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PlugCanonicalHost ensures that all requests are served by a single canonical host.
It will redirect all requests from non-canonical hosts to the canonical one.

Installation

Add plug_canonical_host to the deps function in your project’s mix.exs file:

defp deps do
  [
    …,
    {:plug_canonical_host, "~> 2.0"}
  ]
end

Then run mix do deps.get, deps.compile inside your project’s directory.

Usage

PlugCanonicalHost can be used just as any other plugs. Add PlugCanonicalHost before all of the other plugs you want to happen after successful redirect to your canonical host.

The recommended way to define a canonical host is with an environment variable.

# config/releases.exs
config :my_app,
  canonical_host: System.get_env("CANONICAL_HOST")

# lib/my_app/endpoint.ex
defmodule MyApp.Endpoint do
  plug(:canonical_host)

  defp canonical_host(conn, _opts) do
    :my_app
    |> Application.get_env(:canonical_host)
    |> case do
      host when is_binary(host) ->
        opts = PlugCanonicalHost.init(canonical_host: host)
        PlugCanonicalHost.call(conn, opts)

      _ ->
        conn
    end
  end
end

For example, if your CANONICAL_HOST is www.example.com but your application is accessible via both example.com and www.example.com, all traffic coming through example.com will be redirected (with a 301 HTTP status) to the matching www.example.com URL.

$ curl -sI "http://example.com/foo?bar=1"
#> HTTP/1.1 301 Moved Permanently
#> Location: http://www.example.com/foo?bar=1

Allow multiple hosts

If you want to support multiple hosts, you can make them skip the canonical host behavior but you’re still going to have to specify a canonical host for unknown hosts redirects.

# config/releases.exs
config :my_app,
  canonical_host: System.get_env("CANONICAL_HOST"),
  passthrough_hosts: String.split(System.get_env("PASSTHROUGH_HOSTS"), ",")

# lib/my_app/endpoint.ex
defmodule MyApp.Endpoint do
  plug(:canonical_host)

  defp canonical_host(%Plug.Conn{host: host} = conn, _opts) do
    canonical_host = Application.get_env(:app, :canonical_host)
    passthrough_hosts = Application.get_env(:my_app, :passthrough_hosts)

    cond do
      host in passthrough_hosts ->
        conn
      is_binary(canonical_host) ->
        opts = PlugCanonicalHost.init(canonical_host: canonical_host)
        PlugCanonicalHost.call(conn, opts)
      true ->
        conn
    end
  end
end

Let’s say we have CANONICAL_HOST=www.example.com and PASSTHROUGH_HOSTS=foo.example.com,bar.example.com.

Now, all requests going through www.example.com, foo.example.com or bar.example.com path will skip the canonical host redirect behavior. Other hosts will redirect to www.example.com.

$ curl -sI "http://example.com/foo?bar=1"
#> HTTP/1.1 301 Moved Permanently
#> Location: http://www.example.com/foo?bar=1

$ curl -sI "http://foo.example.com/foo?bar=1"
#> HTTP/1.1 200 OK

$ curl -sI "http://bar.example.com/foo?bar=1"
#> HTTP/1.1 200 OK

$ curl -sI "http://www.example.com/foo?bar=1"
#> HTTP/1.1 200 OK

Exclude certain requests

If you want to exclude certain requests from redirecting to the canonical host, you can use simple pattern matching in your function arguments:

# config/releases.exs
config :my_app,
  canonical_host: System.get_env("CANONICAL_HOST")

# lib/my_app/endpoint.ex
defmodule MyApp.Endpoint do
  plug(:canonical_host)

  defp canonical_host(%Plug.Conn{request_path: "/ignore-me"} = conn, _opts) do
    Plug.Conn.send_resp(conn, 200, "👋")
  end

  defp canonical_host(conn, _opts) do
    :my_app
    |> Application.get_env(:canonical_host)
    |> case do
      host when is_binary(host) ->
        opts = PlugCanonicalHost.init(canonical_host: host)
        PlugCanonicalHost.call(conn, opts)

      _ ->
        conn
    end
  end
end

Now, all requests going to the /ignore-me path will skip the canonical host redirect behavior.

$ curl -sI "http://example.com/foo?bar=1"
#> HTTP/1.1 301 Moved Permanently
#> Location: http://www.example.com/foo?bar=1

$ curl -sI "http://example.com/ignore-me"
#> HTTP/1.1 200 OK

License

The plug logo is based on this lovely icon by Vectors Market, from The Noun Project. Used under a Creative Commons BY 3.0 license.

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