listening on random port

[Hr46ph]

Can someone tell me what I should do about those random ports syncthing insists on listening on? First its 46113/udp and 34583/udp, then its 55338/udp and 58281/udp.

# ss -tulpen|grep syncthing

udp    UNCONN  0    0    0.0.0.0:46113
udp    UNCONN  0    0    0.0.0.0:21027
udp    UNCONN  0    0    [::]:34583
udp    UNCONN  0    0    [::]:21027
tcp    LISTEN  0    128  *:8384
tcp    LISTEN  0    128  *:22000

# systemctl restart syncthing@syncthing.service

# ss -tulpen|grep syncthing

udp    UNCONN  0    0    0.0.0.0:21027
udp    UNCONN  0    0    0.0.0.0:55338
udp    UNCONN  0    0    [::]:21027
udp    UNCONN  0    0    [::]:58281
tcp    LISTEN  0    128  *:8384
tcp    LISTEN  0    128  *:22000

Each restart those two are different. I’ve read https://docs.syncthing.net/users/firewall.html?highlight=firewall, it only mentions 21027/udp and 22000/tcp.

Thanks.

[AudriusButkevicius]

There is no such thing as “listening” on udp. You simply reserve (bind) a port which belongs to your application, and wether you listen or send packets from it is your business.

In this case I suspect it’s NAT discovery that sends packets out of those connections, which is random outbound port the kernel picks.

[calmh (Jakob Borg)]

Also local discovery, iirc. And yeah, you don’t do anything about this, it just is.

[Hr46ph]

Sorry, my bad. I assumed it was the same as with TCP, listening for incoming connections. No need to open up iptables for incoming connections then.

Thanks for the quick reply!