diff --git a/src/include/ipxe/tcp.h b/src/include/ipxe/tcp.h
index 21be3ca8a..f5508fe2b 100644
--- a/src/include/ipxe/tcp.h
+++ b/src/include/ipxe/tcp.h
@@ -26,7 +26,7 @@ struct tcp_header {
 	uint16_t win;		/* Advertised window */
 	uint16_t csum;		/* Checksum */
 	uint16_t urg;		/* Urgent pointer */
-};
+} __attribute__ (( packed ));
 
 /** @defgroup tcpopts TCP options
  * @{