diff --git a/shadowsocks/asyncdns.py b/shadowsocks/asyncdns.py
index 91601ea..fa5be41 100644
--- a/shadowsocks/asyncdns.py
+++ b/shadowsocks/asyncdns.py
@@ -29,7 +29,7 @@ from shadowsocks import common, lru_cache, eventloop, shell
 
 CACHE_SWEEP_INTERVAL = 30
 
-VALID_HOSTNAME = re.compile(br"(?!-)[A-Z\d-]{1,63}(?<!-)$", re.IGNORECASE)
+VALID_HOSTNAME = re.compile(br"(?!-)[A-Z\d\-_]{1,63}(?<!-)$", re.IGNORECASE)
 
 common.patch_socket()