diff --git a/ShadowsocksX-NG/ServerProfile.swift b/ShadowsocksX-NG/ServerProfile.swift
index bd80833..5e558be 100644
--- a/ShadowsocksX-NG/ServerProfile.swift
+++ b/ShadowsocksX-NG/ServerProfile.swift
@@ -131,7 +131,7 @@ class ServerProfile: NSObject {
         }
         
         func validateDomainName(_ value: String) -> Bool {
-            let validHostnameRegex = "^(([a-zA-Z0-9]|[a-zA-Z0-9][a-zA-Z0-9\\-]*[a-zA-Z0-9])\\.)*([A-Za-z0-9]|[A-Za-z0-9][A-Za-z0-9\\-]*[A-Za-z0-9])$"
+            let validHostnameRegex = "^(([a-zA-Z0-9_]|[a-zA-Z0-9_][a-zA-Z0-9\\-_]*[a-zA-Z0-9_])\\.)*([A-Za-z0-9_]|[A-Za-z0-9_][A-Za-z0-9\\-_]*[A-Za-z0-9_])$"
             
             if (value.range(of: validHostnameRegex, options: .regularExpression) != nil) {
                 return true