--- src/usr.sbin/config/mkoptions.c	2005/01/12 00:26:03	1.14
+++ src/usr.sbin/config/mkoptions.c	2005/08/02 13:18:25	1.15
@@ -139,6 +139,12 @@ do_option(char *name)
 		}
 	}
 
+	if (strcmp(name, "COMPAT_DF12") == 0 &&
+	    getenv("NO_COMPAT_DF12") == NULL &&
+	    (value == NULL || strcmp(value, "1") != 0))
+		errx(1, "options COMPAT_DF12 not specified, "
+		     "set NO_COMPAT_DF12 to ignore");
+
 	inf = fopen(file, "r");
 	if (inf == NULL) {
 		outf = fopen(file, "w");