network: detect default ipv6 interface when no ipv4 interface

src/network.js

@@ -19,7 +19,9 @@ exports = module.exports = {
     getIPv4,
     hasIPv6,
     getIPv6,
-    detectIP
+    detectIP,
+
+    getDefaultInterface
 };
 
 const assert = require('node:assert'),
@@ -36,6 +38,8 @@ const assert = require('node:assert'),
 
 const SET_BLOCKLIST_CMD = path.join(__dirname, 'scripts/setblocklist.sh');
 
+let gDefaultIface = null; // cache
+
 function api(provider) {
     assert.strictEqual(typeof provider, 'string');
 
@@ -171,3 +175,31 @@ async function detectIP() {
         ipv6: error6 ? null : ipv6
     };
 }
+
+async function getDefaultInterface() {
+    if (gDefaultIface) return gDefaultIface;
+
+    const contents4 = await fs.promises.readFile('/proc/net/route', { encoding: 'utf8' });
+    const lines4 = contents4.trim().split('\n').slice(1); // skip header
+
+    for (const line of lines4) {
+        const cols = line.trim().split(/\s+/); // Iface, dest, gw, flags, refcount, use, metric, mask, mtu, window, irtt
+        if (cols[1] === '00000000') { // && cols[7] === '00000000'
+            gDefaultIface = cols[0];
+            return gDefaultIface;
+        }
+    }
+
+    const contents6 = await fs.promises.readFile('/proc/net/ipv6_route', { encoding: 'utf8' });
+    const lines6 = contents6.trim().split('\n'); // no header!
+
+    for (const line of lines6) {
+        const cols = line.trim().split(/\s+/); // dest, dest_prefix_len, src, src_prefix_len, next_hop, metric, refcount, use, flags, iface
+        if (cols[0] === '00000000000000000000000000000000' && cols[1] === '00') {
+            gDefaultIface = cols.at(-1);
+            return gDefaultIface;
+        }
+    }
+
+    throw new BoxError(BoxError.EXTERNAL_ERROR, 'Could not detect default interface');
+}