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

[Andrei Borzenkov]

On Wed, Dec 13, 2023 at 10:46 AM Michael Heimpold <mhei at heimpold.de> wrote:
>
> 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--
>

Quoting documentation

--><--
DNS servers from the uplink interface specified in UplinkInterface= will be used
--><--

Try adding

UplinkInterface=br0