Native use system DNS

Fixes #246

app/src/main/java/eu/faircode/netguard/SinkholeService.java

@@ -667,9 +667,10 @@ public class SinkholeService extends VpnService implements SharedPreferences.OnS
 
         if (filter) {
             // TODO multiple DNS servers
-            String dns = prefs.getString("dns", Util.getDefaultDNS(SinkholeService.this));
-            Log.i(TAG, "DNS " + dns);
-            builder.addDnsServer(dns);
+            String sysDns = Util.getDefaultDNS(SinkholeService.this);
+            String vpnDns = prefs.getString("dns", sysDns);
+            Log.i(TAG, "DNS system=" + sysDns + " VPN=" + vpnDns);
+            builder.addDnsServer(vpnDns);
         }
 
         if (tethering) {

app/src/main/java/eu/faircode/netguard/Util.java

@@ -66,6 +66,12 @@ public class Util {
     private static final int NETWORK_TYPE_IWLAN = 18;
     private static final String TAG = "NetGuard.Util";
 
+    private static native String jni_getprop(String name);
+
+    static {
+        System.loadLibrary("netguard");
+    }
+
     public static String getSelfVersionName(Context context) {
         try {
             PackageInfo pInfo = context.getPackageManager().getPackageInfo(context.getPackageName(), 0);
@@ -282,8 +288,7 @@ public class Util {
     }
 
     public static String getDefaultDNS(Context context) {
-        return "8.8.8.8";
-        //return "2001:4860:4860::8888";
+        return jni_getprop("net.dns1");
     }
 
     public static boolean isInteractive(Context context) {

app/src/main/jni/netguard/netguard.c

@@ -26,6 +26,7 @@
 #include <time.h>
 #include <unistd.h>
 #include <pthread.h>
+#include <dlfcn.h>
 
 #include <arpa/inet.h>
 #include <netinet/ip.h>
@@ -247,6 +248,18 @@ Java_eu_faircode_netguard_SinkholeService_jni_1pcap(JNIEnv *env, jclass type,
         log_android(ANDROID_LOG_ERROR, "pthread_mutex_unlock failed");
 }
 
+JNIEXPORT jstring JNICALL
+Java_eu_faircode_netguard_Util_jni_1getprop(JNIEnv *env, jclass type, jstring name_) {
+    const char *name = (*env)->GetStringUTFChars(env, name_, 0);
+
+    char value[250];
+    __system_property_get(name, value);
+
+    (*env)->ReleaseStringUTFChars(env, name_, name);
+
+    return (*env)->NewStringUTF(env, value);
+}
+
 // Private functions
 
 void check_allowed(const struct arguments *args) {
@@ -2155,6 +2168,23 @@ int jniCheckException(JNIEnv *env) {
     return 0;
 }
 
+typedef int (*PFN_SYS_PROP_GET)(const char *, char *);
+
+int __system_property_get(const char *name, char *value) {
+    static PFN_SYS_PROP_GET __real_system_property_get = NULL;
+    if (!__real_system_property_get) {
+        void *handle = dlopen("libc.so", RTLD_NOLOAD);
+        if (!handle)
+            log_android(ANDROID_LOG_ERROR, "dlopen(libc.so): %s", dlerror());
+        else {
+            __real_system_property_get = (PFN_SYS_PROP_GET) dlsym(handle, "__system_property_get");
+            if (!__real_system_property_get)
+                log_android(ANDROID_LOG_ERROR, "dlsym(__system_property_get()): %s", dlerror());
+        }
+    }
+    return (*__real_system_property_get)(name, value);
+}
+
 void log_android(int prio, const char *fmt, ...) {
     if (prio >= loglevel) {
         char line[1024];

app/src/main/jni/netguard/netguard.h

@@ -246,6 +246,8 @@ jobject jniNewObject(JNIEnv *env, jclass cls, jmethodID constructor, const char
 
 int jniCheckException(JNIEnv *env);
 
+int __system_property_get(const char *name, char *value);
+
 void log_android(int prio, const char *fmt, ...);
 
 void log_packet(const struct arguments *args,