Handle offlink default gateways in GNS

[Ben Hillis (benhillis)]

Summary

WSLC session startup can fail with E_UNEXPECTED when a VPN exposes its interface as a /32 and provides a non-link-local gateway outside that prefix. Mark host-supplied default gateways as on-link so the Linux kernel accepts the route through the specified interface.

Fixes #41082

Root cause

The issue trace shows the guest receiving 100.96.5.160/32 followed by default via 100.96.5.161. The kernel rejects the default route with ENETUNREACH because the gateway is outside the interface prefix. The earlier fix in #41003 only covered link-local gateways, while Cloudflare WARP and Tailscale use non-link-local gateways.

Testing

• Full x64 Debug build
• NetworkTests::AddDefaultRouteWithOfflinkGateway
• NetworkTests::RemoveAndAddDefaultRoute

[Copilot]

Pull request overview

This PR addresses a WSLC startup failure (E_UNEXPECTED) caused by Linux rejecting a host-supplied default route when the guest interface is configured as a /32 and the provided gateway is outside that prefix (common with certain VPNs). The fix ensures host-provided default gateways are treated as on-link so the kernel accepts the route through the specified interface.

Changes:

• Mark default routes with an explicit gateway as RTNH_F_ONLINK in the netlink route message so Linux accepts “off-prefix” gateways.
• Add a Windows networking test case that reproduces the /32 + offlink gateway scenario (e.g., 100.96.5.160/32 with default via 100.96.5.161).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/linux/netlinkutil/RoutingTable.cpp	Extends RTNH_F_ONLINK behavior to cover host-supplied default gateways (not just link-local gateways).
test/windows/NetworkTests.cpp	Adds a regression test for adding a default route via an off-prefix (offlink) gateway with a /32 interface address.