ephemerd runs CI jobs from pull requests. One of the rules is that a job can reach the internet but not my LAN. On Linux that’s a few iptables rules in the FORWARD path, and I didn’t think about it much. On Windows I spent about a week finding out how many ways there are to write a firewall rule that Windows accepts, lists and never applies.

I have CI that needs Linux amd64, Linux arm64, Windows and macOS. GitHub-hosted runners for all four get expensive fast, especially the arm64 and Mac minutes. Self-hosted runners fix the bill and create a different problem. A stock actions/runner on a box you own runs whatever the workflow says. It keeps whatever the last job left behind. And it’s one fork PR away from somebody running code on your LAN.

I’ve been building ephemerd, a daemon that runs self-hosted GitHub Actions runners with a fresh environment per job. On a Windows host, Windows jobs run in Hyper-V isolated containers. Linux jobs still need a Linux kernel from somewhere. The obvious answer on Windows in 2026 is WSL2, and that’s what I used first. It worked great from my terminal and fell over the moment ephemerd became a real Windows service.