Index: usr.bin/mkimg/mbr.c
===================================================================
--- usr.bin/mkimg/mbr.c
+++ usr.bin/mkimg/mbr.c
@@ -92,7 +92,12 @@
 	TAILQ_FOREACH(part, &partlist, link) {
 		size = round_track(part->size);
 		dp = dpbase + part->index;
-		dp->dp_flag = (part->index == 0 && bootcode != NULL) ? 0x80 : 0;
+		if (active_partition != 0)
+			dp->dp_flag =
+			    (part->index + 1 == active_partition) ? 0x80 : 0;
+		else
+			dp->dp_flag =
+			    (part->index == 0 && bootcode != NULL) ? 0x80 : 0;
 		mbr_chs(&dp->dp_scyl, &dp->dp_shd, &dp->dp_ssect,
 		    part->block);
 		dp->dp_typ = ALIAS_TYPE2INT(part->type);
Index: usr.bin/mkimg/mkimg.h
===================================================================
--- usr.bin/mkimg/mkimg.h
+++ usr.bin/mkimg/mkimg.h
@@ -59,6 +59,7 @@
 extern u_int nsecs;
 extern u_int secsz;	/* Logical block size. */
 extern u_int blksz;	/* Physical block size. */
+extern uint32_t active_partition;
 
 static inline lba_t
 round_block(lba_t n)
Index: usr.bin/mkimg/mkimg.1
===================================================================
--- usr.bin/mkimg/mkimg.1
+++ usr.bin/mkimg/mkimg.1
@@ -40,6 +40,7 @@
 .Op Fl c Ar capacity
 .Op Fl f Ar format
 .Op Fl o Ar outfile
+.Op Fl a Ar active
 .Op Fl v
 .Op Fl y
 .Op Fl s Ar scheme Op Fl p Ar partition ...
@@ -119,7 +120,7 @@
 partitioning scheme with the
 .Fl s
 option, but without specifying any partitions.
-When the size required to for all the partitions is larger than the
+When the size required for all the partitions is larger than the
 given capacity, then the disk image will be larger than the capacity
 given.
 .Pp
@@ -139,6 +140,26 @@
 .Nm
 utility will create images that are identical.
 .Pp
+The
+.Ar active
+option marks a partition as active, if the partitioning
+scheme supports it.
+Currently, only the
+.Ar mbr
+scheme supports this concept.
+By default,
+.Nm
+will only mark the first partition as active when boot code is
+specified.
+Use the
+.Ar active
+option to override the active partition.
+The number specified corresponds to the number after the 's' in the
+partition's
+.Xr geom 8
+name.
+No partitions are marked active when the value is 0.
+.Pp
 A set of long options exist to query about the
 .Nm
 utility itself.
Index: usr.bin/mkimg/mkimg.c
===================================================================
--- usr.bin/mkimg/mkimg.c
+++ usr.bin/mkimg/mkimg.c
@@ -70,6 +70,7 @@
 u_int nsecs = 1;
 u_int secsz = 512;
 u_int blksz = 0;
+uint32_t active_partition = 0;
 
 static void
 print_formats(int usage)
@@ -145,6 +146,7 @@
 	fprintf(stderr, "\t--schemes\t-  list partition schemes\n");
 	fprintf(stderr, "\t--version\t-  show version information\n");
 	fputc('\n', stderr);
+	fprintf(stderr, "\t-a <num>\t-  mark num'th partion as active\n");
 	fprintf(stderr, "\t-b <file>\t-  file containing boot code\n");
 	fprintf(stderr, "\t-c <num>\t-  capacity (in bytes) of the disk\n");
 	fprintf(stderr, "\t-f <format>\n");
@@ -468,9 +470,14 @@
 
 	bcfd = -1;
 	outfd = 1;	/* Write to stdout by default */
-	while ((c = getopt_long(argc, argv, "b:c:f:o:p:s:vyH:P:S:T:",
+	while ((c = getopt_long(argc, argv, "a:b:c:f:o:p:s:vyH:P:S:T:",
 	    longopts, NULL)) != -1) {
 		switch (c) {
+		case 'a':	/* ACTIVE PARTITION, if supported */
+			error = parse_uint32(&active_partition, 1, 100, optarg);
+			if (error)
+				errc(EX_DATAERR, error, "Partition ordinal");
+			break;
 		case 'b':	/* BOOT CODE */
 			if (bcfd != -1)
 				usage("multiple bootcode given");