Fixes to route mirroring

[FetoiuCatalin]

Summary of the Pull Request

Testing mirrored mode with Proton VPN exposed some issues with route mirroring:

1. Adding routes using netlink messages did not account for default onlink routes (destination everything, no next hop), leading to default onlink routes for IPv6 to fail to be added. Proton VPN uses such a route on the host, which failed to be mirrored and the IPv6 traffic was not going over the VPN as expected
2. When mirroring routes from windows, the metric used in Linux is determined by adding windows interface metric and route metric. It's possible those are both 0 (high priority), leading to a Linux metric with route 0, but 0 is treated as unspecified and defaulted to 1024 instead. Fix this to change the metric 0 to 1 (which is highest priority metric in Linux)

Validation Steps Performed

Testing with ProtonVPN and confirming v6 traffic is correctly routed over the mirrored interface that corresponds to the VPN
Running all NetworkTests
Added new tests that verify adding default onlink routes works as expected for both IPv4 and IPv6

[Copilot]

Pull request overview

This PR addresses gaps in WSL mirrored networking route mirroring, specifically improving handling of default on-link routes (no next hop) and adjusting mirrored route metrics to better match Linux behavior.

Changes:

• Add a Windows NetworkTests coverage case for adding default on-link routes in both IPv4 and IPv6.
• Update mirrored route metric mapping to avoid emitting metric 0.
• Extend Linux netlink route programming to support default on-link routes by sending a default route without RTA_GATEWAY.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
test/windows/NetworkTests.cpp	Adds a new test validating default on-link routes for IPv4/IPv6.
src/windows/service/exe/WslMirroredNetworking.cpp	Adjusts route metric handling to remap metric 0 to 1.
src/linux/netlinkutil/RoutingTable.h	Declares a new helper for default on-link route netlink messages.
src/linux/netlinkutil/RoutingTable.cpp	Implements default on-link default-route netlink messaging and updates default-route logging.

[keith-horton]

Looks good. Thanks!

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated no new comments.

[FetoiuCatalin]

@microsoft-github-policy-service agree company="Microsoft"