Using SSL

To prepare an application to serve requests over SSL, we need to add a little bit of configuration and two environment variables. In order for SSL to actually work, we’ll need a key file and certificate file from a certificate authority. The environment variables that we’ll need are paths to those two files.

The configuration consists of a new https: key for our endpoint whose value is a keyword list of port, path to the key file, and path to the cert (pem) file. If we add the otp_app: key whose value is the name of our application, Plug will begin to look for them at the root of our application. We can then put those files in our priv directory and set the paths to priv/our_keyfile.key and priv/our_cert.crt.

Here’s an example configuration from config/prod.exs.

use Mix.Config

. . .
config :hello, HelloWeb.Endpoint,
  http: [port: {:system, "PORT"}],
  url: [host: ""],
  cache_static_manifest: "priv/static/cache_manifest.json",
  https: [port: 443,
          otp_app: :hello,
          keyfile: System.get_env("SOME_APP_SSL_KEY_PATH"),
          certfile: System.get_env("SOME_APP_SSL_CERT_PATH"),
          cacertfile: System.get_env("INTERMEDIATE_CERTFILE_PATH") # OPTIONAL Key for intermediate certificates

Without the otp_app: key, we need to provide absolute paths to the files wherever they are on the filesystem in order for Plug to find them.

Path.expand("../../../some/path/to/ssl/key.pem", __DIR__)

Forcing requests to use SSL:

In many cases, you’ll want to force all incoming requests to use SSL by redirecting http to https. This can be accomplished by setting the :force_ssl option in your endpoint. It expects a list of options which are forwarded to Plug.SSL. By default it sets the “strict-transport-security” header in https requests, forcing browsers to always use https. If an unsafe request (http) is sent, it redirects to the https version using the :host specified in the :url configuration. To dynamically redirect to the host of the current request,:host must be set nil. For example:

  config :my_app, MyApp.Endpoint,
    force_ssl: [rewrite_on: [:x_forwarded_proto]]

Releasing with Exrm:

In order to build and run a release with exrm, make sure you also include the ssl app in mix.exs:

def application do
    [mod: {Hello, []},
    applications: [:phoenix, :phoenix_html, :cowboy, :logger, :gettext,
                 :phoenix_ecto, :postgrex, :ssl]]

Else you might run into errors: ** (MatchError) no match of right hand side value: {:error, {:ssl, {'no such file or directory', ''}}}