('udp4') and listens on 0.0.0.0
  • IPv4 A record resolves fine via dig
  • I’ve tested sending UDP packets using nc -u and ncat, both directly to the IPv4 address

However, the app never receives any messages — the udpSocket.on("message") handler is never triggered, and nothing logs during tests.

I’ve also verified:

  • WebSocket server on port 8080 is working and logs properly
  • No errors are thrown when binding to UDP
  • No use of processes = ["app"] (I removed that line to be safe)

Is there something I might be missing for Machines in lhr to receive UDP traffic on 3478?

Thanks — happy to provide a repro or minimal config privately if needed.

2

Hi… This actually needs to be the special fly-global-services address, due to the eBPF magic, :honeybee:.

Overall, UDP is idiosyncratic and a little limited on the Fly.io platform. Most of the gotchas are covered here:

https://fly.io/docs/networking/udp-and-tcp/

Hope this helps a bit!

3

Thanks - that’s perfect - you’re a star! :dizzy:

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