feat(otbr): Add option to override backbone interface

[t3hk0d3]

This PR adds option to override interface used for backbone network.

Currently it always uses primary interface, and that's really inconvenient with multi-VLAN setup.

For example i prefer to have my IoT devices to be isolated in a separate network with strict firewall policies enforced on router.

Summary by CodeRabbit

• New Features
  • Added a configurable backbone network interface option with automatic multi-stage fallback (configured → supervisor primary → first available system interface).
• Documentation
  • Added a translation entry describing the new backbone interface setting and behavior.
• Bug Fixes / Reliability
  • Improved logging and error handling during interface resolution.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds an optional backbone_interface configuration field, updates runtime resolution to prefer that value with supervisor and system fallbacks for selecting the interface, and adds a corresponding translation entry.

Changes

Cohort / File(s)	Summary
Configuration Schema openthread_border_router/config.yaml	Added optional backbone_interface: str? to the public configuration schema.
Runtime Logic / Init Script openthread_border_router/rootfs/etc/s6-overlay/s6-rc.d/otbr-agent/run	Implemented multi-stage interface resolution: use config backbone_interface; if empty, query supervisor for primary interface; if supervisor unavailable or empty, choose first enabled interface from ip -br link show up; exit with error if none found. Added logging for chosen fallbacks.
Translations openthread_border_router/translations/en.yaml	Added configuration.backbone_interface entry with name and multi-line description explaining the interface setting and fallback behavior.

Sequence Diagram(s)

sequenceDiagram
    participant Agent as otbr-agent
    participant Config as Configuration
    participant Supervisor as Supervisor API
    participant System as System

    Agent->>Config: Read backbone_interface
    alt configured
        Config-->>Agent: Return configured interface
        Agent->>Agent: Use configured interface
    else not configured
        Agent->>Supervisor: Query primary interface
        alt supervisor responds
            Supervisor-->>Agent: Return primary interface
            Agent->>Agent: Log and use primary interface
        else supervisor unavailable
            Agent->>System: Run `ip -br link show up`
            alt enabled interfaces found
                System-->>Agent: Return first enabled interface
                Agent->>Agent: Log and use fallback interface
            else none found
                Agent->>Agent: Exit with error
            end
        end
    end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main feature being added: the ability to configure which network interface is used for the backbone, matching the core objective of making the backbone interface configurable.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@openthread_border_router/rootfs/etc/s6-overlay/s6-rc.d/otbr-agent/run`:
- Around line 81-88: The fallback selection for backbone_if in the run script
incorrectly assumes loopback is always first (it uses awk 'NR==2'); update the
block that sets backbone_if to pick the first non-loopback interface instead
(e.g. pipe ip -br link show up through a filter to exclude "lo" and take the
first match) so backbone_if="$(ip -br link show up | awk '$1 != "lo" {print $1;
exit}')" (or equivalent grep -v/awk combo); change the code that assigns
backbone_if and keep the existing error and log handling (references:
backbone_if variable and the fallback block in the run script).

In `@openthread_border_router/translations/en.yaml`:
- Around line 13-17: The description for backbone_interface is awkwardly
phrased; update the sentence "If not specified it would fallback to the primary
interface." to present tense with a comma and correct spacing: change it to "If
not specified, it falls back to the primary interface." Ensure the
backbone_interface.description entry is edited accordingly (use "falls back" as
two words and insert the comma after "specified").

🧹 Nitpick comments (1)

openthread_border_router/rootfs/etc/s6-overlay/s6-rc.d/otbr-agent/run (1)

26-79: Verify optional backbone_interface handling when unset.
If bashio::config returns null for missing keys, Line 26 sets backbone_if to null, so Line 76 won’t trigger fallback and otbr-agent will be passed an invalid interface. Guard with bashio::config.has_value and quote the -z check.

🛠️ Suggested update

-backbone_if=$(bashio::config 'backbone_interface')
+backbone_if=""
+if bashio::config.has_value 'backbone_interface'; then
+    backbone_if="$(bashio::config 'backbone_interface')"
+fi
@@
-if [ -z ${backbone_if} ]; then
+if [ -z "${backbone_if}" ]; then

[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@openthread_border_router/rootfs/etc/s6-overlay/s6-rc.d/otbr-agent/run`:
- Around line 76-79: Quote the variable test and only log the "using primary
interface" message when the supervisor API actually returned a value: change the
conditional to test [ -z "${backbone_if}" ] and call bashio::api.supervisor to
populate backbone_if before emitting a warning; if backbone_if is non-empty log
the existing warning via bashio::log.warning referencing the resolved
backbone_if, otherwise log a different error/warning that the supervisor
returned no interface. Ensure references to backbone_if, bashio::api.supervisor
and bashio::log.warning are updated accordingly.

[GollyJer]

would love to have this ability.

[t3hk0d3]

@agners Do you think a more sophisticated fallback for default network interface would be necessary? Previously it was just hardcoded "eth0" (that no longer in use with modern systemd/Linux kernels).

Current behaviour is to pick first interface if primary interface is not available from Supervisor API.
Maybe more correct behaviour would to give an fatal error instead?