Getting docker to start on boot
In this post, we look at what is needed to get docker running from initial boot of the device. This is the first step in making the device self-contained.
Docker needs a network
Docker really likes having a properly configured network before it starts, so all we need to do is make sure that the g_ether module loaded at boot time. Luckily, the people over at yocto towers have thought of this. There are two ways to achieve this, the simple one where we simply add this to our local.conf file.
echo KERNEL_MODULE_AUTOLOAD += " g_ether" >> conf/local.conf
But this would mean thay anybody who wanted to build this project would need to add that configuration to their .local.conf as well, so my preferred approach is to add it to our pre-existing linux-stable_5.0.bbappend in the meta-usbbuilder project. I have added this to the git repo here.
The next step to to have the network auto-configure. We need to edit the /etc/network/interfaces file to remove auto eth0 to prevent the wired ethernet from coming up, and add auto usb0 to bring up the gadget USB ethernet interface. The best way to achieve this is with a do_install_append function added to an init-ifupdown_1.0.bbappend because the init-ifupdown package is responsible for adding the interfaces file to our image.
This has also been added to the meta-usbbuilder project and you can browse the changes here.
In order to get the DNS entry being processed properly, we need to add the resolveconf package to our distro. This has been done here by adding it to the core-image-minimal.bbappend.
Entropy - Entropy - Entropy
Experience tells me that the CPU backed random number entropy on the ODroid-c2 is not up to the job required by the container runtime, so we need to give it a little helping hand.
# You can check your RNG Entropy by executing this command
cat /proc/sys/kernel/random/entropy_avail
37
anything less than ~200 is a problem. I am showing 37!
So I have added the rng-tools package that enables the use of hardware random number generators that will improve the entropy hugely! I followed the process outlined above by adding the package to the meta-usbbuilder here
There are also a couple of fixes in the runtime configuration in the overlay recipes found here that disables the jitter driver and fixes up an issue with the init script generation.
Rebuild core-image-minimal and push the resulting image on the sd-card and test.
# Test RNG Entropy once the device has been updated
cat /proc/sys/kernel/random/entropy_avail
4035
As you can see, my entropy is now 4035… thats much more acceptable :¬)
Conclusion time
After a bit of bitbake magic, we now have the gadget starting all the services I need at boot time, another step in the right direction.
Next steps will be to configure docker to accept remote connections, and in the back of my mind I am really wanting to move to systemd, but that probably says more about my mental health than anything else.