只需在应用端点顶部的重定向中输入plug即可。
在lib/app/endpoint.ex中：

defmodule App.Endpoint do
  use Phoenix.Endpoint, otp_app: :app

  socket "/socket", App.UserSocket

  plug App.Plugs.WWWRedirect
  # ...
end

在lib/app/plugs/www_redirect.ex中：

defmodule App.Plugs.WWWRedirect do
  import Plug.Conn

  def init(options) do
    options
  end

  def call(conn, _options) do
    if bare_domain?(conn.host) do
      conn
        |> Phoenix.Controller.redirect(external: www_url(conn))
        |> halt
    else
      conn # Since all plugs need to return a connection
    end
  end

  # Returns URL with www prepended for the given connection. Note this also
  # applies to hosts that already contain "www"
  defp www_url(conn) do
    "#{conn.scheme}://www.#{conn.host}"
  end

  # Returns whether the domain is bare (no www)
  defp bare_domain?(host) do
    !Regex.match?(~r/\Awww\..*\z/i, host)
  end
end

请注意，由于lib中的任何内容都没有重新加载，因此需要重新启动服务器才能使其生效。

您还可以使用plug_canonical_host来处理它，并确保您的Elixir应用程序只能通过其规范URL访问。

为了完成fny的anser，我将添加完整的请求路径和查询字符串：

defmodule App.Plugs.WWWRedirect do
  import Plug.Conn

  def init(options) do
    options
  end

  def call(conn, _options) do
    if bare_domain?(conn.host) do
      conn
      |> Phoenix.Controller.redirect(external: www_url(conn))
      |> halt
    else
      # Since all plugs need to return a connection
      conn
    end
  end

  # Returns URL with www prepended for the given connection. Note this also
  # applies to hosts that already contain "www"
  defp www_url(%{query_string: ""} = conn) do
    "#{conn.scheme}://www.#{conn.host}#{conn.request_path}"
  end

  defp www_url(conn) do
    "#{conn.scheme}://www.#{conn.host}#{conn.request_path}?#{conn.query_string}"
  end

  # Returns whether the domain is bare (no www)
  defp bare_domain?(host) do
    !Regex.match?(~r/\Awww\..*\z/i, host)
  end
end