Auto detect prefix length as well

Unfortunately NetworkInterface requires INTERNET permission. (this can actually be bypassed very easily but let's prefer usage of public API)

mobile/src/main/AndroidManifest.xml

@@ -4,6 +4,7 @@
 
     <uses-permission android:name="android.permission.ACCESS_WIFI_STATE"/>
     <uses-permission android:name="android.permission.CHANGE_WIFI_STATE"/>
+    <uses-permission android:name="android.permission.INTERNET"/>
 
     <application
         android:name=".App"

mobile/src/main/java/be/mygod/vpnhotspot/ArpCache.kt

@@ -1,21 +0,0 @@
-package be.mygod.vpnhotspot
-
-import java.io.File
-
-class ArpCache(downstream: String? = null) : HashMap<String, String>() {
-    companion object {
-        private val spaces = " +".toPattern()
-        private val mac = "^([0-9a-f]{2}:){5}[0-9a-f]{2}$".toPattern()
-    }
-
-    init {
-        File("/proc/net/arp").bufferedReader().useLines {
-            for (line in it) {
-                val parts = line.split(spaces)
-                // IP address       HW type     Flags       HW address            Mask     Device
-                if (parts.size >= 4 && (downstream == null || parts.getOrNull(5) == downstream) &&
-                        mac.matcher(parts[3]).matches()) put(parts[3], parts[0])
-            }
-        }
-    }
-}

mobile/src/main/java/be/mygod/vpnhotspot/HotspotService.kt

@@ -16,6 +16,7 @@ import android.support.v4.content.LocalBroadcastManager
 import android.util.Log
 import android.widget.Toast
 import be.mygod.vpnhotspot.App.Companion.app
+import java.net.NetworkInterface
 
 class HotspotService : Service(), WifiP2pManager.ChannelListener {
     companion object {
@@ -63,26 +64,7 @@ class HotspotService : Service(), WifiP2pManager.ChannelListener {
                 val info = intent.getParcelableExtra<WifiP2pInfo>(WifiP2pManager.EXTRA_WIFI_P2P_INFO)
                 val net = intent.getParcelableExtra<NetworkInfo>(WifiP2pManager.EXTRA_NETWORK_INFO)
                 val group = intent.getParcelableExtra<WifiP2pGroup>(WifiP2pManager.EXTRA_WIFI_P2P_GROUP)
-                val hostAddress = info.groupOwnerAddress?.hostAddress
+                if (downstream == null) onGroupCreated(info, group)
-                val downstream = group.`interface`
-                if (info.groupFormed && info.isGroupOwner &&
-                        downstream != null && hostAddress != null && this.downstream == null) {
-                    this.downstream = downstream
-                    this.hostAddress = hostAddress
-                    if (noisySu("echo 1 >/proc/sys/net/ipv4/ip_forward",
-                            "ip route add default dev $upstream scope link table 62",
-                            "ip route add $hostAddress/$subnetPrefixLength dev $downstream scope link table 62",
-                            "ip route add broadcast 255.255.255.255 dev $downstream scope link table 62",
-                            "ip rule add from $hostAddress/$subnetPrefixLength lookup 62",
-                            "iptables -N vpnhotspot_fwd",
-                            "iptables -A vpnhotspot_fwd -i $upstream -o $downstream -m state --state ESTABLISHED,RELATED -j ACCEPT",
-                            "iptables -A vpnhotspot_fwd -i $downstream -o $upstream -j ACCEPT",
-                            "iptables -I FORWARD -j vpnhotspot_fwd",
-                            "iptables -t nat -A PREROUTING -i $downstream -p tcp -d $hostAddress --dport 53 -j DNAT --to-destination $dns",
-                            "iptables -t nat -A PREROUTING -i $downstream -p udp -d $hostAddress --dport 53 -j DNAT --to-destination $dns")) {
-                        doStart(group)
-                    } else startFailure("Something went wrong, please check logcat.")
-                }
                 this@HotspotService.group = group
                 binder.data?.onGroupChanged()
                 showNotification(group)
@@ -99,7 +81,7 @@ class HotspotService : Service(), WifiP2pManager.ChannelListener {
      *  https://android.googlesource.com/platform/frameworks/base/+/android-4.0.1_r1/wifi/java/android/net/wifi/p2p/WifiP2pService.java#1028
      *  https://android.googlesource.com/platform/frameworks/opt/net/wifi/+/a8d5e40/service/java/com/android/server/wifi/p2p/WifiP2pServiceImpl.java#2547
      */
-    private val subnetPrefixLength get() = app.pref.getString("service.subnetPrefixLength", "24")
+    private var subnetPrefixLength: Short = 24
     private val dns get() = app.pref.getString("service.dns", "8.8.8.8:53")
 
     var status = Status.IDLE
@@ -183,6 +165,35 @@ class HotspotService : Service(), WifiP2pManager.ChannelListener {
                         Intent(this, MainActivity::class.java), PendingIntent.FLAG_UPDATE_CURRENT))
                 .build())
 
+    private fun onGroupCreated(info: WifiP2pInfo, group: WifiP2pGroup) {
+        val owner = info.groupOwnerAddress
+        val hostAddress = owner?.hostAddress
+        val downstream = group.`interface`
+        if (!info.groupFormed || !info.isGroupOwner || downstream == null || hostAddress == null) return
+        this.downstream = downstream
+        this.hostAddress = hostAddress
+        var subnetPrefixLength = NetworkInterface.getByName(downstream)?.interfaceAddresses
+                ?.singleOrNull { it.address == owner }?.networkPrefixLength
+        if (subnetPrefixLength == null) {
+            Log.w(TAG, "Unable to find prefix length of interface $downstream, 24 is assumed")
+            subnetPrefixLength = 24
+        }
+        this.subnetPrefixLength = subnetPrefixLength
+        if (noisySu("echo 1 >/proc/sys/net/ipv4/ip_forward",
+                "ip route add default dev $upstream scope link table 62",
+                "ip route add $hostAddress/$subnetPrefixLength dev $downstream scope link table 62",
+                "ip route add broadcast 255.255.255.255 dev $downstream scope link table 62",
+                "ip rule add from $hostAddress/$subnetPrefixLength lookup 62",
+                "iptables -N vpnhotspot_fwd",
+                "iptables -A vpnhotspot_fwd -i $upstream -o $downstream -m state --state ESTABLISHED,RELATED -j ACCEPT",
+                "iptables -A vpnhotspot_fwd -i $downstream -o $upstream -j ACCEPT",
+                "iptables -I FORWARD -j vpnhotspot_fwd",
+                "iptables -t nat -A PREROUTING -i $downstream -p tcp -d $hostAddress --dport 53 -j DNAT --to-destination $dns",
+                "iptables -t nat -A PREROUTING -i $downstream -p udp -d $hostAddress --dport 53 -j DNAT --to-destination $dns")) {
+            doStart(group)
+        } else startFailure("Something went wrong, please check logcat.")
+    }
+
     private fun clean() {
         if (receiverRegistered) {
             unregisterReceiver(receiver)

mobile/src/main/java/be/mygod/vpnhotspot/MainActivity.kt

@@ -66,14 +66,14 @@ class MainActivity : AppCompatActivity(), ServiceConnection, Toolbar.OnMenuItemC
     inner class ClientAdapter : RecyclerView.Adapter<ClientViewHolder>() {
         private var owner: WifiP2pDevice? = null
         private lateinit var clients: MutableCollection<WifiP2pDevice>
-        private lateinit var arpCache: ArpCache
+        private lateinit var arpCache: Map<String, String>
 
         fun fetchClients() {
             if (data.running) {
                 val binder = binder!!
                 owner = binder.service.group.owner
                 clients = binder.service.group.clientList
-                arpCache = ArpCache(binder.service.downstream)
+                arpCache = NetUtils.arp(binder.service.downstream)
             } else owner = null
             notifyDataSetChanged()  // recreate everything
         }

mobile/src/main/java/be/mygod/vpnhotspot/NetUtils.kt

@@ -0,0 +1,17 @@
+package be.mygod.vpnhotspot
+
+import java.io.File
+
+object NetUtils {
+    private val spaces = " +".toPattern()
+    private val mac = "^([0-9a-f]{2}:){5}[0-9a-f]{2}$".toPattern()
+
+    fun arp(iface: String? = null) = File("/proc/net/arp").bufferedReader().useLines {
+        // IP address       HW type     Flags       HW address            Mask     Device
+        it.map { it.split(spaces) }
+                .drop(1)
+                .filter { it.size >= 4 && (iface == null || it.getOrNull(5) == iface) &&
+                        mac.matcher(it[3]).matches() }
+                .associateBy({ it[3] }, { it[0] })
+    }
+}

mobile/src/main/res/xml/pref_settings.xml

@@ -8,13 +8,6 @@
             android:title="Upstream interface"
             android:summary="%s"
             android:defaultValue="tun0"/>
-        <AutoSummaryEditTextPreference
-            android:key="service.subnetPrefixLength"
-            android:title="Subnet prefix length"
-            android:summary="%s"
-            android:inputType="number"
-            android:maxLength="3"
-            android:defaultValue="24"/>
         <AutoSummaryEditTextPreference
             android:key="service.dns"
             android:title="Downstream DNS server:port"