Hello, I have an ipv4 only network at home and I found a very cheap ipv6 only VPS (ipv4 is 3$/month more for some reason). I thought I could use it anyway with syncthing thanks to relays, but it does not work! I setup the VPS while being on another ipv6 network. I tested syncthing connection to the VPS from there and it works.
Is this a limit relays cannot overcome?
Relays are essentially v4 only to assist in v4 Nat traversal. Our relay announcement protocol doesn’t cater for dual stack as would be needed to tie together v4 and v6 clients, unfortunately.
@calmh What if we add relay manually in listenAddress? Will it work if the relay address is different on two nodes but relay id is identical, or if we have a domain name with both A and AAAA record pointing to the same host? 
In my testing strelaysrv itself has no problem handling IPv6.
I believe that should work, yes