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: "example.com"],
  cache_static_manifest: "priv/static/cache_manifest.json",
  https: [
    port: 443,
    cipher_suite: :strong,
    otp_app: :hello,
    keyfile: System.get_env("SOME_APP_SSL_KEY_PATH"),
    certfile: System.get_env("SOME_APP_SSL_CERT_PATH"),
    # OPTIONAL Key for intermediate certificates:
    cacertfile: System.get_env("INTERMEDIATE_CERTFILE_PATH")
  ]

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__)

The options under the https: key are passed to the Plug adapter, typically Plug.Cowboy, which in turn uses Plug.SSL to select the TLS socket options. Please refer to the documentation for Plug.SSL.configure/1 for more information on the available options and their defaults. The Plug HTTPS Guide and the Erlang/OTP ssl documentation also provide valuable information.

SSL in Development

If you would like to use HTTPS in development, a self-signed certificate can be generated by running: mix phx.gen.cert. This requires Erlang/OTP 20 or later.

With your self-signed certificate, your development configuration in config/dev.exs can be updated to run an HTTPS endpoint:

config :my_app, MyApp.Endpoint,
  ...
  https: [
    port: 4001,
    cipher_suite: :strong,
    keyfile: "priv/cert/selfsigned_key.pem",
    certfile: "priv/cert/selfsigned.pem"
  ]

This can replace your http configuration, or you can run HTTP and HTTPS servers on different ports.

Force 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 configuration. 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 (HTTP) request is sent, it redirects to the HTTPS version using the :host specified in the :url configuration. For example:

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

To dynamically redirect to the host of the current request, set :host in the :force_ssl configuration to nil.

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

In these examples, the rewrite_on: key specifies the HTTP header used by a reverse proxy or load balancer in front of the application to indicate whether the request was received over HTTP or HTTPS. For more information on the implications of offloading TLS to an external element, in particular relating to secure cookies, refer to the Plug HTTPS Guide. Keep in mind that the options passed to Plug.SSL in that document should be set using the force_ssl: endpoint option in a Phoenix application.

HSTS

HSTS or "strict-transport-security" is a mechanism that allows a website to declare itself as only accessible via a secure connection (HTTPS). It was introduced to prevent man-in-the-middle attacks that strip SSL/TLS. It causes web browsers to redirect from HTTP to HTTPS and refuse to connect unless the connection uses SSL/TLS.

With force_ssl: [hsts: true] set, the Strict-Transport-Security header is set with a max age that defines the length of time the policy is valid for. Modern web browsers will respond to this by redirecting from HTTP to HTTPS for the standard case but it does have other consequenses. RFC6797 which defines HSTS also specifies that the browser should keep track of the policy of a host and apply it until it expires. It also specifies that traffic on any port other than 80 is assumed to be encrypted as per the policy.

This can result in unexpected behaviour if you access your application on localhost, for example https://localhost:4000, as from that point forward and traffic coming from localhost will be expected to be encrypted, except port 80 which will be redirected to port 443. This has the potential to disrupt traffic to any other local servers or proxies that you may be running on your computer. Other applications or proxies on localhost will refuse to work unless the traffic is encrypted.

If you do inadvertently turn on HSTS for localhost you may need to reset the cache on your browser before it will accept any HTTP traffic from localhost. For Chrome you need to Empty Cache and Hard Reload which is available from the reload menu that appears when you click and hold the reload icon from the Developer Tools Panel. For Safari you will need to clear your cache, remove the entry from ~/Library/Cookies/HSTS.plist (or delete that file entirely) and restart Safari. Alternately you can set the :expires option on force_ssl to 0 which should expired the entry to turn off HSTS. More information on the options for HSTS are available at Plug.SSL.

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