There is no explanation for /ipv6 dhcp-relay at http://wiki.mikrotik.com/wiki/Manual:IPv6

This is essential for MS Windows to configure IPv6 address of DNS server.

It is probably an experimental feature that may not work at all, seeing that there is no GUI support
for it but only commandline.

I think for now you need to use IPv4 DNS only. This does not mean you cannot use IPv6.

DHCPv6 relay is not for Windows DNS server assignment. It is something similar to an IPv6 DHCP relay.

However, if you need to distribute IPv6 DNS server addresses to the windows host you have configure these things:
1) For prefix you want to distribute to windows hosts (or default) set other-configuration=yes in the '/ipv6 nd' menu.
2) add dummy DHCP-PD server in the '/ipv6 dhcp-server' menu on the interface where you are announcing your prefix.
3) configure '/ip dns' servers attribute to hold IPv6 addresses


wiki entry for DHCPv6 relay will be fixed and the winbox support added.

Does that actually work? When the DHCP-v6 server will send DHCP server info to a client, will it send
its own IPv6 LAN address or will it send the IPv6 addresses from the /ip dns info? Can the Windows
client still get its address using SLAAC and then request a DNS server address using DHCPv6?
(as I think the DHCPv6 server cannot assign addresses from the local prefix)

I don't quite understand the configuration. My ISP provides DHCPv6-PD. I would like my WAN router to relay DHCPv6 request from LAN router. So how should I configure it on the WAN router? The DHCPv6 server address is link local

That is an entirely different topic. For DHCPv6-PD from your provider you use DHCPv6 client, not relay.
Just configure it, specifying the interface name of the link to your provider (PPPoE interface if it is used)
and enter some arbitrary name for the pool. The client will request the prefix and put it in the pool.
Under IPv6 addresses assign the internal interfaces (LAN) an address from this same pool.

That is an entirely different topic. For DHCPv6-PD from your provider you use DHCPv6 client, not relay.
Just configure it, specifying the interface name of the link to your provider (PPPoE interface if it is used)
and enter some arbitrary name for the pool. The client will request the prefix and put it in the pool.
Under IPv6 addresses assign the internal interfaces (LAN) an address from this same pool.

I have a downstream DHCPv6-PD client. RouterOS DHCPv6 relay has 0 documentation and does NOT work at all.

Probably in this case you would have to use a dhcpv6 client to get a pool from upstream and a dhcpv6 server to send prefixes downstream.
However, I have not tried it in practice.

How many of you have actually attempted use of the DHCPv6 relay? It successfully hands out prefixes from the server, but doesn't populate a route for those prefixes. Quite the headache. I can statically route the prefix from the server down to the relay, but if I try to use any routing protocol whatsoever, there ends up being a routing loop and an unreachable prefix.

I`m trying to setup IPv6 DHCP Relay but it looks like it doen`t do anything. The wiki is still missing so i don`t know which values to configure

It is an unusual setup and it probably doesn't even work.
The common method is to use IPv6 DHCP client to get a prefix from the ISP (DHCPv6-PD) and put that in a pool,
then configure your interfaces with addresses "from pool" and set the Advertise option.
Also configure the IP DNS to have the proper IPv6 DNS servers.
The clients will then use IPv6 stateless autoconfiguration to get their IPv6 address, the default gw, and the DNS servers.
DHCPv6 is not used by the clients in this case.

wiki page for /ipv6 dhcp-relay still missing.