Setting for apps behind a load balancer

Phoenix Forum Questions / Help
1

Hi,

I’m trying to understand better how to configure the app for production. My DevOps skills are somewhat limited, so bare with me if I write something dumb :slight_smile:

We have the following setup:

  • Domain: www.super-app.com
  • There is a load balancer, which proxy passes to the app on port 4000
  • The app has an internal DNS name. super-app.internal.cloud

Now, I’m wondering what the correct setting is:

In production.exs we have this:

config :super_app, SuperAppWeb.Endpoint,
  # This is critical for ensuring web-sockets properly authorize.
  url: [host: "localhost", port: 4000]

What is the correct setting here? Should put the public domain in there? I’m 99% sure that yes, but want to double check.

2

The url config is used by Phoenix to generate any public facing URLs. So this must be indeed your public domain name. In a similar fashion, I have the following config for my Phoenix app on Heroku (and thus also serving behind a load balancer);

config :example, ExampleWeb.Endpoint,
  url: [scheme: "https", host: System.fetch_env!("HOST"), port: 443],
  http: [port: String.to_integer(System.get_env("PORT", "4000"))]

The HOST option is my domain name and I only configure the PORT on which Phoenix’s server listens to.

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3

Assuming the LB terminates TLS: don’t forget to make your Phoenix app aware that requests were actually made over HTTPS, based on the X-Forwarded-Proto (or similar) header. Otherwise Plug.Session will believe the request was made over HTTP and won’t set the ‘secure’ flag on session cookies (which would be bad).

The force_ssl endpoint option can be used for that, as explained here and here.

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4

If the LB forwards the HOST header it will be taken from the conn and doesn’t need to be hardcoded in the config.

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