How to configure upstream proxy for federation

If you want to proxify all http requests (e.g. for TOR) that pleroma makes to an upstream proxy server, edit you config file (dev.secret.exs or prod.secret.exs) and add the following:

config :pleroma, :http,
  proxy_url: "127.0.0.1:8123"

The other way to do it, for example, with Tor you would most likely add something like this:

config :pleroma, :http, proxy_url: {:socks5, :localhost, 9050}

The Gun HTTP adapter supports SOCKS5. SOCKS4 proxies require another HTTP adapter. For regular HTTP proxies, Gun forwards plain HTTP requests directly and uses CONNECT tunnels for HTTPS requests.