xapi/nm: Send non-empty dns to networkd when using IPv6 autoconf

[psafont]

Because Autoconf is not DHCP, networkd uses the dns value to write to
resolv.conf. This is done on ocaml/networkd/bin/network_server.ml line 745
This allows to have non-empty resolv.conf when using IPv6 autoconf.

This mismatch was caused byt technical debt that cause this check to be duplicated, the second removes it so
xapi decides the instances where these values are written and networkd follows that decision.

I've tested new isntallations using IPv4 and couldn't see any regression, two users have tested the patch and now the DNS entries don't get overriden when using IPv6 in Autoconf mode: xcp-ng/xcp#641 (comment)

I'd like Rob to review this approach in any case

[minglumlu]

From the discussion, especially xcp-ng/xcp#641 (comment)
I understand we still need to handle a case of SLAAC + RDNSS. The networkd will need to capture the DNS server addresses from the Router Advertisements messages and write them into /etc/resolv.conf file.

[psafont]

I understand we still need to handle a case of SLAAC + RDNSS. The networkd will need to capture the DNS server addresses from the Router Advertisements messages and write them into /etc/resolv.conf file.

Yes, this patch fixes the case where DNS is configured using IPv4 mechanisms, but not written to /etc/resolv.conf because IPv6 is configured in Autoconf mode.

I'm working on a design to make configuration of IP and DNS independent from each other under IPv6 mode, but that will take longer to get done. In any case RDNSS needs to be implemented independently

[minglumlu]

I'd like Rob to review this approach in any case

Rob is in vacation. He will be back in 2 weeks.

but not written to /etc/resolv.conf because IPv6 is configured in Autoconf mode

Without these DNS server addresses, would the applications not be able to find DNS server to resolve their DNS name? XS doesn't have resolved running in dom0? Or I'm missing something. Anyway this could be resolved in your design.

[psafont]

Without these DNS server addresses, would the applications not be able to find DNS server to resolve their DNS name? XS doesn't have resolved running in dom0? Or I'm missing something. Anyway this could be resolved in your design.

The glibc function gethostbynameuses the resolv.conf file to know what IP to send the DNS requests to. So having the empty file means that some processes are unsable to send DNS requests if the file is empty.

This is an independent matter from the new design, the new design is meant to configure the IP addresses (yes, plural) of each PIF independently from its DNS configuration.

[0lini]

An example for something that doesn't work when DNS is empty, is yum update.