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FEL/NFS boot explanation

Warning: Armbian has dropped support for old legacy 3.x kernels. Therefore FEL is no longer supported/available. This page is archived and not maintained.

What is FEL/NFS boot?

FEL/NFS boot mode is a possibility to test freshly created Armbian distribution without using SD card. It is implemented by loading u-boot, kernel, initrd, boot script and .bin/.dtb file via USB FEL mode and providing root filesystem via NFS share.

NOTE: this mode is designed only for testing. To use root on NFS permanently, use ROOTFS_TYPE=nfs option. NOTE: “hot” switching between kernel branches (default <-> dev/next) is not supported


  • Allwinner device that supports FEL mode. Check wiki to find out how to enter FEL mode with your device
  • USB connection between build host and board OTG port (VM USB passthrough or USB over IP may work too)
  • Network connection between build host and board. For target board wired Ethernet connection is required (either via onboard Ethernet or via USB ethernet adapter that has required kernel modules built-in)
  • NFS ports on build host should be reachable from board perspective (you may need to open ports in firewall or change network configuration of your VM)
  • Selected kernel should have built-in support for DHCP and NFS root filesystem
  • CLEAN_LEVEL="make,debs" to always update u-boot configuration
  • DHCP server in local network
  • UART console connected to target board

Build script options



Text Only

Shutdown and reboot

Once you start FEL boot, you will see this prompt:

Text Only
[ o.k. ] Press any key to boot again, <q> to finish [ FEL ]

Pressing q deletes current rootfs and finishes build process, so you need to shut down or reboot your board to avoid possible problems unmounting/deleting temporary rootfs. All changes to root filesystem will persist until you exit FEL mode.

To reboot again into testing system, switch your board into FEL mode and press any key other than q.

Because kernel and .bin/.dtb file are loaded from rootfs each time, it’s possible to update kernel or its configuration (via apt-get, dtc, fex2bin/bin2fex) from within running system.

Advanced configuration

If you don’t have DHCP server in your local network or if you need to alter kernel command line, use lib/scripts/fel-boot.cmd.template as a template and save modified script as userpatches/fel-boot.cmd. Check this for configuring static IP for NFS root.

  • Note: As of 2020-11-25, the above link is broken. However I am not sure what to replace it with. If you know (or find out) please submit a PR. - TRS-80

Set FEL_DTB_FILE to relative path to .dtb or .bin file if it can’t be obtained from u-boot config (mainline kernel) or boot/script.bin (legacy kernel)

You may need to set these additional options (it’s a good idea to put them in userpatches/lib.config:

Set FEL_NET_IFNAME to name of your network interface if you have more than one non-loopback interface with assigned IPv4 address on your build host

Set FEL_LOCAL_IP to IP address that can be used to reach NFS server on your build host if it can’t be obtained from ifconfig (i.e. port forwarding to VM guest)

Set FEL_AUTO=yes to skip prompt before trying FEL load


You can even create userpatches/ and define there 2 functions: fel_post_prepare and fel_pre_load. All normal build variables like $BOARD, $BRANCH and so on can be used in these functions to define specific actions.

fel_post_prepare is executed once after setting up u-boot script and NFS share, you can use it to add extra stuff to boot.scr (like gpio set or setenv machid) based on device name.

fel_pre_load is executed before calling sunxi-fel, you can use it to implement logic to select one of multiple connected boards; to pass additional arguments to sunxi-fel you can use FEL_EXTRA_ARGS variable.

An example is provided as scripts/