diff --git a/shadowsocks/asyncdns.py b/shadowsocks/asyncdns.py
index 4354b1d..46459c9 100644
--- a/shadowsocks/asyncdns.py
+++ b/shadowsocks/asyncdns.py
@@ -307,7 +307,7 @@ class DNSResolver(object):
 
     def _parse_hosts(self):
         etc_path = '/etc/hosts'
-        if os.environ.__contains__('WINDIR'):
+        if 'WINDIR' in os.environ:
             etc_path = os.environ['WINDIR'] + '/system32/drivers/etc/hosts'
         try:
             with open(etc_path, 'rb') as f:
@@ -324,7 +324,7 @@ class DNSResolver(object):
         except IOError:
             self._hosts['localhost'] = '127.0.0.1'
 
-    def add_to_loop(self, loop):
+    def add_to_loop(self, loop, ref=False):
         if self._loop:
             raise Exception('already add to loop')
         self._loop = loop
@@ -333,21 +333,21 @@ class DNSResolver(object):
                                    socket.SOL_UDP)
         self._sock.setblocking(False)
         loop.add(self._sock, eventloop.POLL_IN)
-        loop.add_handler(self.handle_events, ref=False)
+        loop.add_handler(self.handle_events, ref=ref)
 
     def _call_callback(self, hostname, ip, error=None):
         callbacks = self._hostname_to_cb.get(hostname, [])
         for callback in callbacks:
-            if self._cb_to_hostname.__contains__(callback):
+            if callback in self._cb_to_hostname:
                 del self._cb_to_hostname[callback]
             if ip or error:
                 callback((hostname, ip), error)
             else:
                 callback((hostname, None),
                          Exception('unknown hostname %s' % hostname))
-        if self._hostname_to_cb.__contains__(hostname):
+        if hostname in self._hostname_to_cb:
             del self._hostname_to_cb[hostname]
-        if self._hostname_status.__contains__(hostname):
+        if hostname in self._hostname_status:
             del self._hostname_status[hostname]
 
     def _handle_data(self, data):
@@ -408,7 +408,7 @@ class DNSResolver(object):
                 arr.remove(callback)
                 if not arr:
                     del self._hostname_to_cb[hostname]
-                    if self._hostname_status.__contains__(hostname):
+                    if hostname in self._hostname_status:
                         del self._hostname_status[hostname]
 
     def _send_req(self, hostname, qtype):
@@ -422,15 +422,17 @@ class DNSResolver(object):
             self._sock.sendto(req, (server, 53))
 
     def resolve(self, hostname, callback):
+        if type(hostname) != bytes:
+            hostname = hostname.encode('utf8')
         if not hostname:
             callback(None, Exception('empty hostname'))
         elif is_ip(hostname):
             callback((hostname, hostname), None)
-        elif self._hosts.__contains__(hostname):
+        elif hostname in self._hosts:
             logging.debug('hit hosts: %s', hostname)
             ip = self._hosts[hostname]
             callback((hostname, ip), None)
-        elif self._cache.__contains__(hostname):
+        elif hostname in self._cache:
             logging.debug('hit cache: %s', hostname)
             ip = self._cache[hostname]
             callback((hostname, ip), None)
@@ -453,3 +455,53 @@ class DNSResolver(object):
         if self._sock:
             self._sock.close()
             self._sock = None
+
+
+def test():
+    dns_resolver = DNSResolver()
+    loop = eventloop.EventLoop()
+    dns_resolver.add_to_loop(loop, ref=True)
+
+    global counter
+    counter = 0
+
+    def make_callback():
+        global counter
+
+        def callback(result, error):
+            global counter
+            # TODO: what can we assert?
+            print(result, error)
+            counter += 1
+            if counter == 10:
+                loop.remove_handler(dns_resolver.handle_events)
+                dns_resolver.close()
+        a_callback = callback
+        return a_callback
+
+    assert(make_callback() != make_callback())
+
+    dns_resolver.resolve(b'google.com', make_callback())
+    dns_resolver.resolve('google.com', make_callback())
+    dns_resolver.resolve('example.com', make_callback())
+    dns_resolver.resolve('ipv6.google.com', make_callback())
+    dns_resolver.resolve('www.facebook.com', make_callback())
+    dns_resolver.resolve('ns2.google.com', make_callback())
+    dns_resolver.resolve('not.existed.google.com', make_callback())
+    dns_resolver.resolve('invalid.@!#$%^&$@.hostname', make_callback())
+    dns_resolver.resolve('toooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'long.hostname', make_callback())
+    dns_resolver.resolve('toooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'ooooooooooooooooooooooooooooooooooooooooooooooooooo'
+                         'long.hostname', make_callback())
+
+    loop.run()
+
+
+if __name__ == '__main__':
+    test()
\ No newline at end of file