If you have a Nest thermostat first or second generation they recently became dumber. Google decided to pull the plug on the servers that operate these devices, turning them into dumb thermostats. A bounty on fulu inspired codykociemba on github to start the NoLongerEvil-Thermostat project.
If you want to smarten up your thermostat again or you never had one because of privacy, you can now use Linux to self host and make the nest work for you and not you being a product for Google. The thermostat has a DFU-enabled OMAP loader. To access it, you have to plug it into USB and then reboot it. There is a narrow window for the loader to grab it, so you have to be running the software before you reboot or you’ll miss it.
After that, the flash is relatively fast, but the Nest will look dead for a brief time. Then the No Longer Evil logo will show. Now it redirects all the network traffic to a custom URL. If you don’t want to run your own server you can control your thermostat from the nolongerevil.com. The project is open source, but all the important bits are in a binary U-Boot image file, so it would take some work to validate it. To get you started, the command to dump the content is probably: dumpimage -T kernel -p 0 -o kernel uImage. Or, you could watch it with Wireshark for a bit.