Nerves.Firmware.SSH
This project contains the necessary infrastruction to support "over-the-air" firmware updates with Nerves by using ssh.
The default settings make it quick to integrate into Nerves projects for
development work. Later on, if your deployed devices can be reached by ssh
,
it's even possible to use tools like Ansible or even shell scripts to update a
set of devices all at once.
Installation
First, add nerves_firmware_ssh
to your list of dependencies in mix.exs
:
def deps do
[{:nerves_firmware_ssh, "~> 0.3"}]
end
Next, update your config/config.exs
with one or more authorized keys. These
come from files like your ~/.ssh/id_rsa.pub
or ~/.ssh/id_ecdsa.pub
that were
created when you created your ssh
keys. If you haven't done this, the following
article
may be helpful. Here's an example:
config :nerves_firmware_ssh,
authorized_keys: [
"ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAAAgQDBCdMwNo0xOE86il0DB2Tq4RCv07XvnV7W1uQBlOOE0ZZVjxmTIOiu8XcSLy0mHj11qX5pQH3Th6Jmyqdj",
"ssh-rsa AAAAB3NzaC1yc2EAAAADAQABAAACAQCaf37TM8GfNKcoDjoewa6021zln4GvmOiXqW6SRpF61uNWZXurPte1u8frrJX1P/hGxCL7YN3cV6eZqRiF"
]
Here's another way that may work well for you that avoids needing to commit your keys:
config :nerves_firmware_ssh,
authorized_keys: [
File.read!(Path.join(System.user_home!, ".ssh/id_rsa.pub"))
]
The first firmware bundle that you create after adding nerves_firmware_ssh
will need to be installed in a non-ssh way. The usual route is to burn a MicroSD
card for most targets, but you may have another way of getting the new image
onto the device.
Pushing firmware updates to devices
This project provides a mix
task for updating devices. Here's an example:
mix upload [destination IP or hostname]
This task uses the current mix
target and environment settings to find the
firmware bundle (.fw
file) to upload. It does not build it, so you will need
to run mix firmware
first.
The upload
task has some limitations especially if ssh
requires credentials.
If this task doesn't work for you, see the next section for
creating a shell script with the equivalent commands.
Pushing firmware updates to devices via commandline ssh
Regular commandline ssh
can push firmware to devices and is desirable if you
want to integrate firmware updates into other scripts or programs like Ansible.
To get started, run mix
to generate a starter shell script with the ssh
invocation:
mix firmware.gen.script
And then run:
./upload.sh [destination IP] [.fw file]
The destination IP and .fw file can frequently be guessed so the script attempts to do that for you.
See the nerves_firmware_ssh protocol section below and the ssh(1) man page for more details.
Troubleshooting
If you're not able to connect, try the following:
- Make sure that there's network connectivity and that you can ping the device.
- Check that the authorized keys are set correctly. On the target, run
Application.get_all_env(:nerves_firmware_ssh)
and compare keys. - Are you trying to use OpenSSH's
ssh-agent
? See ERL-469 and try not using it. - Add
:runtime_tools
to the:extra_applications
key in yourmix.exs
and run:ssh_dbg.messages()
on the target and try to connect. You should get some diagnostic data from the Erlang:ssh
application that may help. - File an issue
or try the
#nerves
channel on the Elixir Slack.
Oops, I have the wrong keys on the device
Do not panic! You can temporarily change them if you have access to the console. Here's what to do:
Application.stop(:nerves_firmware_ssh)
Application.stop(:ssh)
Application.put_env(:nerves_firmware_ssh, :authorized_keys, ["ssh-rsa AAAAB3NzaC1yc2EAA..."])
Application.start(:ssh)
Application.start(:nerves_firmware_ssh)
You should be able to update the devices firmware now.
Device keys
Devices also have keys. This prevents man-in-the-middle attacks. For
development, nerves_firmware_ssh
uses hardcoded device keys that are contained
in its priv
directory. The private key portion is also in the clear in
source control, so you should not rely on device authentication in this default
configuration. This is for convenience since man-in-the-middle attacks and
device authentication are usually not concerns for everyday development tasks.
If your device uses ssh
for other services (e.g., for providing a remote
command prompt), you'll likely want to use the same keys for both services. If
the /etc/ssh
directory exists in the device's root filesystem,
nerves_system_ssh
will automatically use keys from there. To generate them,
add a rootfs-additions directory to your project (see the Nerves
documentation
and run something like the following:
mkdir -p rootfs-additions/etc/ssh
ssh-keygen -t rsa -f rootfs-additions/etc/ssh/ssh_host_rsa_key
This setup also hardcodes the ssh server keys for all devices and keeps them in the clear, so it doesn't improve security, but makes working with devices more convenient since there's one set of keys.
Another method is to either symlink /etc/ssh
on the device to a writable
location on the device (Nerves devices have read-only root filesystems) or to
specify an alternative location for device keys in your config.exs
:
config :nerves_firmware_ssh,
authorized_keys: [
],
system_dir: "/mnt/device/ssh"
This requires that you add a manufacturing step to your device production that
creates a public/private key pair, writes it to your device in a hypothetical
/mnt/device
partition, and saves the public key portion. How to do this isn't
covered here.
The nerves_firmware_ssh protocol
nerves_firmware_ssh
makes use of the ssh
subsystem feature for operation.
This is similar to sftp
. The subsystem is named nerves_firmware_ssh
. See the
-s
option on ssh(1).
The data sent over ssh
contains a header and then the contents of one or more
.fw
files. The header is terminated by a newline (\n
) and is a comma
separated list of operations. Currently supported operations are:
Operation | Description |
---|---|
fwup($FILESIZE) | Stream $FILESIZE bytes to fwup on the device |
reboot | Reboot the device |
After the header, all data required by operations in the header is concatenated and streamed over. Here's an example header:
fwup(10000),reboot\n
For this case, 10,000 bytes of data should be sent after the header. That data
will be streamed into fwup
. After fwup
completes, the device will be
rebooted. If any error occurs with the fwup
step, processing stops and the
device will not be rebooted.
The data coming back from the server is the output of the invoked commands. This is primarily textual output suitable for reading by humans. If automating updates, this output should be logged to help debug update failures if any.
Example manual invocation
Use mix firmware.gen.script
to generate a script that's portable and has a few
workarounds for platforms. The general idea, though, is to do something like
this:
FILENAME=myapp.fw
FILESIZE=$(stat -c%s "$FILENAME")
printf "fwup:$FILESIZE,reboot\n" | cat - $FILENAME | ssh -s -p 8989 target_ip_addr nerves_firmware_ssh
License
All source code is licensed under the Apache License, 2.0.