It’s possible to pass fd option to :gen_tcp.listen: gen_tcp — OTP 29.0.2 (kernel 11.0.2). So something like

# not sure about this ...
config :app, App.Endpoint,
  http: [transport_options: %{socket_opts: [fd: SYSTEMD_FIRST_SOCKET_FD]}]

might work.

So working from bottom up,

• :gen_tcp’s fd option would probably make it work
• ranch accepts socket options via :socket_opts
• plug uses :transport_options to configure that part of :ranch
• phoenix uses :http option to configure that part of plug

There are plenty of solutions to bypass the problem I’m tackling (like the one you are offering), yet in that case, I want specifically to try to solve the systemd activation thing & see if we can make it work.

Doing this because I think nginx + systemd will remain quite a common solution for entry-level deployments.

Thanks for the proposition, though, this is still helpful (& feel free to share your Envoy setup in a separate thread, by the way, it can definitely be interesting!).

I will experiment the coming week, on a client setup for which I need to do maintenance (Ansible + Elixir). Will report back with my findings!