Syncthing on Port 5351

I have three computers using syncthing, all working fine. Syncthing seem to want to send a lot of traffic to port 5351 on my firewall. What is it trying to do, is it necessary, and how do I shut it off?

Syncthing doesn’t use port 5351, and I can’t see any relays running on that port.

Are you sure that Syncthing is attempting to connect to that port?

Yes. I ran “lsof | grep 5351” and caught it in the act.

Could you post exactly what that printed?

$ lsof | grep 5351
lsof: WARNING: can't stat() tracefs file system /sys/kernel/debug/tracing
      Output information may be incomplete.
syncthing  1189                      charles   58u     IPv4             669777       0t0        UDP hawk.localdomain:60722->freeman.localdomain:5351 
syncthing  1189  1190 syncthing      charles   58u     IPv4             669777       0t0        UDP hawk.localdomain:60722->freeman.localdomain:5351 
syncthing  1189  1191 syncthing      charles   58u     IPv4             669777       0t0        UDP hawk.localdomain:60722->freeman.localdomain:5351 
syncthing  1189  1192 syncthing      charles   58u     IPv4             669777       0t0        UDP hawk.localdomain:60722->freeman.localdomain:5351 
syncthing  1189  1193 syncthing      charles   58u     IPv4             669777       0t0        UDP hawk.localdomain:60722->freeman.localdomain:5351 
....

I don’t suppose you’ve configured freeman.localdomain to listen on port 5351? Or configured Syncthing on that machine to connect to freeman.localdomain on port 5351?

freeman is the firewall. Syncthing doesn’t run on it. freeman doesn’t have anything on it that uses port 5351, so it blocks that port.

You’ve tried grepping Syncthing’s config.xml for “5351” I assume?

Yes, both of them. No show.

D’oh, 5351 is NAT-PMP. Settings → Connections → Uncheck “Enable NAT traversal” I guess?

Well, I tried that. We will see if that makes a difference. I also shut off global discovery and enable relaying. I don’t know what they do, so I don’t think I need them.

Global discovery is used to let devices which aren’t on the same LAN find each others’ addresses (local discovery is used if they are on the same LAN). Relaying is used if two devices can’t connect directly.

Ah, thank you. About what I figured when I looked at them.

Two hours on, and freeman is not reporting any new packets at 5351. I think that does it. Thank you.