diff --git a/config.h b/config.h
index 5824b05..858c5ff 100644
--- a/config.h
+++ b/config.h
@@ -49,6 +49,7 @@ static const Rule rules[] = {
 	/* class              instance  title           tags mask  isfloating  isterminal  noswallow  monitor */
 	{ "st-256color",      NULL,     NULL,           0,         0,          1,           0,        -1 },
 	{ NULL,               NULL,     "Event Tester", 0,         0,          0,           1,        -1 }, /* xev */
+	{ "Xephyr",           NULL,     NULL,           0,         0,          0,           1,        -1 }, /* lightdm --test-mode */
 };
 
 /* layout(s) */