A phoenix plug to prevent search engine on specific page

For a current project I needed to prevent search engine to index the admin dashboard.

There is different way to achieve that. The method I found the most conveniant with phoenix is to add a X-Robots-tag header using a plug.

The X-Robots-tag header can take several value the one we are intrested are:

noindex Do not show this page, media, or resource in search results. If you don't specify this directive, the page, media, or resource may be indexed and shown in search results.
nofollow Do not follow the links on this page. If you don't specify this directive, Google may use the links on the page to discover those linked pages. Learn more about nofollow.

First, create a new module in your phoenix app.

defmodule AppWeb.NoIndexPlug do
  @moduledoc """
  A Plug to add the the X-Robots-Tag: noindex, nofollow
  Read more on
  https://developers.google.com/search/docs/crawling-indexing/robots-meta-tag
  """

  @behaviour Plug

  def init(_params), do: nil

  def call(conn, _params) do
    Plug.Conn.put_resp_header(conn, "X-Robots-Tag", "noindex, nofollow")
  end
end

Add the plug in your router.

defmodule AppWeb.Router do

  ...

  pipeline :admin do
    plug AppWeb.NoIndexPlug
    plug :browser
  end

  ...

  scope "/admin", AppWeb do
    pipe_through :admin

    live "/", AdminLive.Index, :index

    ...
  end
end

Let's verify using a curl that the header is here. We use -I option to only return headers

curl -I http://localhost:4000/admin
> HTTP/1.1 200 OK
> X-Robots-Tag: noindex, nofollow
...

Sucess !

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