Add fallback static DNS resolver.

buildSrc/src/main/java/org/signal/StaticIpResolver.java

@@ -0,0 +1,63 @@
+package org.signal;
+
+import org.gradle.internal.impldep.org.eclipse.jgit.annotations.NonNull;
+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.Type;
+
+import java.net.InetAddress;
+import java.net.UnknownHostException;
+import java.util.Arrays;
+
+public final class StaticIpResolver {
+
+  private StaticIpResolver() {}
+
+  public static String resolveToBuildConfig(String hostName) {
+    String[] ips = resolve(hostName);
+    StringBuilder builder = new StringBuilder();
+    builder.append("new String[]{");
+    for (int i = 0; i < ips.length; i++) {
+      builder.append("\"").append(ips[i]).append("\"");
+      if (i < ips.length - 1) {
+        builder.append(",");
+      }
+    }
+    return builder.append("}").toString();
+  }
+
+  private static String[] resolve(String hostName) {
+    try {
+      Resolver resolver = new SimpleResolver("1.1.1.1");
+      Lookup   lookup   = doLookup(hostName);
+
+      lookup.setResolver(resolver);
+
+      Record[] records = lookup.run();
+
+      if (records != null) {
+        return Arrays.stream(records)
+                     .filter(r -> r.getType() == Type.A)
+                     .map(r -> (ARecord) r)
+                     .map(ARecord::getAddress)
+                     .map(InetAddress::getHostAddress)
+                     .toArray(String[]::new);
+      } else {
+        throw new IllegalStateException("Failed to resolve host! " + hostName);
+      }
+    } catch (UnknownHostException e) {
+      throw new IllegalStateException("Failed to resolve host! " + hostName);
+    }
+  }
+
+  private static @NonNull Lookup doLookup(@NonNull String hostname) throws UnknownHostException {
+    try {
+      return new Lookup(hostname);
+    } catch (Throwable e) {
+      throw new UnknownHostException();
+    }
+  }
+}