neutron: document DNS resolution for OVN

[ideaship]

What

Adds a DNS resolution for instances (OVN) section to the Neutron configuration
guide, documenting how Neutron decides which DNS servers instances are handed
and how to set them on an OSISM deployment.

Why

The precedence itself is upstream-documented — subnet dns_nameservers, then
dns_servers in the [ovn] section, then the resolvers of the host running
neutron-server. The third level is the problem: on an OSISM deployment it is
not a useful fallback, and nothing in the guide says so.

osism.commons.resolvconf points /etc/resolv.conf at the systemd-resolved
stub (127.0.0.53) and configures the real upstream resolvers in
resolved.conf instead. Neutron does not filter loopback addresses out of that
file, and the containers run in the host network namespace, so 127.0.0.53 is
what gets advertised to instances — where it resolves nothing. Instances on a
subnet without dns_nameservers therefore end up with no working DNS at all,
including internal name resolution.

How

• States the three-level precedence and links to the upstream admin guide.
• Names per-subnet dns_nameservers as the recommended approach.
• Explains why the host-resolvers level does not work on OSISM, and gives the
  deployment-wide fix: [ovn] dns_servers via
  environments/kolla/files/overlays/neutron/ml2_conf.ini.
• Notes that neutron_dnsmasq_dns_servers, which defaults to public resolvers,
  applies only to the ML2/OVS DHCP agent and has no effect under OVN.

Markdown-only addition to an existing page; yarn build has not been run
locally.

Related

Independent of, but found in the same investigation as:

• openstack: warn that overlays match by exact name #1042

🤖 Generated with Claude Code

[github-actions]

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Detailed Issues

⚠️ SPELL / lychee - 1 error

📝 Summary
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🔍 Total..........881
🔗 Unique.........714
✅ Successful.....834
⏳ Timeouts.........0
🔀 Redirected.......3
👻 Excluded........46
❓ Unknown..........0
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Errors in docs/concepts/index.md
[ERROR] file://docs/concepts/metalbox#sonic-ztp (at 70:1) | File not found. Check if file exists and path is correct

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