Add DNS fallback system.

app/src/main/java/org/thoughtcrime/securesms/net/CustomDns.java

@@ -0,0 +1,62 @@
+package org.thoughtcrime.securesms.net;
+
+import androidx.annotation.NonNull;
+
+import com.annimon.stream.Stream;
+
+import org.xbill.DNS.ARecord;
+import org.xbill.DNS.Lookup;
+import org.xbill.DNS.Record;
+import org.xbill.DNS.Resolver;
+import org.xbill.DNS.SimpleResolver;
+import org.xbill.DNS.TextParseException;
+import org.xbill.DNS.Type;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.List;
+
+import okhttp3.Dns;
+
+/**
+ * A {@link Dns} implementation that specifies the hostname of a specific DNS.
+ */
+public class CustomDns implements Dns {
+
+  private final String dnsHostname;
+
+  public CustomDns(@NonNull String dnsHostname) {
+    this.dnsHostname = dnsHostname;
+  }
+
+  @Override
+  public @NonNull List<InetAddress> lookup(@NonNull String hostname) throws UnknownHostException {
+    Resolver resolver = new SimpleResolver(dnsHostname);
+    Lookup   lookup   = doLookup(hostname);
+
+    lookup.setResolver(resolver);
+
+    Record[] records = lookup.run();
+
+    if (records != null) {
+      List<InetAddress> ipv4Addresses = Stream.of(records)
+                                              .filter(r -> r.getType() == Type.A)
+                                              .map(r -> (ARecord) r)
+                                              .map(ARecord::getAddress)
+                                              .toList();
+      if (ipv4Addresses.size() > 0) {
+        return ipv4Addresses;
+      }
+    }
+
+    throw new UnknownHostException(hostname);
+  }
+
+  private static @NonNull Lookup doLookup(@NonNull String hostname) throws UnknownHostException {
+    try {
+      return new Lookup(hostname);
+    } catch (TextParseException e) {
+      throw new UnknownHostException();
+    }
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/net/SequentialDns.java

@@ -0,0 +1,45 @@
+package org.thoughtcrime.securesms.net;
+
+import androidx.annotation.NonNull;
+
+import org.thoughtcrime.securesms.logging.Log;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.Arrays;
+import java.util.List;
+import java.util.Locale;
+
+import okhttp3.Dns;
+
+/**
+ * Iterates through an ordered list of {@link Dns}, trying each one in sequence.
+ */
+public class SequentialDns implements Dns {
+
+  private static final String TAG = Log.tag(SequentialDns.class);
+
+  private List<Dns> dnsList;
+
+  public SequentialDns(Dns... dns) {
+    this.dnsList = Arrays.asList(dns);
+  }
+
+  @Override
+  public @NonNull List<InetAddress> lookup(@NonNull String hostname) throws UnknownHostException {
+    for (Dns dns : dnsList) {
+      try {
+        List<InetAddress> addresses = dns.lookup(hostname);
+        if (addresses.size() > 0) {
+          return addresses;
+        } else {
+          Log.w(TAG, String.format(Locale.ENGLISH, "Didn't find any addresses for %s using %s. Continuing.", hostname, dns.getClass().getSimpleName()));
+        }
+      } catch (UnknownHostException e) {
+        Log.w(TAG, String.format(Locale.ENGLISH, "Failed to resolve %s using %s. Continuing.", hostname, dns.getClass().getSimpleName()));
+      }
+    }
+    Log.w(TAG, "Failed to resolve using any DNS.");
+    throw new UnknownHostException(hostname);
+  }
+}

app/src/main/java/org/thoughtcrime/securesms/net/StaticDns.java

@@ -0,0 +1,35 @@
+package org.thoughtcrime.securesms.net;
+
+import androidx.annotation.NonNull;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.Collections;
+import java.util.List;
+import java.util.Map;
+
+import okhttp3.Dns;
+
+/**
+ * A super simple {@link Dns} implementation that maps hostnames to a static IP addresses.
+ */
+public class StaticDns implements Dns {
+
+  private final Map<String, String> hostnameMap;
+
+  public StaticDns(@NonNull Map<String, String> hostnameMap) {
+    this.hostnameMap = hostnameMap;
+  }
+
+  @Override
+  public @NonNull List<InetAddress> lookup(@NonNull String hostname) throws UnknownHostException {
+    String ip = hostnameMap.get(hostname);
+
+    if (ip != null) {
+      return Collections.singletonList(InetAddress.getByName(ip));
+    } else {
+      throw new UnknownHostException(hostname);
+    }
+
+  }
+}