diff --git a/os/xdmcp.c b/os/xdmcp.c
index 4d4690c70..2a62e9a28 100644
--- a/os/xdmcp.c
+++ b/os/xdmcp.c
@@ -34,6 +34,7 @@
 #include <netdb.h>
 #endif
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <X11/X.h>