Add option to disable IPv6 tethering

Partially addresses #6, #20.

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

@@ -49,6 +49,7 @@ class TetheringService : IpNeighbourMonitoringService(), VpnMonitor.Callback {
                     // system tethering already has working forwarding rules
                     // so it doesn't make sense to add additional forwarding rules
                     val routing = Routing(upstream, downstream).rule().forward().masquerade().dnsRedirect(dns)
+                    if (app.pref.getBoolean("service.disableIpv6", false)) routing.disableIpv6()
                     routings[downstream] = routing
                     if (!routing.start()) failed = true
                 } catch (e: SocketException) {

mobile/src/main/java/be/mygod/vpnhotspot/net/Routing.kt

@@ -47,6 +47,12 @@ class Routing(val upstream: String?, private val downstream: String, ownerAddres
         return this
     }
 
+    fun disableIpv6(): Routing {
+        startScript.add("echo 1 >/proc/sys/net/ipv6/conf/$downstream/disable_ipv6")
+        stopScript.add("echo 0 >/proc/sys/net/ipv6/conf/$downstream/disable_ipv6")
+        return this
+    }
+
     /**
      * Since Android 5.0, RULE_PRIORITY_TETHERING = 18000.
      * This also works for Wi-Fi direct where there's no rule at 18000.

mobile/src/main/res/values-zh-rCN/strings.xml

@@ -60,6 +60,8 @@
     <string name="settings_service_repeater_oc_summary">"自动 (1\u201114 = 2.4GHz, 15\u2011165 = 5GHz)"</string>
     <string name="settings_service_repeater_strict">严格模式</string>
     <string name="settings_service_repeater_strict_summary">只允许通过 VPN 隧道的包通过，不适用于系统共享</string>
+    <string name="settings_service_disable_ipv6">禁用 IPv6 共享</string>
+    <string name="settings_service_disable_ipv6_summary">防止 IPv6 VPN 泄漏。</string>
     <string name="settings_service_dns">备用 DNS 服务器[:端口]</string>
     <string name="settings_service_clean">清理/重新应用路由规则</string>
     <string name="settings_misc">杂项</string>

mobile/src/main/res/values/strings.xml

@@ -64,6 +64,8 @@
     <string name="settings_service_repeater_strict">Strict mode</string>
     <string name="settings_service_repeater_strict_summary">Only allow packets that goes through VPN tunnel. Does not
         apply to system tethering.</string>
+    <string name="settings_service_disable_ipv6">Disable IPv6 tethering</string>
+    <string name="settings_service_disable_ipv6_summary">Enabling this option will prevent VPN leaks via IPv6.</string>
     <string name="settings_service_dns">Fallback DNS server[:port]</string>
     <string name="settings_service_clean">Clean/reapply routing rules</string>
     <string name="settings_misc">Misc</string>

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

@@ -6,6 +6,10 @@
             android:key="service.repeater.strict"
             android:title="@string/settings_service_repeater_strict"
             android:summary="@string/settings_service_repeater_strict_summary"/>
+        <SwitchPreference
+            android:key="service.disableIpv6"
+            android:title="@string/settings_service_disable_ipv6"
+            android:summary="@string/settings_service_disable_ipv6_summary"/>
         <AutoSummaryEditTextPreference
             android:key="service.dns"
             android:title="@string/settings_service_dns"