LocalDiscovery flip flopping and network design tips

James Hartig james at levenlabs.com
Tue Feb 14 19:59:45 CET 2017

On Tue, Feb 14, 2017 at 1:46 PM, Guus Sliepen <guus at tinc-vpn.org> wrote:
> On Tue, Feb 14, 2017 at 11:21:34AM -0500, James Hartig wrote:
>> Those 2 boxes are in the same subnet and have addresses of and
>>, respectively, on their eth0 interface. Port 655 on tcp and udp
>> is open to the world. The tinc_test_2 box has a ConnectTo of tinc_test_1.
>> When tinc_test_2 is started, it prints out:
>>   UDP address of tinc_test_1 set to port 655
>>   UDP address of tinc_test_1 set to port 655
>>   UDP address of tinc_test_1 set to port 655
>>   UDP address of tinc_test_1 set to port 655
>> repeatedly for a minute or so before finally settling on
>> Is there a reason it's flip flopping? Is that expected? Am I doing
>> something wrong?
> No, you are not doing anything wrong. Although I've not seen this kind
> of flip-flopping behavior myself, it is possible this behaviour occurs,
> although it should only happen in the first 10 seconds or so. When
> LocalDiscovery is enabled, tinc tries to send probe packets to both the
> address it learned from its TCP connections and to the local network.
> When receiving a valid packet, it notes the source address of that
> packet. If it is different from the source address of the previous valid
> UDP packet, a log message is printed about it.

When you say "and to the local network" what IP does it try to send to
on the local network? The subnet address? Is this configurable with
the LocalDiscoveryAddress?

For instance if I have a host:
Subnet =
Address =

Will it try to contact over the local network, or if I
specify a LocalDiscoveryAddress on tinc_test_5 of, will it
use instead? Apologizes for my confusion. Since I'm setting
Subnet to be the internal IP, is there any difference in that versus

>> Additionally, we have multiple Google Compute regions with their own
>> subnets and external DCs with their own subnets and we'd like to install
>> tinc on all servers but keep inner-Google traffic to the internal IPs and
>> not over external IPs since it's an order of magnitude cheaper. My first
>> thinking is a hub and spoke model. We have 2 boxes in each subnet that have
>> port 655 open to the world, and all the other servers have 655 open to
>> internal ips only. With LocalDiscovery (as well as IndirectData = yes on
>> "non-public" servers) this works work pretty well, as far as I can tell.
>> But it wouldn't solve the inner-Google traffic between subnets since Google
>> Subnet0 would talk over public to Google Subnet1. What's the best way of
>> doing something like this? I was thinking maybe 2 instances of tinc on the
>> "public" boxes, but Google servers only have a single interface, eth0, that
>> has the internal IP, so I couldn't listen on the external and internal IPs
>> separately.
> You can use BindToAddress to have tinc bind to a specific IP address. So
> you can have two tinc daemons, one binding to the internal IP address,
> and another to the external IP address, even if they are residing on the
> same network interface. Would that help?

I can't use BindToAddress as far as I know because there isn't an
interface on the box with the external IP. The servers only have 1
interface, eth0, that has a single IP, eg, and Google
forwards packets to their public IP, eg, to the
internal IP, eg, so the server itself doesn't actually know
its public IP.

I think if my understanding above of the local network stuff is
correct, and since it's unicast in 1.1, then it should work fine.
Google Subnet0 and Google Subnet1 can talk to each other's private
IPs, so theoretically discovery should work and it should use their
private IPs.

> --
> Met vriendelijke groet / with kind regards,
>      Guus Sliepen <guus at tinc-vpn.org>
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