networkd: learned DNS server not propagated to rdnss option in RA

[Michael Heimpold]

Dear all,

I'm trying to setup an embedded system as IPv6 router using IPv6
prefix delegation. The system has two network interfaces, eth0 and
eth1. eth0 is the upstream interface, but it is part of a bridge interface,
but I think this does not play an important role here.
The upstream interfaces acquires an IPv6 via RA and also learns
the DNS server from the upstream router. IPv6 and DNS resolving
works properly on the system.
(note: the upstream interface is dual-stack, IPv4 is also working as usual).

The goal is to pass routable IPv6 addesses down to a system connected
via eth1 ("client"). In my understanding, this is what IPv6 prefix delegation
is for and it is working so far, that the client acquires an IPv6 and I can
ping IPv6 addresses on the Internet etc.
However, when I look at the IPv6 RA sent out on eth1 interface, the rdnss 
option is missing, so that the client does not learn any DNS server over the
link.

Please find below my configuration files. My understanding of the
documentation is, that this should work out-of-the-box since all related
config options have reasonable defaults (EmitDNS, UseDNS...), so that
learned DNS server from upstream interfaces should propagate to the RAs
of the downstream interface. But this is not the case.
If I manually configure DNS=... option in the "IPv6SendRA" section, then a
rdnss field is part of the RA and then the client learns the DNS server and
I also have DNS on the client working. So this is not an issue on the
client itself.

On my system, I'm using systemd-networkd v250.5 from Yocto kirkstone,
but I also tested v254.4 with no difference in behavior.

My question is whether I have a wrong understanding or might this be a bug?
Any hints are appreciated.

Thanks,
Michael


--snip: /lib/systemd/network/br0.network--
[Match]
Name=br0

[Network]
DHCP=yes
IPv6AcceptRA=yes

[DHCPv4]
ClientIdentifier=mac
UseHostname=no

[DHCPv6]
UseHostname=no
--snap--

--snip: /etc/systemd/network/eth1.network--
[Match]
Name=eth1

[Network]
DHCP=no
IPv6AcceptRA=no
IPForward=ipv6
IPv6SendRA=yes
DHCPPrefixDelegation=yes
--snap--

--snip: output of resolvectl on the embedded system (shortened)--
...

Link 5 (br0)
    Current Scopes: DNS LLMNR/IPv4 LLMNR/IPv6
         Protocols: +DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/
unsupported
       DNS Servers: 192.168.8.254 fd00::c614:6dff:fed6:c2e9
--snap--