Setting up a Pi as a Wifi access point
First, why am I doing this? The simple answer is, it's fun A better answer is, I can set up a mesh wifi, so that any device can access the wifi without having to choose which access point to use. I can do that by purchasing mesh wifi routers, at a couple of hundred pounds each, or I can use a Pi3 costing £30, and several Pi Zero Ws, costing £10 each. Plus, it's more fun.
You'll need a Raspberry pi. I used a 3B, but I think you can do it with any Pi.
First, download the OpenWrt image from here. I downloaded 19.07.7, because it was the latest one at the time. For a Pi, you want I had a Pi 3B. I don't know whether to use "Install" or "Upgrade" so I used "Upgrade". I think you're supposed to use "Upgrade" if you upgrade your version, "Install" for a new install.
I gunziped the download. That gave me an image, which I "burned" onto an SD card. I used the smallest card I have, 8gb, because the whole image is way less than that. I use dcfldd for this, which is like dd, but nicer.
I put the SD card into the Pi, and started it up. That worked. But the IP was 192.168.1.1, and my network is 10.x.x.x. I could either set up a computer that was in the 192.168 range, or ... you'll see. So I set up an old Windows Vista laptop in the 192.168 range, and pointed the browser at 192.168.1.1, and the OpenWrt user interface popped up. But whether I used Safari or Internet Explorer, it wouldn't accept as permanent, any changes I made to OpenWrt.
So I went the other way. On the Pi, I did this:
# uci set network.lan.ipaddr='10.168.1.1'
# uci set network.lan.netmask='255.0.0.0'
# uci set network.lan.proto='static'
# uci commit
# /etc/init.d/network restart
That changed the IP address to be in the 10 range, and that meant I could access the Pi from any of my computers. And then I could, using the OpenWrt configuration interface, give it a DNS server and a gateway, which would thus allow the Pi to access the whole of the internet.
By default, there is no password. So I created one - I don't want strangers to be able to log in to my Wifi router to change the configuration.