fix(android): recover default-interface index via Os.if_nametoindex

[garmr-ulfr]

Problem

On Android, every sing-box outbound (direct, the auto-selector, and proxy outbounds) and DNS failed with no available network interface, while the UI still reported the VPN as Connected. Observed on Android 10 / API 29 (Samsung SM-G960U1); the condition persisted for the whole session with zero successful connections, and the auto-selector churned through servers marking each hard_demoted — a consequence, not the cause.

Root cause

DefaultNetworkMonitor.checkDefaultInterfaceUpdate is the only path that delivers the default network interface to sing-box (via updateDefaultInterface). It resolved the interface index solely through NetworkInterface.getByName(name).index, which relies on the interface-enumeration syscall. That syscall is restricted on some Android versions/ROMs, where getByName returns null or throws. On failure the 10-retry loop exhausted and returned without ever calling updateDefaultInterface, leaving sing-box with no default interface — so every outbound failed to bind.

The sibling PlatformInterfaceWrapper.getInterfaces() was already hardened for this exact case with an Os.if_nametoindex fallback; checkDefaultInterfaceUpdate was not.

Fix

Fall back to Os.if_nametoindex(interfaceName) when getByName cannot resolve the index, mirroring the hardened sibling, and retry only when the index is still unresolvable (<= 0). Also early-return on a null interface name instead of burning the retry budget on getByName(null).

Testing

Needs a build + on-device check on an API 29 device to confirm.

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Pull request overview

This PR addresses an Android-specific failure mode where sing-box never receives a default network interface index, causing all outbounds/DNS to fail with “no available network interface” even while the VPN appears connected. It hardens DefaultNetworkMonitor.checkDefaultInterfaceUpdate to recover the interface index on restricted Android ROMs by falling back to Os.if_nametoindex.

Changes:

• Add Os.if_nametoindex(interfaceName) fallback when NetworkInterface.getByName cannot resolve an interface index.
• Avoid retrying when the interface name is null by early-returning before entering the retry loop.
• Retry only when the resolved index is still not usable (<= 0).

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