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A plug to force SSL connections.

If the scheme of a request is `https`, it'll add a `strict-transport-security`
header to enable HTTP Strict Transport Security.

Otherwise, the request will be redirected to a corresponding location
with the `https` scheme by setting the `location` header of the response.
The status code will be 301 if the method of `conn` is `GET` or `HEAD`,
or 307 in other situations.

## x-forwarded-proto

If your Plug application is behind a proxy that handles HTTPS, you will
need to tell Plug to parse the proper protocol from the `x-forwarded-proto`
header. This can be done using the `:rewrite_on` option:

    plug Plug.SSL, rewrite_on: [:x_forwarded_proto]

The command above will effectively change the value of `conn.scheme` by
the one sent in `x-forwarded-proto`.

Since rewriting the scheme based on `x-forwarded-proto` can open up
security vulnerabilities, only provide the option above if:

  * your app is behind a proxy
  * your proxy strips `x-forwarded-proto` headers from all incoming requests
  * your proxy sets the `x-forwarded-proto` and sends it to Plug

## Options

  * `:rewrite_on` - rewrites the scheme to https based on the given headers
  * `:hsts` - a boolean on enabling HSTS or not, defaults to `true`
  * `:expires` - seconds to expires for HSTS, defaults to `31_536_000` (a year).
  * `:preload` - a boolean to request inclusion on the HSTS preload list
     (for full set of required flags, see:
     [Chromium HSTS submission site](,
    defaults to `false`
  * `:subdomains` - a boolean on including subdomains or not in HSTS,
    defaults to `false`
  * `:host` - a new host to redirect to if the request's scheme is `http`,
    defaults to ``. It may be set to a binary or a tuple
    `{module, function, args}` that will be invoked on demand

## Port

It is not possible to directly configure the port in `Plug.SSL` because
HSTS expects the port to be 443 for SSL. If you are not using HSTS and
wants to redirect to HTTPS on another port, you can sneak it alongside
the host, for example: `host: ""`.
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