diff --git a/doc/arm/Bv9ARM-book.xml b/doc/arm/Bv9ARM-book.xml
index 9445c81e10..7b5e0ddf15 100644
--- a/doc/arm/Bv9ARM-book.xml
+++ b/doc/arm/Bv9ARM-book.xml
@@ -6042,6 +6042,12 @@ options {
 		  server cookie.
 		  The default is <userinput>no</userinput>.
 		</para>
+		<para>
+		  Set this to <userinput>yes</userinput> to test that DNS
+		  COOKIE clients correctly handle BADCOOKIE or if you are
+		  getting a lot of forged DNS requests with DNS COOKIES
+		  present.
+		</para>
 	      </listitem>
 	    </varlistentry>