Index: head/UPDATING
===================================================================
--- head/UPDATING
+++ head/UPDATING
@@ -52,6 +52,13 @@
 ****************************** SPECIAL WARNING: ******************************
 
 20170912:
+	The default serial number format for CTL LUNs has changed.  This will
+	affect users who use /dev/diskid/* device nodes, or whose FibreChannel
+	or iSCSI clients care about their LUNs' serial numbers.  Users who
+	require serial number stability should hardcode serial numbers in
+	/etc/ctl.conf .
+
+20170912:
 	For 32-bit arm compiled for hard-float support, soft-floating point
 	binaries now always get their shared libraries from
 	LD_SOFT_LIBRARY_PATH (in the past, this was only used if
Index: head/sys/cam/ctl/ctl_backend_block.c
===================================================================
--- head/sys/cam/ctl/ctl_backend_block.c
+++ head/sys/cam/ctl/ctl_backend_block.c
@@ -2324,7 +2324,7 @@
 	cbe_lun->be = &ctl_be_block_driver;
 
 	if ((params->flags & CTL_LUN_FLAG_SERIAL_NUM) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%4d",
+		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%04d",
 			 softc->num_luns);
 		strncpy((char *)cbe_lun->serial_num, tmpstr,
 			MIN(sizeof(cbe_lun->serial_num), sizeof(tmpstr)));
@@ -2338,7 +2338,7 @@
 			sizeof(params->serial_num)));
 	}
 	if ((params->flags & CTL_LUN_FLAG_DEVID) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%4d", softc->num_luns);
+		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%04d", softc->num_luns);
 		strncpy((char *)cbe_lun->device_id, tmpstr,
 			MIN(sizeof(cbe_lun->device_id), sizeof(tmpstr)));
 
Index: head/sys/cam/ctl/ctl_backend_ramdisk.c
===================================================================
--- head/sys/cam/ctl/ctl_backend_ramdisk.c
+++ head/sys/cam/ctl/ctl_backend_ramdisk.c
@@ -1096,7 +1096,7 @@
 	cbe_lun->lun_config_status = ctl_backend_ramdisk_lun_config_status;
 	cbe_lun->be = &ctl_be_ramdisk_driver;
 	if ((params->flags & CTL_LUN_FLAG_SERIAL_NUM) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%4d",
+		snprintf(tmpstr, sizeof(tmpstr), "MYSERIAL%04d",
 			 softc->num_luns);
 		strncpy((char *)cbe_lun->serial_num, tmpstr,
 			MIN(sizeof(cbe_lun->serial_num), sizeof(tmpstr)));
@@ -1110,7 +1110,7 @@
 			    sizeof(params->serial_num)));
 	}
 	if ((params->flags & CTL_LUN_FLAG_DEVID) == 0) {
-		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%4d", softc->num_luns);
+		snprintf(tmpstr, sizeof(tmpstr), "MYDEVID%04d", softc->num_luns);
 		strncpy((char *)cbe_lun->device_id, tmpstr,
 			MIN(sizeof(cbe_lun->device_id), sizeof(tmpstr)));