Added setting to prefer IPv4 addresses

app/src/main/java/eu/faircode/email/ApplicationEx.java

@@ -103,7 +103,7 @@ public class ApplicationEx extends Application {
         if (Helper.hasWebView(this))
             CookieManager.getInstance().setAcceptCookie(false);
 
-        MessageHelper.setSystemProperties();
+        MessageHelper.setSystemProperties(this);
         ContactInfo.init(this);
 
         try {

app/src/main/java/eu/faircode/email/FragmentOptionsMisc.java

@@ -55,6 +55,7 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
     private SwitchCompat swDoubleBack;
     private SwitchCompat swEnglish;
     private SwitchCompat swWatchdog;
+    private SwitchCompat swPreferIp4;
     private SwitchCompat swUpdates;
     private SwitchCompat swCrashReports;
     private SwitchCompat swDebug;
@@ -68,7 +69,7 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
     private Group grpDebug;
 
     private final static String[] RESET_OPTIONS = new String[]{
-            "badge", "subscriptions", "subscribed_only", "biometrics_timeout", "double_back", "english", "watchdog", "updates", "crash_reports", "debug"
+            "badge", "subscriptions", "subscribed_only", "biometrics_timeout", "double_back", "english", "watchdog", "prefer_ip4", "updates", "crash_reports", "debug"
     };
 
     private final static String[] RESET_QUESTIONS = new String[]{
@@ -93,6 +94,7 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
         swDoubleBack = view.findViewById(R.id.swDoubleBack);
         swEnglish = view.findViewById(R.id.swEnglish);
         swWatchdog = view.findViewById(R.id.swWatchdog);
+        swPreferIp4 = view.findViewById(R.id.swPreferIp4);
         swUpdates = view.findViewById(R.id.swUpdates);
         swCrashReports = view.findViewById(R.id.swCrashReports);
         swDebug = view.findViewById(R.id.swDebug);
@@ -172,6 +174,15 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
             }
         });
 
+        swPreferIp4.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
+            @Override
+            public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
+                prefs.edit().putBoolean("prefer_ip4", checked).apply();
+                System.setProperty("java.net.preferIPv4Stack", Boolean.toString(checked));
+                ServiceSynchronize.reload(getContext(), "prefer_ip4=" + checked);
+            }
+        });
+
         swUpdates.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
             @Override
             public void onCheckedChanged(CompoundButton compoundButton, boolean checked) {
@@ -310,6 +321,7 @@ public class FragmentOptionsMisc extends FragmentBase implements SharedPreferenc
         swDoubleBack.setChecked(prefs.getBoolean("double_back", true));
         swEnglish.setChecked(prefs.getBoolean("english", false));
         swWatchdog.setChecked(prefs.getBoolean("watchdog", true));
+        swPreferIp4.setChecked(prefs.getBoolean("prefer_ip4", false));
         swUpdates.setChecked(prefs.getBoolean("updates", true));
         swUpdates.setVisibility(Helper.isPlayStoreInstall(getContext()) ? View.GONE : View.VISIBLE);
         swCrashReports.setChecked(prefs.getBoolean("crash_reports", false));

app/src/main/java/eu/faircode/email/MessageHelper.java

@@ -20,10 +20,13 @@ package eu.faircode.email;
 */
 
 import android.content.Context;
+import android.content.SharedPreferences;
 import android.net.MailTo;
 import android.text.TextUtils;
 import android.webkit.MimeTypeMap;
 
+import androidx.preference.PreferenceManager;
+
 import com.sun.mail.util.FolderClosedIOException;
 import com.sun.mail.util.MessageRemovedIOException;
 
@@ -86,7 +89,7 @@ public class MessageHelper {
 
     static final int DEFAULT_ATTACHMENT_DOWNLOAD_SIZE = 256 * 1024; // bytes
 
-    static void setSystemProperties() {
+    static void setSystemProperties(Context context) {
         System.setProperty("mail.mime.decodetext.strict", "false");
 
         System.setProperty("mail.mime.ignoreunknownencoding", "true"); // Content-Transfer-Encoding
@@ -99,6 +102,12 @@ public class MessageHelper {
         // https://docs.oracle.com/javaee/6/api/javax/mail/internet/MimeMultipart.html
         System.setProperty("mail.mime.multipart.ignoremissingboundaryparameter", "true"); // javax.mail.internet.ParseException: In parameter list
         System.setProperty("mail.mime.multipart.ignoreexistingboundaryparameter", "true");
+
+        // https://docs.oracle.com/javase/7/docs/api/java/net/doc-files/net-properties.html
+        SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(context);
+        boolean prefer_ip4 = prefs.getBoolean("prefer_ip4", false);
+        Log.i("Prefer ip4=" + prefer_ip4);
+        System.setProperty("java.net.preferIPv4Stack", Boolean.toString(prefer_ip4));
     }
 
     static Properties getSessionProperties(String realm, boolean insecure) {
@@ -194,11 +203,6 @@ public class MessageHelper {
         props.put("mail.mime.allowutf8", "false"); // SMTPTransport, MimeMessage
         props.put("mail.mime.address.strict", "false");
 
-        if (false) {
-            Log.i("Prefering IPv4");
-            System.setProperty("java.net.preferIPv4Stack", "true");
-        }
-
         return props;
     }
 

app/src/main/res/layout/fragment_options_misc.xml

@@ -158,6 +158,29 @@
                 app:layout_constraintTop_toBottomOf="@id/tvEnglishHint"
                 app:switchPadding="12dp" />
 
+            <androidx.appcompat.widget.SwitchCompat
+                android:id="@+id/swPreferIp4"
+                android:layout_width="0dp"
+                android:layout_height="wrap_content"
+                android:layout_marginTop="12dp"
+                android:text="@string/title_advanced_prefer_ip4"
+                app:layout_constraintEnd_toEndOf="parent"
+                app:layout_constraintStart_toStartOf="parent"
+                app:layout_constraintTop_toBottomOf="@id/swWatchdog"
+                app:switchPadding="12dp" />
+
+            <TextView
+                android:id="@+id/tvPreferIp4Hint"
+                android:layout_width="0dp"
+                android:layout_height="wrap_content"
+                android:layout_marginEnd="48dp"
+                android:text="@string/title_advanced_prefer_ip4_hint"
+                android:textAppearance="@style/TextAppearance.AppCompat.Small"
+                android:textStyle="italic"
+                app:layout_constraintEnd_toEndOf="parent"
+                app:layout_constraintStart_toStartOf="parent"
+                app:layout_constraintTop_toBottomOf="@id/swPreferIp4" />
+
             <androidx.appcompat.widget.SwitchCompat
                 android:id="@+id/swUpdates"
                 android:layout_width="0dp"
@@ -167,7 +190,7 @@
                 android:text="@string/title_advanced_updates"
                 app:layout_constraintEnd_toEndOf="parent"
                 app:layout_constraintStart_toStartOf="parent"
-                app:layout_constraintTop_toBottomOf="@id/swWatchdog"
+                app:layout_constraintTop_toBottomOf="@id/tvPreferIp4Hint"
                 app:switchPadding="12dp" />
 
             <androidx.appcompat.widget.SwitchCompat

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

@@ -260,6 +260,7 @@
     <string name="title_advanced_double_back">Double \'back\' to exit</string>
     <string name="title_advanced_english">Force English language</string>
     <string name="title_advanced_watchdog">Periodically check if FairEmail is still active</string>
+    <string name="title_advanced_prefer_ip4">Prefer IPv4 addresses</string>
     <string name="title_advanced_updates">Check for updates</string>
     <string name="title_advanced_crash_reports">Send error reports</string>
     <string name="title_advanced_debug">Debug mode</string>
@@ -300,6 +301,7 @@
     <string name="title_advanced_badge_hint">Only available on supported launchers</string>
     <string name="title_advanced_subscribed_only_hint">Enabling this will delete all local folders without subscription</string>
     <string name="title_advanced_english_hint">This will restart the app</string>
+    <string name="title_advanced_prefer_ip4_hint">Enable this only if IPv6 connectivity does not work properly</string>
     <string name="title_advanced_debug_hint">Enable extra logging and show debug information at various places</string>
     <string name="title_advanced_cleanup_hint">This will delete all temporary files</string>