ULA Address is not assigned in DHCPv6 client mode

I have 2 LAN interfaces configured in LEDE 17.01.4.:
lan and lan6

lan is configured for a static IPv4 address and lan6 (@lan) is a DHCPv6 client.

In this configuration my main internet router assigns a DNSv6 server address which is an ULA-address. Unfortunately no interface got an ULA address assignment.

But if I configure an IPv6 assignment length of e.g. 64 to the lan interface (even with builtin IPv6 management disabled on that interface) an ULA-address is being assigned to that interface.

Is this the workaround to a bug that no ULA-address or route are being assigned to the lan6 interface?

Thanks for an explanation!

Why isn't the DNSv6 server's ULA address reachable using the global IPv6 address as source address? If the route to the ULA address worked there should be no need to assign a ULA address to the LEDE device.

Here is my (anonymized) IPv6 routing table:

Network Target Source Metric Table
lan ::/0 2003:b013:1305:9800:92f6:b1a0:beef:beef 640 main
lan 2003:b013:1305:9800::/56 2003:b013:1305:9800:92f6:b1a0:beef:beef 640 main
lan 2003:b013:1305:9800::/64 256 main
lan fd23:4567::/64 1024 main
(eth0) ff00::/8 256 local
DMZ ff00::/8 256 local
lan ff00::/8 256 local

When not configuring lan as mentioned it not only gets no IPv6 ULA address but the bold ULA-net routing entry is also missing which leads to the upstream router ULA-DNS address not being reachable (pingable).
Of course I could also configure a static IPv6 route but I think thats not how this is meant to be.
Or: Should the ULA-DNS be reachable through the default gateway? But why is it limited to this source address?

Have you got other global IPv6 addresses which can't use the default route?

No. Or at least not yet. I'm planning to set up an OpenVPN Connection to another site.

Why are you trying to route a LOCAL IPv6 addeess (the ULA)?

Because the upstream router advertises it's DNS-service via it's ULA-address.


OK...emphasis on the word LOCAL...you shouldn't route ULAs, and if so (or even possible), it's surely against best practice. Also, that router would need a route back to you, if it's your ISP, that definitely won't work.

I would advise using the ULA of your router's DNS instance, since it's local to the other DNS server. You should ensure that you've added the entry in LAN Interface or the system DNS to make it work. Placing it in the WAN DNS entries will not work in this case since you only use LAN.