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TUN_WRAPPER_FIX.md

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TUN Wrapper Fix - Exposing Subpackage Functions

Problem

The error showed:

Failed to start Go TUN bridge: mobile.Mobile.startTunBridge [long, long, class java.lang.String]

This means mobile.Mobile.startTunBridge() method didn't exist in the AAR.

Root Cause

Gomobile only exports functions from the main package, not subpackages.

mobile/
├── mobile.go          ← Gomobile exports functions from HERE
└── tun/
    └── tun_api.go     ← Functions here are NOT exported by gomobile

Even though mobile/tun/tun_api.go has exported functions like StartTunBridge(), gomobile doesn't automatically include them because they're in a subpackage.

Solution

Added wrapper functions in mobile/mobile.go that call the TUN subpackage functions:

import "github.com/kianmhz/GooseRelayVPN/mobile/tun"

// Wrapper functions
func StartTunBridge(tunFd int64, mtu int64, socksAddr string) error {
    return tun.StartTunBridge(int32(tunFd), int32(mtu), socksAddr)
}

func StopTunBridge() {
    tun.StopTunBridge()
}

func IsTunBridgeRunning() bool {
    return tun.IsTunBridgeRunning()
}

func GetTunBandwidth() (up int64, down int64) {
    return tun.GetTunBandwidth()
}

func GetDNSMapping(fakeIP string) string {
    return tun.GetDNSMapping(fakeIP)
}

func GetDNSMappingCount() int {
    return tun.GetDNSMappingCount()
}

func GetTunVersion() string {
    return tun.GetVersion()
}

How Gomobile Works

What gets exported:

  • ✅ Functions in mobile/mobile.go (main package)
  • ✅ Functions in mobile/*.go files (same package)

What doesn't get exported:

  • ❌ Functions in mobile/tun/*.go (subpackage)
  • ❌ Functions in mobile/subdir/*.go (any subpackage)

Solution: Add wrapper functions in the main package that call subpackage functions.

Architecture

┌─────────────────────────────────────────────────────────┐
│ Android App (Kotlin)                                    │
│                                                         │
│  mobile.Mobile.startTunBridge()                         │
│         │                                               │
│         ▼                                               │
│  ┌──────────────────────────────────────────┐          │
│  │ mobile/mobile.go (Wrapper)               │          │
│  │                                          │          │
│  │  func StartTunBridge(...) error {       │          │
│  │      return tun.StartTunBridge(...)     │──────┐   │
│  │  }                                       │      │   │
│  └──────────────────────────────────────────┘      │   │
│                                                    │   │
│  ┌──────────────────────────────────────────┐     │   │
│  │ mobile/tun/tun_api.go (Implementation)   │◄────┘   │
│  │                                          │          │
│  │  func StartTunBridge(...) error {       │          │
│  │      // Actual TUN bridge logic         │          │
│  │  }                                       │          │
│  └──────────────────────────────────────────┘          │
└─────────────────────────────────────────────────────────┘

Benefits of This Approach

  1. Clean separation - TUN code stays in mobile/tun/ subpackage
  2. Easy to maintain - Update mobile/tun/ without touching main code
  3. Gomobile compatible - Wrapper functions are in main package
  4. Type conversion - Wrapper handles int64 ↔ int32 conversion

Usage in Kotlin

No changes needed in Kotlin code - it already uses mobile.Mobile:

import mobile.Mobile

// These now work because wrappers exist in mobile.Mobile
Mobile.startTunBridge(fd.toLong(), 1500L, "127.0.0.1:1080")
Mobile.stopTunBridge()
Mobile.isTunBridgeRunning()
Mobile.getTunBandwidth()
Mobile.getDNSMapping("198.18.0.1")
Mobile.getTunVersion()

Testing

After rebuilding the AAR:

  1. Push changes to GitHub
  2. Wait for CI to build
  3. Download APK
  4. Enable "Fake DNS" and connect
  5. Check logs for:
    • Starting Go TUN bridge with DNS interception...
    • Go TUN bridge started (DNS will be resolved remotely)
    • [TUN-BRIDGE] Starting bridge

Why This Wasn't Needed Before

In my original approach, I tried to build mobile/tun/ as a separate AAR with its own gomobile bind command. That would have worked, but had the "no exported names" issue.

The current approach (single AAR) is simpler but requires these wrapper functions.

Summary

Problem: Gomobile doesn't export subpackage functions
Solution: Add wrapper functions in main package
Result: TUN functions now accessible from mobile.Mobile

This is the final piece needed to make the TUN bridge work!