I don’t think you can monitor the traffic as it’d encrypted, unless you somehow provide certificates to wireshark to undo TLS and forward secrecy, which I think is not possible. Even if you decrypted the traffic, what do you expect to do with it?
I don’t care so much for the content of the traffic. I realize that with the encryption in place it will all be gobbelygook. But I can monitor IP packets moving back and forth on the wire. And knowing how syncthing communicates would help in trying to determine my problem. Maybe. In the meantime, I’ve established that its unlikely to be syncthing that has an issue. I appear to have issues with ssh as well. The NAS can successfully connect via SSH to the RPi, and the RPi’s can successfully connect over SSH to each other as well as my other Synology NAS. But none of the remote RPi’s can connect over SSH to the problem NAS. I’ve setup another RPi next to the problem NAS on the same local network and that can connect over SSH and syncthing fine. So you’re initial post about the network seems to be likely. Or there’s something on my NAS blocking access from remote networks. More troubleshooting tomorrow.
An update. This doesn’t appear to be a syncthing issue. I’ve tried many things over the past week and everything points to the Synology NAS having an issue. For some reason I cannot sync from remote endpoints to the NAS. It works fine as long as the traffic is local. My testing revealed that for some reason, I can access the NAS remotely using telnet and SMB, but not syncthing or SSH. Whilst these protocols are all very different, one of the main differences I can see is that the former two are plaintext protocols, whereas syncthing and SSH are encrypted protocols. I tested telnet to port 23023 and SSH to port 23023 (not at the same time, can’t have two daemons listening to the same port), and telnet would work and SSH not. SSH would just sit there waiting for something. Shutting down SSH and trying to connect would instantly give me an error. I suspect the problems with SSH and syncthing are related and I have engaged the NAS vendor to see if they can find out what is broken. Everything points to the NAS. For the moment I’m using another (Synology) NAS that works fine.
So it doesn’t look like its a syncthing problem.
Thanks Audrius and Jakob for trying to help me!
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