Do ipv6 announce messages loop back?

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1

I am getting firewall block messages every minute in the kernel log of a Kubuntu machine Ox1

[UFW BLOCK] IN=eth0 OUT= MAC= SRC=fe80:0000:0000:0000:82fa:5bff:fe31:fa27 DST=ff12:0000:0000:0000:0000:0000:0000:8384 LEN=118 TC=0 HOPLIMIT=1 FLOWLBL=464139 PROTO=UDP SPT=37113 DPT=21027 LEN=78

which seems to be local announce messages being sent from Ox1 ethernet adaptor. Do local announce messages loop back?

Also I am getting messages:

[UFW BLOCK] IN=wlan0 OUT= MAC= SRC=fe80:0000:0000:0000:4685:00ff:fe44:3f72 DST=ff12:0000:0000:0000:0000:0000:0000:8384 LEN=118 TC=0 HOPLIMIT=1 FLOWLBL=237879 PROTO=UDP SPT=37113 DPT=21027 LEN=78

which seems to be local announce messages being sent from Ox1 wlan adaptor however the wlan adaptor is not currently connected. Why is syncthing generating messages from an unconnected adaptor?

2

It’s sending stuff to all adapters regardless it seems.

3

Yes but the mystery to me here is why a message from this same computer

> SRC=fe80:0000:0000:0000:82fa:5bff:fe31:fa27

is coming in on eth0

> IN=eth0

to itself (ie from Ox1 to Ox1)

And also why messages appear to be coming in on a disconnected interface:

IN=wlan0

again to itself from Ox1 to Ox1.

The only way I can think of this happening is if Ox1 sent messages on the loopback interface,but then it should show:

IN = lo

(fe80:0000:0000:0000:82fa:5bff:fe31:fa27 is the ipv6 address of this same machine, Ox1)

Any ideas?

4

It depends on a bunch of things, but it’s not unusual for IPv6 multicasts to be received again on the interface they were sent, yes.

5

Thanks. Surprising. Maybe the router is relaying them.

I guess I need to take a year off to understand ipv6. :confused:

6

It may, but it’s probably internal to the computer. One rationale is that multicast is often used for various kinds of discovery, and one program on the computer (like a word processor) may be interested on what another (a printer driver) has to announce. I think you can run two Syncthing instances and have local discovery between them work, as well.

7

You must be correct that it is internal to the computer - the 2nd case I mentioned where a packet was coming in on a disconnected interface (wlan0) shows that.

Thanks for you insight.

8

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