Index: head/usr.bin/ar/ar.1
===================================================================
--- head/usr.bin/ar/ar.1
+++ head/usr.bin/ar/ar.1
@@ -210,6 +210,7 @@
 .Ar .
 This ensures that checksums on the resulting archives are reproducible
 when member contents are identical.
+This option is enabled by default.
 If multiple
 .Fl D
 and
Index: head/usr.bin/ar/ar.c
===================================================================
--- head/usr.bin/ar/ar.c
+++ head/usr.bin/ar/ar.c
@@ -104,6 +104,8 @@
 
 	bsdar = &bsdar_storage;
 	memset(bsdar, 0, sizeof(*bsdar));
+	/* Enable deterministic mode by default. */
+	bsdar->options |= AR_D;
 
 	if ((bsdar->progname = getprogname()) == NULL)
 		bsdar->progname = "ar";