[systemd-devel] systemd-resolved: Missing(?) DNS server info from systemd-networkd

[javud]

Hi there,

currently I try to debug an resolved-networkd issues, which is hard to 
detect for myself.

Some technical facts:
- systemd 247 (247.3-7)
- 5.10.0-12-amd64 #1 SMP Debian 5.10.103-1 (2022-03-07) x86_64 GNU/Linux
   within a LXC container

Description: I have setup an IPv6 only network with DHCPv6 to exchange 
IP/Hostname relations. When the container boots up, it queries for an 
IPv6 address via DHCPv6 (Rapid commit)...
> # cat /etc/systemd/network/eth0.network 
> [Match]
> Name=eth0
> [Network]
> DHCP=ipv6
This works as expected (checked with wireshark). After the DHCPv6 
response, `networkctl` shows me the received DNS server...
> # networkctl status eth0 | grep DNS 
>                            DNS: fdxy:abcd:abcd:abcd::1

but `resolved` did not get this DNS server...
> # resolvectl status eth0
> Link 2 (eth0)
> Current Scopes: none
>      Protocols: -DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported

and therefore `/etc/resolv.conf` is "empty". If I restart 
`systemd-resolved` (`# systemctl restart systemd-resolved`) everything 
works as expected, afterwards.
> # resolvectl status eth0
> Link 2 (eth0)
> Current Scopes: DNS
>      Protocols: +DefaultRoute +LLMNR -mDNS -DNSOverTLS DNSSEC=no/unsupported
>    DNS Servers: fdxy:abcd:abcd:abcd::1

During some debugging, resolved-networkd has worked properly, if I 
edited the systemd-service units, adding 
'Environment=SYSTEMD_LOG_LEVEL=debug'. Every time restarting the 
container, the DHCPv6-DNS nameserver exchange worked. When I removed all 
additional systemd-unit edits, it 'failed' to set the DNS nameserver again.

My first guess is, that resolved misses somehow the networkd DNS 
nameserver information. If I wait a second starting resolved (adding 
'ExecStartPre=/bin/sleep 1' to the systemd-resolved-unit) it works as 
expected.

Does someone has an idea, how I can debug this a little bit more and 
maybe even fix it, without some strange workarounds like enabling debug 
logging mode or slow down the startup process of resolved?

thx a lot, javud

some further information:
My system will not have an separate IPv6 route set, after the successful 
DHCP response, but this should not matter anyway, cause the DNS server 
adresse is reachable via the default route.
> # ip -6 r
> fe80::/64 dev eth0 proto kernel metric 256 pref medium
> default via fe80::abcd:abff:fecd:abcd dev eth0 proto ra metric 1024 expires 1599sec mtu 1500 pref medium