SD card

OpenWrt can be installed and run from devices that use an SD card. Common devices with this include Raspberry Pi and NanoPi.

Install Balena Etcher, a free open source program to flash SD cards on Windows, Linux, or macOS.

1. Download the image for your device which will be named similar to openwrt-*-sysupgrade.img.gz
2. Select the image
3. Select your SD card
4. Flash the image

1. Download the image for your device which will be named similar to openwrt-*-sysupgrade.img.gz
2. Decompress it:

   gzip -d openwrt-*-sysupgrade.img.gz

3. Connect an SD card to your computer and look at lsblk or dmesg to identify it. In most cases, it would be something like /dev/sdX. ¹⁾
4. Double check you identified your SD card correctly. If the /dev/sdX you have chosen corresponds to your hard drive, the next step would destroy your system.
5. Copy the image to the SD card with:

   dd if=openwrt-*-sysupgrade.img of=/dev/sdX

Most SD card devices have multiple images available which differ in the filesystem used.

• Not optimized for flash memory (journaling increases flash wear)
• SD card can be easily mounted externally for modification
• Updates and changes can be made directly to the partition
• Linux desktop standard

• Compressed
• Newer images include a hidden F2FS filesystem, which is optimized for flash memory
• Needs special mount procedure to externally modify
• All changes are done in an overlay partition
• Due to overlay partition it is simple to reset system to defaults

• ubifs-sdcard.img.gz

If you insert your newly flashed SD card into a Linux computer it will be easy to mount the read only squashfs partition but it won't know about the overlay, which is not even in the partition table but instead located immediately after the squashfs filesystem in the same partition. In fact, before you've booted the SD card on your device, the overlay won't even exist.

So, first you need to make sure you've booted your image. You then need to mount the overlay as a loopback device. You can discover the offset by running losetup on the device, or calculate the offset yourself by inspecting the filesystem.

# Setup the loop back device.
# See libfstools/rootdisk.c for source of partition offset logic.
DEVICE= ### Set this appropriately - e.g. /dev/sda
PARTITION="$DEVICE"2
FS_SIZE="$(sudo unsquashfs -s "$PARTITION" | grep -o 'Filesystem size [0-9]* bytes' | grep -o '[0-9][0-9]*')"
FS_OFFSET="$(expr '(' "$FS_SIZE" + 65535 ')' / 65536 '*' 65536)" 
LOOP_DEVICE="$(sudo losetup -f --show -o "$FS_OFFSET" "$PARTITION")"

# Now mount both partitions (remember, you may need to unmount any automatic mounts)
mkdir -p /mnt/base /mnt/overlay /mnt/combined
sudo mount "$PARTITION" /mnt/base            
sudo mount "$LOOP_DEVICE" /mnt/overlay
sudo mount -o noatime,lowerdir=/mnt/base,upperdir=/mnt/overlay/upper,workdir=/mnt/overlay/work -t overlay overlayfs /mnt/combined

This should leave you with a writable filesystem in /mnt/combined which will work as it does on OpenWrt.

To use the whole available space of your SD card, you may have to resize your partition.

First, make sure the partition is not mounted, then do something like:

DEVICE= ### Set this appropriately - e.g. /dev/sda
PARTITION="$DEVICE"2
sudo cfdisk "$DEVICE"  # select resize, then write

If you've never booted the image that's all there is to it. OpenWrt will create an overlay which uses the rest of this partition on the first boot.

However, if you already booted the image OpenWrt will have created an overlay that is smaller, so you'll need to resize the filesystem. Expand the partition as above, then:

# Create a loop device pointing to the FS
# See libfstools/rootdisk.c for source of partition offset logic.
FS_SIZE="$(sudo unsquashfs -s "$PARTITION" | grep -o 'Filesystem size [0-9]* bytes' | grep -o '[0-9][0-9]*')"
FS_OFFSET="$(expr '(' "$FS_SIZE" + 65535 ')' / 65536 '*' 65536)" 
LOOP_DEVICE="$(sudo losetup -f --show -o "$FS_OFFSET" "$PARTITION")"

# Now to resize... you may need to use fsck first though.
sudo fsck "$LOOP_DEVICE"
sudo resize2fs "$LOOP_DEVICE"
sudo fsck "$LOOP_DEVICE"

If you get an error from resize2fs about a bad superblock, you probably have an F2FS filesystem. Use resize.f2fs instead of resize2fs.

You can use gparted to resize and extend the partitions. To do it online, follow the procedure described on github or expanding_root_partition_and_filesystem.

Example, to resize /dev/mmcblk0p2 mounted on /, install parted, tune2fs and resize2fs then:

parted
p
resizepart 2 32GB
q

Next, you may need to repair your device (perhaps say yes to all interactive queries):

mount -o remount,ro /                  #Remount root as Read Only
tune2fs -O^resize_inode /dev/mmcblk0p2    #Remove reserved GDT blocks
fsck.ext4 /dev/mmcblk0p2                  #Fix part, answer yes to remove GDT blocks remnants

Now, reboot and then resize the partition:

resize2fs /dev/mmcblk0p2

Note that these operations may alter partition UUID. Either preserve the partition UUID or edit your boot parameters. For example on raspberry pi you must update

/boot/cmdline.txt
/boot/partuuid.txt

Examples:

• Sunxi

Howto:

1. Partition and format the SD card. Details devicespecific? → Devicepage
2. Copy bootloader, kernel, rootfs (and if necessary other data) to SD card. Details devicespecific? → Devicepage
3. Possibly resize filesystem in order to use the complete available space on the SD card