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](https://linux-sunxi.org/FEL/USBBoot) and providing root filesystem via NFS share.
- Allwinner device that supports FEL mode. Check [wiki](https://linux-sunxi.org/FEL) 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
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.
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](https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/plain/Documentation/filesystems/nfs/nfsroot.txt) for configuring static IP for NFS root
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/fel-hooks.sh` 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/fel-hooks.sh.example`.
