Phoenix in Production on EC2 not rendering in HTTPS with AWS Load Balancer

I have followed this tutorial to set up my phoenix app on EC2, and later I added the load balancer for SSL.

I used ACM (Amazon Certificate Manager) to get the public certificate and applied on the Amazon Load Balancer (ALB).

I'm still a bit fuzzy on the port mapping, so I suppose it might be the cause.

# config/prod.exs

host = System.get_env("HOST") || "example.com"

config :app_web, AppWeb.Endpoint,
  force_ssl: [rewrite_on: [:x_forwarded_proto]],
  load_from_system_env: true,
  http: [port: 80],
  url: [host: host, port: 80],
  url: [host: host, port: 443, scheme: "https"],
  server: true,
  secret_key_base: System.get_env("SECRET_KEY_BASE")

# docker-compose.yml
version: '2'
services:
  kroo:
    image: [image url]
    environment:
      - HOST=0.0.0.0
    ports:
      - '443:443'
      - '80:80'

$ docker ps
PORTS
0.0.0.0:80->80/tcp, 0.0.0.0:443->443/tcp

$ docker logs 
01:56:30.177 [info] Running AppWeb.Endpoint with cowboy 2.7.0 at 0.0.0.0:80 (http)
01:56:30.177 [info] Access AppWeb.Endpoint at https://example.com
Running Release tasks
[]
01:56:31.316 [info] Already up
01:56:33.085 [info] Plug.SSL is redirecting GET / to https://example.com with status 301

When I don't include force_ssl: [rewrite_on: [:x_forwarded_proto]], I'm able to have the page displayed fine in http, but when I include force_ssl, it redirects the https which is working fine, but I'm getting unable to connect error.

My confusion is that, since the load balancer is taking care of the SSL, I don't have the key and the certificate for SSL, which is why I don't have https: [] option in prod.exs.

Could someone point out what I'm doing wrong here?

Thanks

UPDATE: I finally got it working, below is my working configs in case anyone would find it helpful.

# config/prod.exs

# https config is not needed since ALB is handling the SSL
# Phoenix app serving in http is fine
config :app_web, AppWeb.Endpoint,
  load_from_system_env: true,
  http: [port: 8080],
  url: [host: "example.com"],
  server: true,
  secret_key_base: System.get_env("SECRET_KEY_BASE")

# docker-compose.yml
# map phoenix port 8080 to docker 8080
  ports:
    - '8080:8080'

Since I'm not providing SSL certificates, but I still want to force ssl, like @jamesvl suggested in answer, use your load balancer to redirect http traffic to https.

If you need help setting up SSL on ALB, I followed this guide

If somehow your app still not showing up under your domain, make sure that you have an A Record with an alias map to the DNS name of your load balancer

Solution

I would suggest setting the listen port of your docker container to something other than 80, and don't listen on 443 at all.

Rationale

I think the issue may lie in the fact that your http: configuration is listening on port 80.

With force_ssl: enabled, you're indicating that you want http connections to go to port 443, but when something arrives on 443 (via the load balancer), you send it to your (listening) port 80... which redirects it back to 443?

Fix

Let Phoenix listen on an arbitrary port (say... 4010) for http only connections. (Since the load balancer does your SSL termination, all your communication with the load balancer will be over http.) This involves changing your Docker container to forward connections to that port as well - you don't want to listen on 80 or 443 at all in your container.

Your url: configuration would then be looking only at headers, redirecting http requests to https as needed.

By the way, Amazon's ALB can also do 80 -> 443 redirection for you if you setup the rules; this saves Phoenix from even having to have a config url: setup for port 80 at all