diff --git a/doc/SOCKS4.protocol.txt b/doc/SOCKS4.protocol.txt
new file mode 100644
index 0000000..06aee8a
--- /dev/null
+++ b/doc/SOCKS4.protocol.txt
@@ -0,0 +1 @@
+Please reference to "https://www.openssh.com/txt/socks4.protocol".
\ No newline at end of file