network: detect default ipv6 interface when no ipv4 interface

src/metrics.js

@@ -14,6 +14,7 @@ const apps = require('./apps.js'),
     docker = require('./docker.js'),
     fs = require('node:fs'),
     net = require('node:net'),
+    network = require('./network.js'),
     os = require('node:os'),
     { Readable } = require('node:stream'),
     safe = require('safetydance'),
@@ -141,18 +142,7 @@ async function readDiskMetrics() {
 }
 
 async function readNetworkMetrics() {
-    const contents = await fs.promises.readFile('/proc/net/route', { encoding: 'utf8' });
-    const lines = contents.trim().split('\n').slice(1); // skip header
-
-    let defaultIface = null;
-    for (const line of lines) {
-        const [iface, destination] = line.split(/\s+/);
-        if (destination === '00000000') {
-            defaultIface = iface; // default route
-            break;
-        }
-    }
-    if (!defaultIface) throw new BoxError(BoxError.EXTERNAL_ERROR, 'Could not detect default interface');
+    const defaultIface = await network.getDefaultInterface();
 
     const [rx, tx] = await Promise.all([
         fs.promises.readFile(`/sys/class/net/${defaultIface}/statistics/rx_bytes`, { encoding: 'utf8' }),