Walkthrough: Turning a Raspberry Pi into an appliance

time to read 7 min | 1340 words

I’m currently playing with a Secret Project (code-named Hugin right now) and for that purpose, I literally ordered all the available Raspberry Pi in Israel. That last statement sounds like a joke, but we checked six to eight places, and our order quantity exceeds the inventory in the country. They are flying the units to us as you read this.

I would love to hear what you think I’m doing, by the way. Please share  your thoughts on the matter in the comments.

For Hugin, I’m playing with Pi Zero 2 W, which is about the size of a lighter. They are small, and somewhat underpowered, but really cool. They also run RavenDB surprisingly well, but I’ll touch on that in a later post.

The drawback of the Zero is that basically it has two ports: a micro USB and a mini-HDMI. There is also a micro USB for power, but for doing stuff with it, just those two ports. If you are like me, you have more micro USB power cables than you know what to do with. However, micro USB on-the-go connectors or mini-HDMI are far rarer these days.

I want this to be useful and easy, so I started thinking about how I could make it simpler to work with. Then I realized that the Zero model I’m using (2 W) has built-in wifi, and that meant that I could start getting smart. The idea is that we can turn the Zero into an access point, so all you’ll need is to plug it into power (using a micro USB cable you likely already have), wait half a minute, and connect to the machine.

Once I had the idea, I delved deep into figuring out how to make it work. I managed, and the entire process is pretty simple from a user perspective, but it was anything but to make it work.

For the rest of this post, I will be working with the Raspberry Pi Zero 2 W, using Raspberry Pi OS Lite (Legacy, 32 bits) (Debian Bullseye). I tested this on a range of Pis (I apparently got lots, from Raspberry Pi 3 B to the Raspberry Pi 400), and it worked on everything I tried.

I actually tried quite hard to get it working on the Raspberry Pi OS (the non-legacy, which is Debian Bookworm). However, I couldn’t get it to behave the way I wanted it to. Setting up a wifi hotspot on Bookworm is easy, but getting it to bind DNS and DHCP to a particular device was beyond my capabilities.

From my reading, it doesn’t look like I’m the only one running into issues here.

The basic idea is that on connecting to a WiFi network, most devices will check connectivity and display the captive portal page if needed. In this case, we simply provide the captive portal page to our application. Hence, the only thing you need to do is to connect to the hotspot, and everything else is handled for you.

This blog post was really helpful figuring things out.

How this works, however, is a whole other matter. I’m assuming that you are running on a clean slate, booting for the first time on the clean image of Raspberry Pi Lite (Bullseye). The first thing to do is to set up the wifi, DNS, and DHCP, like so:

sudo raspi-config nonint do_wifi_country IL
sudo rfkill unblock wifi
sudo apt-get install -y nginx dnsmasq dhcpcd

We first set up the country for wifi, unblock it, and install nginx,dnsmasq and dhcpcd. Our next step it update /etc/wpa_supplicant/wpa_supplicant.conf to create the actual hotspot:

ctrl_interface=DIR=/var/run/wpa_supplicant GROUP=netdev

We define the MyHotSpot network as an open (key_mgmt=NONE) access point (mode=2). We need to plug this into the DHCP configuration in /etc/dhcpcd.conf:

option rapid_commit
option domain_name_servers, domain_name, domain_search, host_name
option classless_static_routes
option interface_mtu
require dhcp_server_identifier
slaac private

env wpa_supplicant_conf=/etc/wpa_supplicant/wpa_supplicant.conf
interface wlan0
static ip_address=

The last part is the most important bit. We pull the wpa_supplicant configuration that we previously defined, apply it to the WiFi device (wlan0), and register a static IP for that interface. Basically, the WiFi interface will use that IP address as the gateway for clients connecting to it. Those clients need to get their own IP addresses, and that is the role of dnsmasq (no idea why it isn’t a dhcpcd that does it, it’s literally in the name). Here is the relevant configuration file /etc/dnsmasq.conf:


# Resolve everything to the portal's IP address.

# Android Internet Conectivity Test Domains

There is a lot going on here. We define the DHCP range from which clients will get their IPs and set the router for this connection. We also define option 114 (and 160, which is a legacy one) to instruct the client that it needs to first visit that URL before it connecting to the wider internet.

Finally, we set up DNS in such a way that all DNS entries go to the server, except for a certain set of known domains used by some Android phones to check for an internet connection. We’ll touch on that in a bit.

In short, all of this configuration basically tells the Zero to create a WiFi hotspot with IP, assign connected devices IP addresses in the range .., set  the DNS server for those devices to, and resolve any DNS query  to IP Also, if they care to, there is a specific URL users need to visit to get things started. In short, we are trying to guide the user to take us to the right place.

One problem we have, however, is that we didn’t set up anything to respond to HTTP requests. That is why we installed nginx earlier. We configure it using /etc/nginx/sites-available/default:

server {
    listen 80 default_server;
    listen [::]:80 default_server;
    server_name _;
    location / {
        return 302 http://awesome.appliance;
server {
  listen *:80;
  server_name awesome.appliance;

  root /var/appliance/web;
  autoindex on;

The idea here is simple. Everything before basically directs the client to the server, all domains go to it, etc. So when a connection comes, we tell nginx that it should return a 302 response (redirect) to the portal endpoint we have.

If the client is requesting the http://awesome.applianceaddress, however, we serve an actual website.

All of this together ends up with an open access point that, upon connection, will direct you to a web page. This is a walled garden, of course, since we assume that the Zero is connected only to the power.

Now that this is solved, you need to figure out what function you want the appliance to actually have.