This tutorial explains, how to build openwrrt to have it run on NFS.

Prerequisite

Follow https://wiki.openwrt.org/doc/howto/build to set up a build environment.

Configure the system

Special configuration options to be used

Unless otherwise specified all statements are to be considered relative to your openwrt build root.

Use make menuconfig to configure the system. At least the following items have to be changed. AFAIK this works only for network devices that don't need any special firmware to be loaded.

Target System (x86)
Subtarget (x86_64)
Target images --->
  [*] tar.gz
Global Build Settings --->
  [*] Compile the kernel with rootfs on NFS
[*] Image configuration --->
  [*] Preinit configuration options --->
    (disabled) Preinit network interface

Also make sure that KERNEL_IP_PNP_DHCP=y is in your resulting .config.

The target image is extracted on the nfs root. NFS support has to be enabled in the kernel. The preinit network interface has to be set to disabled so the openwrt preinit script does not disable the network device on boot.

Compile the network devices drivers

The network device drivers have to be compiled in the kernel. This means editing the .config file per hand.

An example would be to use an Intel e1000 or e1000e network card. Kernel drivers to be included in the kernel are to be prepended with CONFIG_KERNEL_. This would result in appending the following lines to your .config:

CONFIG_KERNEL_NET_VENDOR_INTEL=y
CONFIG_KERNEL_E1000=y
CONFIG_KERNEL_E1000E=y

This is not possible with make menuconfig - you have to edit the .config with your text editor of choice.

Change the network configuration

Create the following file:

$ mkdir -p files/etc/config
$ vim files/etc/config/network

The file should contain the following plus every other network configuration required. It is mandatory not to include the network device used for the NFS root.

config interface loopback
        option ifname lo
        option proto static
        option ipaddr 127.0.0.1
        option netmask 255.0.0.0

Build openwrt

Now you can build openwrt with make V=s. This generates two files we need in bin/targets/x86/64:

• the kernel in lede-x86-64-vmlinuz
• the nfsroot to be in lede-x86-64-generic-rootfs.tar.gz

Deploy openwrt

Setup the NFS root

Just extract the afformentioned .tar.gz to a folder which is shared via NFS.

Setup the PXE server

Assuming a PXE server with tftp is already installed and running the aformentioned vmlinuz image has to be copied to the tftp. The pxelinux.cfg entry is equivalent to a normal linux PXE NFS root.

LABEL openwrt
  KERNEL openwrt/vmlinuz
  APPEND panic=0 root=/dev/nfs nfsroot=192.168.1.1:/var/nfs/openwrt,nolock,v3,intr,hard,noacl ip=dhcp rw

panic=0 disables the automatic reboots on kernel panics - helpful during debugging. The rest of the parameters are standard kernel command line parameters.

Done

Now boot the client and enjoy.