Chromebook Netboot Setup Guide

What is Netboot?

Netboot firmware is a special firmware that instead of loading kernel from storage, it downloads a light weight kernel (vmlinuz) from TFTP server. And the kernel will download ChromeOS images from factory server. This is helpful when you need to re-flash ChromeOS images frequently. Also, this can be used in some early phase of projects (e.g. Proto builds), when images might be changed during the build, so you don't want to pre-flash it by copy machine.

Overview

The netboot imaging flow contains the following steps:

Enter netboot mode with netboot or dev firmware.
Download vmlinuz kernel from TFTP server in netboot mode.
Download images (with cros_payload) from the factory server in vmlinuz kernel.

Therefore, you need to setup the following components to do a netboot imaging:

Flash the devices with netboot or dev firmware to enter netboot mode.
Setup a TFTP server to deploy netboot kernel.
Setup a factory server with image resources.
Specify netboot settings (e.g., factory server IP).

Details of each component can be found in corresponding sections in this document.

Prerequisition

A Linux machine, which will be running TFTP and factory server
An USB Ethernet dongle for DUT
Connect DUT and Linux machine by ethernet,
- Assume that the ethernet device on Linux machine is eth2
- Assume that eth2 is set up 192.168.200.1/24
Assume that the board we are going to use is zork, and the model name is morphius
Assume that the Linux machine is running Ubuntu (otherwise the network config files might need to be changed according to your Linux distribution).

Put Device Into Netboot Mode

1. Prepare netboot firmware

Find firmware blob image-${MODEL}.net.bin which should be available in firmware archive, or you can build it locally.

Local build netboot firmware:

$ emerge-${BOARD} sys-boot/chromeos-bootimage

You have to set the path to the bootfile and argsfile on local built netboot firmware with the following command:

$ bin/factory_env py/tools/netboot_firmware_settings.py \
  -i /build/${BOARD}/firmware/image-${MODEL}.net.bin \
  --bootfile chrome-bot/${MODEL}/vmlinuz \
  --argsfile chrome-bot/${MODEL}/cmdline
  -o image-${MODEL}.net.bin

2. Flash netboot firmware

You can use Servo to flash the netboot firmware:

(chroot)$ cros ap flash -b $BOARD -i image-${MODEL}.net.bin

or run flashrom on DUT:

(dut)$ flashrom -p host -w image-${MODEL}.net.bin

After flash netboot firmware,the device will boot into netboot mode automatically.

Alternatively, you can also install dev firmware (image-${MODEL}.dev.bin) to the device, and press Ctrl + N in the developer screen to enter netboot mode. This effectively runs the same netboot code as full netboot firmware.

Setup TFTP server

You can choose to use Dome or dnsmasq to setup the TFTP server.

Setup TFTP server with Dome

If your network infrastructure already has a DHCP server, you can setup the TFTP server from Dome UI with the following steps:

Upload the vmlinuz kernel (and netboot_cmdline file if needed) to Umpire.
Select the “NETBOOT” button (use this bundle's netboot resource) at the bundle.
Enable TFTP server in Dome config page.

By default this will create a TFTP folder at /cros_docker/tftp/, and the netboot kernel and cmdline files will be copied to /cros_docker/tftp/chrome-bot/${PROJECT}/, where ${PROJECT} is the project name of the Umpire instance. So if the project name on Umpire does not match the model name, you have to override the path to the netboot kernel and cmdline to download them from TFTP server; see Netboot Settings section for more details.

Setup TFTP server and DHCP server with `dnsmasq`

Decide a folder to put TFTP files, for example, /var/tftp

sudo mkdir /var/tftp
sudo chown "${USER}" /var/tftp
mkdir -p "/var/tftp/chrome-bot/${MODEL}"

Create a dnsmasq setup config in TFTP root, for example, /var/tftp/dnsmasq.conf, with the following contents:

interface=eth2
tftp-root=/var/tftp
enable-tftp
dhcp-leasefile=/tmp/dnsmasq.leases
dhcp-range=192.168.200.50,192.168.200.150,12h
port=0

Assuming that you are running Ubuntu, you can setup static IP for eth2 by adding a file /etc/network/interfaces.d/eth2.conf (you can change the name eth2.conf to whatever you like):

auto eth2
allow-hotplug eth2
iface eth2 inet static
    address 192.168.200.1
    netmask 255.255.255.0

Reload the configuration by running:

sudo ifup eth2

Then ip addr show eth2 and check if it has inet address 192.168.200.1/24 assigned. If not, reboot Linux machine and see if that works.

Make sure you have dnsmasq installed on Linux machine (e.g. Ubuntu / Debian)

sudo apt-get install dnsmasq

In the tftp-root, create sub folder under chrome-bot using the model name. For example, morphius model should be /var/tftp/chrome-bot/morphius/.

Copy the netboot kernel into tftp model folder with name vmlinuz.

If you are setting up with a factory zip, the netboot kernel is in path factory_shim/netboot/vmlinuz. So you have to copy it into tftp model folder manually. For example:

cp factory_shim/netboot/vmlinuz /var/tftp/chrome-bot/morphius/vmlinuz

If you are setting up with a factory bundle (prepared by finalize_bundle command and is usually a tar.bz2 archive), the tftp folder is already prepared in netboot/tftp. So you have to copy everything to your tftp root, or start dnsmasq server from there. For example:

cp -r netboot/tftp/* /var/tftp/

Note: Some boards might call vmlinuz as “vmlinux.bin”.

Start DHCP & TFTP server

sudo dnsmasq -d -C /var/tftp/dnsmasq.conf

Prepare Images

You should download the recovery image, test image and factory.zip from CPFE in following steps,

Click Image Files on the left
Select board (e.g. zork)
Select Image type
- RECOVERY_IMAGE for recovery image (signed)
- TEST_IMAGE_ARCHIVE for test image
- FACTORY_IMAGE_ARCHIVE for factory.zip

As we mentioned above, you can extract netboot firmware and vmlinuz from factory.zip. Or, if you'd like to use a specific version of firmware, you can download it by selecting FIRMWARE_IMAGE_ARCHIVE in above steps.

If you want to build the netboot kernel from source, do this inside chroot:

# You can skip the `cros-workon` and `emerge` commands if you don't have
# local changes.
cros-workon-${BOARD} factory factory_installer

# Replace ${VER} with the kernel version used by your board, and you only need
# to build the kernel once. If you don't know the version number, run
# `emerge-${BOARD} -pv virtual/linux-sources`.
emerge-${BOARD} chromeos-kernel-${VER}
emerge-${BOARD} factory factory_installer

cd ~/trunk/src/scripts
./build_images --board "${BOARD}"
./make_netboot.sh --board "${BOARD}"

# If you need to add any USE flags while building kernel, add USE flags to
# environment variables:
USE="..." ./make_netboot.sh --board "${BOARD}"

And find the netboot kernel in ../build/images/${BOARD}/latest/netboot/vmlinuz.

Deploy to Factory Server

You have to first setup a Factory Server and create a project. When ready, login to the Dome web interface, select your project and upload the images for deployment.

To do that, you can create a complete Bundle, or deploy only files you need to testing and development.

Required resources:
- test_image: Extracted from TEST_IMAGE_ARCHIVE
- release_image: RECOVERY_IMAGE
- toolkit: Extracted from FACTORY_IMAGE_ARCHIVE/toolkit
- netboot_kernel: Extracted from FACTORY_IMAGE_ARCHIVE/factory_shim/netboot
complete, project_config, hwid, and firmware are optional resources, but hwid and firmware are required in typical factory scenarios.

Netboot Settings

This section introduces the settings during the netboot process and how to customize them.

Factory server IP

The factory server (Umpire) IP (e.g., http://192.168.200.1:8080) for the netboot kernel to download the image files. To specify this, you can do one of the following approaches:

Append omahaserver=http://192.168.200.1:8080 in the kernel boot options (cmdline).
Use --factory-server-url argument in netboot_firmware_settings.py. This will append the argument (omahaserver=) in kernel boot options (cmdline) for you.
Create a omahaserver_${BOARD}.conf under the TFTP root (for example, ${TFTP_ROOT}/omahaserver_zork.conf), and set its content to the factory server IP (for example, http://192.168.200.1:8080). This will override the omahaserver settings in kernel boot options.

Default value: CHROMEOS_AUSERVER (typically http://10.0.0.1:8080).

This is a required argument for netboot process.

TFTP server IP for firmware

The TFTP server IP to download the netboot kernel and kernel boot option file (cmdline) from. To specify this, you can do one of the following approaches:

Modify the siaddr field (next-server) in DHCP message.
Use --tftpserverip argument in netboot_firmware_settings.py. This will override the setting in DHCP message.

Default value: IP address of DHCP server.

This is a required argument if you have separate DHCP server and TFTP server.

Board name (Optional)

The board name of the device. This changes the path to the factory server IP config file on TFTP server (${TFTP_ROOT}/omahaserver_${BOARD}.conf), so you can override the board name to connect to different factory server. To specify this, you can do one of the following approaches:

Append cros_board=${BOARD} in the kernel boot options (cmdline).
Use --board argument in netboot_firmware_settings.py. This will append the argument (cros_board=) in kernel boot options (cmdline) for you.

Default value: board name (CHROMEOS_RELEASE_BOARD) of the device.

Netboot kernel boot options (Optional)

The boot options (cmdline) for the netboot kernel. To specify this, you can do one of the following approaches:

Use --kernel_arg argument in netboot_firmware_settings.py.
Upload netboot_cmdline resource to the factory server. **Note that this will override all boot options specified by netboot_firmware_settings.py. **

Default value:

lsm.module_locking=0 cros_netboot_ramfs cros_factory_install cros_secure cros_netboot

See Debugging and Skip Complete Prompt section for more information on kernel boot options.

TFTP server IP for netboot kernel (Optional)

If you want to specify a different TFTP server IP for downloading omahaserver_${BOARD}.conf in netboot kernel, you can append tftpserverip=${TFTP_SERVER_IP} into the kernel boot options (cmdline).

Default value: TFTP server IP

Path to netboot kernel (Optional)

The path to the netboot kernel on TFTP server. To Specify this, you can do one of the following approaches:

Modify the file (filename) field in DHCP message.
Use --bootfile argument in netboot_firmware_settings.py. This overrides the setting in the DHCP message.

Default value: chrome-bot/{MODEL}/vmlinuz.

Path to netboot kernel boot options file (Optional)

The path to the netboot kernel boot options file (cmdline) on TFTP server. You can specify this by --argsfile argument in netboot_firmware_settings.py.

Default value: chrome-bot/{MODEL}/cmdline.

Debugging

Create an additional cmdline in TFTP model folder to override default kernel boot options. For example, in ${TFTP_ROOT}/chrome-bot/morphius/cmdline:

lsm.module_locking=0 cros_netboot_ramfs cros_factory_install cros_secure
cros_netboot tftpserverip=192.168.200.2 console=ttyS2,115200n8 loglevel=7
earlyprintk cros_debug

Note all optional must be in one line. No newline is allowed.

The console parameter might be different from board to board, if you are not sure which should be used, please refer to “Care & Feeding” document for your project.

Skip Complete Prompt

By default, you'll see a big “OK” when the netboot completes, and you have to press ENTER key to reboot into the factory software.

If you want to disable the prompt, you can append nocompleteprompt to the kernel boot options (cmdline).

Setting GBB Flags

When the device boots from vmlinuz, vmlinuz will connect to factory server and download images from the server. chromeos-firmwareupdate will be extracted from recovery image, which will be used to install the real firmware. chromeos-firmwareupdate will preserve the GBB flag from netboot firmware. Therefore, you might need to change GBB flag of netboot firmware.

If you are setting up with a factory zip, the netboot image is in path factory_shim/netboot/image-${MODEL}.net.bin. You can change GBB flag by futility. For example,

futility gbb -s --flags 0x1239 factory_shim/netboot/image-${MODEL}.net.bin

If you are setting up with a factory bundle, the netboot image is in path netboot/image-${MODEL}.net.bin. You can change GBB flag by futility. For example,

futility gbb -s --flags 0x1239 netboot/image-${MODEL}.net.bin