UDP port 5060: packets from specific source IP not reaching container

Questions / Help
1

App: laima-voice-server, machine d8d5205f11de48 (ams)

We have a SIP server on UDP port 5060. UDP packets from our own IP (78.61.59.57, Lithuania) arrive at our container normally. However, UDP packets from our SIP provider’s server (185.45.155.33, AS199790 IP Telecom Bulgaria) never arrive — confirmed by server logs showing zero incoming packets from that IP.

Our SIP provider confirmed they ARE sending INVITE packets to our dedicated IP (137.66.10.45:5060) but receive no response. Packets are captured on their side but never reach our container.

Additional note: in the Fly.io dashboard under Services, only the HTTP service is shown — our [[services]] entries for UDP port 5060 and RTP ports 10000-10018 are not displayed. Not sure if this indicates they are not active at the network level.

This worked until 2026-05-12 when we did several rapid deploys and machine restarts (server was unresponsive for ~25 min). After recovery, the server runs fine but UDP from 185.45.155.33 stopped arriving.

We released our old dedicated IP (213.188.214.14) and allocated a new one (137.66.10.45) — problem persists.

Could you check if Fly.io’s network is dropping UDP from AS199790 / 185.45.155.33, or if there’s a misconfiguration in our UDP service setup?

4

Hi Igor,

We don’t drop UDP from specific ASs randomly. You could perhaps check this if you have access to another server somewhere on the Internet and see if you can hit your Fly-hosted UDP service.
Sometimes I do this by just starting nc in echo mode (e.g. use a wrapper script to start your actual service and nc in echo mode, for diagnostics) and creating a [service] block for nc on an arbitrary port. Then you can use nc in udp mode to send (and check you get echos back) from your Fly machines. (they’re not containers! they’re micro VMs!)

If your UDP service is reachable from other locations, and was working with the same config, then I’m fairly certain it’s not a misconfiguration. Perhaps the above can help diagnose things.