allow user to override system dnsserver with config.

config item:
    dns_server     list

    eg:
        dns_server:[8.8.8.8,8.8.4.4]

shadowsocks/asyncdns.py

@@ -248,16 +248,19 @@ def __str__(self):
 
 class DNSResolver(object):
 
-    def __init__(self):
+    def __init__(self, server_list=None):
         self._loop = None
         self._hosts = {}
         self._hostname_status = {}
         self._hostname_to_cb = {}
         self._cb_to_hostname = {}
         self._cache = lru_cache.LRUCache(timeout=300)
         self._sock = None
-        self._servers = None
-        self._parse_resolv()
+        if server_list is None:
+            self._servers = None
+            self._parse_resolv()
+        else:
+            self._servers = server_list
         self._parse_hosts()
         # TODO monitor hosts change and reload hosts
         # TODO parse /etc/gai.conf and follow its rules

shadowsocks/server.py

@@ -57,7 +57,12 @@ def main():
 
     tcp_servers = []
     udp_servers = []
-    dns_resolver = asyncdns.DNSResolver()
+
+    if 'dns_server' in config:  # allow override settings in resolv.conf
+        dns_resolver = asyncdns.DNSResolver(config['dns_server'])
+    else:
+        dns_resolver = asyncdns.DNSResolver()
+
     port_password = config['port_password']
     del config['port_password']
     for port, password in port_password.items():