During my most recent hospitalization, besides making a video about the IV pump, I was messing around with a Raspberry Pi 4G LTE router project I could stash in my hospital go-bag (I am hospitalized about 2.5x per year, on average).
It's been a busy start to the year on my Raspberry Pi PCIe Devices website. Not only have we finally made some significant progress learning about the BCM2711's PCIe bus (both good and bad), I've also added a few dozen new Raspberry Pi CM4-based boards to the site.
In my YouTube video today, I go through four of them in depth, showing how they're built and what they're used for.
The four projects I cover in depth are:
And there are some other projects I'm watching closely as they journey down the road towards production:
tl;dr: This website is currently being hosted off-grid, on a cluster of Raspberry Pis, via 4G LTE—or at some points through the same tunnel via WiFi if signal strength gets too low. Here's the GitHub repo for the project.
Note: The website was down for a few hours this morning, as shortly after this post I started getting a 40-50 Mbps flood of POST requests (over 6 million in a 30 minute time frame)... and yeah, no way the little Pi cluster could handle that. Thanks, Internet. It's back up through Cloudflare now, and I'll post more on this 'fun' experience later.
A couple weeks ago, after months of preparation, I took my 4-node Turing Pi 2 cluster (see my earlier review) to my cousin's farm, and ran this website (JeffGeerling.com) on it, live on the Internet—completely disconnected from grid power or hard-wired Internet.
For a recent project, I needed to add cellular connectivity to a Raspberry Pi (actually, an entire cluster... but that's a story for a future time!).
I figured I'd document the process in this blog post so people who follow in my footsteps don't need to spend quite as much time researching. This post is the culmination of 40+ hours of reading, testing, and head-scratching.
There doesn't seem to be any good central resource for "4G LTE and Linux" out there, just a thousand posts about the ABC's of getting an Internet connection working through a 4G modem—but with precious little explanation about why or how it works. (Or why someone should care about random terms like PPP, ECM, QMI, or MBIM, or why someone would choose qmi_wwan over cdc_ether, or ... I could go on).
Hopefully you can learn something from my notes. Or point out places where I'm glaringly wrong :)
As I start using Raspberry Pis for more and more network routing activities—especially as the Compute Module 4 routers based on Debian, OpenWRT, and VyOS have started appearing—I've been struggling with one particular problem: how can I set routing priorities for network interfaces?
Now, this is a bit of a loaded question. You could dive right into routing tables and start adding and deleting routes from the kernel. You could mess with subnets, modify firewalls, and futz with iptables.
But in my case, my need was simple: I wanted to test the speed of a specific interface, either from one computer to another, or over the Internet (e.g. via speedtest-cli).
The problem is, even if you try limiting an application to a specific IP address (each network interface has its own), the Linux kernel will choose whatever network route it deems the best.