Embedded System Ramdisk from USB

Sometimes it is convenient to boot an embedded System without a network. For example shipping one to customers for factory tests.

Documentation only valid for acc7.


requires an USB Pendrive, any size will do and root permissions on a linux system. Connect pendrive, run parted to create a boot and a localinit partition.

root@host# parted /dev/sdb
(parted) mklabel msdos
(parted) mkpart primary fat32 2048s 512M
(parted) set 1 boot on
(parted) print
Model:   (scsi)
Disk /dev/sdb: 1992MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Disk Flags: 

Number  Start   End    Size   Type     File system  Flags
 1      1049kB  512MB  511MB  primary  fat32        boot, lba

create filesystems:

root@host# mkfs.vfat -n BOOT /dev/sdb1


Install syslinux and the bootsector (Where is the bootsector? rpm -ql syslinux-nonlinux)

root@host# syslinux -i /dev/sdb1
root@host# cat /usr/share/syslinux/mbr.bin > /dev/sdb

now mount /dev/sdb1 somewhere. it will contain an ldlinux.c32 and ldlinux.sys.


Create a ramdisk from opkg packages that contains the localinit package. Sample config


Copy kernel and initramfs to /dev/sdb1

Add a syslinux.cfg (in the /dev/sdb1 filesystem)

DEFAULT default

LABEL default
  KERNEL kernel
  APPEND initrd=initramfs


the default embedded network script is still executed. It will request an ip via dhcp. If no ip is received within 30 seconds, the system will reboot.

This can be configured via kernel commandline parameters. Either specify a kernel ip config (ip=...) or dhcp=TIMEOUT:PANIC_TIMEOUT. Where timeout is the number of seconds to wait for an ip and panic_timeout the number of seconds before a system reset is executed. A panic_timeout of zero disables the reset.

for example the following syslinux.cfg

DEFAULT default

LABEL default
  KERNEL kernel
  APPEND initrd=initramfs dhcp=30:0


if the localinit package is included. On boot the system will try to mount a partition with label "BOOT". If you followed the partitioning guide above, it is the same partition as the ramdisk. Note that windows only displays the first partition on a pendrive. The location of the overlay filesystem can be customized using a kernel parameter localinit to either specify a device or a filesystem label. For example
  KERNEL kernel
  APPEND initrd=initramfs localinit=/dev/sda4

LABEL foobar
  KERNEL kernel
  APPEND initrd=initramfs localinit=LABEL=foobar

The file /opt/localinit/overlay.tar.gz will be unpacked over the ramdisk. And the file /opt/localinit/overlay.sh will be copied to /etc/init/boot.d/99-overlay and executed. This should make it possible to use a vfat filesystem on the overlay partition, as all posix attributes are in the tarball.

Replacing uid/gid during tarball creation cd overlay; tar cvzf ../overlay.tar.gz --owner=0 --group=0 *.

For an example disk image that can be written to a pendrive see EmbeddedRamdiskUSB
Topic revision: r7 - 01 Apr 2019, ChristophHandel
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