diff --git a/shadowsocks/udprelay.py b/shadowsocks/udprelay.py
index caebe12..7f6c21f 100644
--- a/shadowsocks/udprelay.py
+++ b/shadowsocks/udprelay.py
@@ -70,6 +70,7 @@ import threading
 import socket
 import logging
 import struct
+import encrypt
 import eventloop
 
 BUF_SIZE = 65536
@@ -92,7 +93,8 @@ def parse_header(data):
             addrlen = ord(data[1])
             if len(data) >= 2 + addrlen:
                 dest_addr = data[2:2 + addrlen]
-                dest_port = struct.unpack('>H', data[2 + addrlen:4 + addrlen])[0]
+                dest_port = struct.unpack('>H', data[2 + addrlen:4 +
+                                          addrlen])[0]
                 header_length = 4 + addrlen
             else:
                 logging.warn('[udp] header is too short')
@@ -112,6 +114,10 @@ def parse_header(data):
     return (addrtype, dest_addr, dest_port, header_length)
 
 
+def client_key(a, b, c, d):
+    return '%s:%s:%s:%s' % (a, b, c, d)
+
+
 class UDPRelay(object):
     def __init__(self, listen_addr='127.0.0.1', listen_port=1080,
                  remote_addr='127.0.0.1', remote_port=8387, password=None,
@@ -125,24 +131,84 @@ class UDPRelay(object):
         self._timeout = timeout
         self._is_local = is_local
         self._eventloop = eventloop.EventLoop()
-        self._cache = {}  # TODO replace this dictionary with an LRU cache
+        self._cache = {}  # TODO replace ith an LRU cache
+        self._client_fd_to_server_addr = {}  # TODO replace ith an LRU cache
 
     def _handle_server(self):
         server = self._server_socket
-        data = server.recvfrom(BUF_SIZE)
+        data, r_addr = server.recvfrom(BUF_SIZE)
         if self._is_local:
             frag = ord(data[2])
             if frag != 0:
                 logging.warn('drop a message since frag is not 0')
+                return
             else:
                 data = data[3:]
         else:
-            decrypt
-        
+            # decrypt data
+            data = encrypt.encrypt_all(self._password, self._method, 0, data)
+            if not data:
+                return
+        header_result = parse_header(data)
+        if header_result is None:
+            return
+        addrtype, dest_addr, dest_port, header_length = header_result
+
+        if self._is_local:
+            server_addr, server_port = self._remote_addr, self._remote_port
+        else:
+            server_addr, server_port = dest_addr, dest_port
+
+        key = client_key(r_addr[0], r_addr[1], dest_addr, dest_port)
+        client = self._cache.get(key, None)
+        if not client:
+            # TODO async getaddrinfo
+            addrs = socket.getaddrinfo(server_addr, server_port, 0,
+                                       socket.SOCK_DGRAM, socket.SOL_UDP)
+            if addrs:
+                af, socktype, proto, canonname, sa = addrs[0]
+                client = socket.socket(af, socktype, proto)
+                client.setblocking(False)
+                self._cache[key] = client
+                self._client_fd_to_server_addr[client.fileno()] = r_addr
+            else:
+                # drop
+                return
+        self._eventloop.add(client, eventloop.MODE_IN)
+        if self._is_local:
+            data = encrypt.encrypt_all(self._password, self._method, 1, data)
+            if not data:
+                return
+        client.sendto(data, (server_addr, server_port))
 
     def _handle_client(self, sock):
-        # TODO
-        pass
+        data, r_addr = sock.recvfrom(BUF_SIZE)
+        if not self._is_local:
+            addrlen = len(r_addr[0])
+            if addrlen > 255:
+                # drop
+                return
+            data = '\x03' + chr(addrlen) + r_addr[0] + \
+                   struct.pack('>H', r_addr[1]) + data
+            response = encrypt.encrypt_all(self._password, self._method, 1,
+                                           data)
+            if not response:
+                return
+        else:
+            data = encrypt.encrypt_all(self._password, self._method, 0,
+                                       data)
+            if not data:
+                return
+            # addrtype, dest_addr, dest_port, header_length = parse_header(data)
+            response = '\x00\x00\0x00' + data
+        client_addr = self._client_fd_to_server_addr.get(sock.fileno(), None)
+        if client_addr:
+            self._server_socket.sendto(response, client_addr)
+        else:
+            pass
+            # self._eventloop.remove(sock)
+            # sock.close()
+            # TODO remove it from cache else we can't close it
 
     def _run(self):
         server_socket = self._server_socket
@@ -171,3 +237,4 @@ class UDPRelay(object):
         t = threading.Thread(target=self._run)
         t.setDaemon(True)
         t.start()
+        self._thread = t