diff --git a/lib/BasicResolver.php b/lib/BasicResolver.php
index c88c998..7b5030a 100644
--- a/lib/BasicResolver.php
+++ b/lib/BasicResolver.php
@@ -14,7 +14,7 @@ use LibDNS\Records\Question;
 use LibDNS\Records\QuestionFactory;
 use function Amp\call;
 
-class BasicResolver implements Resolver {
+final class BasicResolver implements Resolver {
     const CACHE_PREFIX = "amphp.dns.";
 
     /** @var \Amp\Dns\ConfigLoader */
@@ -52,9 +52,10 @@ class BasicResolver implements Resolver {
         $this->gcWatcher = Loop::repeat(5000, function () {
             $now = \time();
 
-            foreach ($this->servers as $server) {
+            foreach ($this->servers as $key => $server) {
                 if ($server->getLastActivity() < $now - 60) {
-                    unset($this->servers);
+                    $server->close();
+                    unset($this->servers[$key]);
                 }
             }
         });
@@ -192,6 +193,7 @@ class BasicResolver implements Resolver {
                     $server = yield $this->getServer($protocol . "://" . $nameservers[$i]);
 
                     if (!$server->isAlive()) {
+                        $this->servers[$protocol . "://" . $nameservers[$i]]->close();
                         unset($this->servers[$protocol . "://" . $nameservers[$i]]);
 
                         /** @var \Amp\Dns\Server $server */
diff --git a/lib/Config.php b/lib/Config.php
index 9db9d25..dff02d7 100644
--- a/lib/Config.php
+++ b/lib/Config.php
@@ -2,7 +2,7 @@
 
 namespace Amp\Dns;
 
-class Config {
+final class Config {
     private $nameservers;
     private $knownHosts;
     private $timeout;
diff --git a/lib/Record.php b/lib/Record.php
index 768fa69..800090f 100644
--- a/lib/Record.php
+++ b/lib/Record.php
@@ -5,7 +5,7 @@ namespace Amp\Dns;
 use LibDNS\Records\ResourceQTypes;
 use LibDNS\Records\ResourceTypes;
 
-class Record {
+final class Record {
     const A = ResourceTypes::A;
     const AAAA = ResourceTypes::AAAA;
     const AFSDB = ResourceTypes::AFSDB;
diff --git a/lib/Server.php b/lib/Server.php
index 2076d22..c0a22d7 100644
--- a/lib/Server.php
+++ b/lib/Server.php
@@ -14,6 +14,7 @@ use LibDNS\Messages\MessageTypes;
 use LibDNS\Records\Question;
 use function Amp\call;
 
+/** @internal */
 abstract class Server {
     /** @var ResourceInputStream */
     private $input;
@@ -154,6 +155,18 @@ abstract class Server {
         });
     }
 
+    public function close() {
+        if ($this->input === null) {
+            return;
+        }
+
+        $this->input->close();
+        $this->output->close();
+
+        $this->input = null;
+        $this->output = null;
+    }
+
     protected function read(): Promise {
         return $this->input->read();
     }
diff --git a/lib/TcpServer.php b/lib/TcpServer.php
index 206b2c1..ad44a95 100644
--- a/lib/TcpServer.php
+++ b/lib/TcpServer.php
@@ -13,6 +13,7 @@ use LibDNS\Encoder\EncoderFactory;
 use LibDNS\Messages\Message;
 use function Amp\call;
 
+/** @internal */
 class TcpServer extends Server {
     /** @var \LibDNS\Encoder\Encoder */
     private $encoder;
diff --git a/lib/UdpServer.php b/lib/UdpServer.php
index 21d234d..4f8b61a 100644
--- a/lib/UdpServer.php
+++ b/lib/UdpServer.php
@@ -9,6 +9,7 @@ use LibDNS\Encoder\EncoderFactory;
 use LibDNS\Messages\Message;
 use function Amp\call;
 
+/** @internal */
 class UdpServer extends Server {
     /** @var \LibDNS\Encoder\Encoder */
     private $encoder;
diff --git a/lib/UnixConfigLoader.php b/lib/UnixConfigLoader.php
index 1c7af9d..bbf47b8 100644
--- a/lib/UnixConfigLoader.php
+++ b/lib/UnixConfigLoader.php
@@ -7,7 +7,7 @@ use Amp\File\FilesystemException;
 use Amp\Promise;
 use function Amp\call;
 
-class UnixConfigLoader implements ConfigLoader {
+final class UnixConfigLoader implements ConfigLoader {
     private $path;
     private $hostLoader;
 
diff --git a/lib/WindowsConfigLoader.php b/lib/WindowsConfigLoader.php
index 8c2e38b..113ad79 100644
--- a/lib/WindowsConfigLoader.php
+++ b/lib/WindowsConfigLoader.php
@@ -7,7 +7,7 @@ use Amp\WindowsRegistry\KeyNotFoundException;
 use Amp\WindowsRegistry\WindowsRegistry;
 use function Amp\call;
 
-class WindowsConfigLoader implements ConfigLoader {
+final class WindowsConfigLoader implements ConfigLoader {
     public function loadConfig(): Promise {
         return call(function () {
             $keys = [