How to assign subdomain with elixir?

Questions / Help
1

Hello all,
Q1) how can I assign subdomain to a user from elixir code ( say when he want to create a different instance of an app)?
Q2) How can I query whether a particular subdomain exists or not?
I found “dig” could be a possible solution, just wanna be sure.
thanks.

2

how can I assign subdomain to a user from elixir code ( say when he want to create a different instance of an app)?

Save their subdomain into your database and reference it to the user.

How can I query whether a particular subdomain exists or not?

Ask the database.

The plugs for this could look like (adapted from Subdomains With Phoenix - Gazler)

defmodule MyApp.Plugs.SubdomainExtractor do
  @moduledoc "Extracts subdomains and puts them into `conn.private`"
  @behaviour Plug
  import Plug.Conn, only: [put_private: 3]

  def init(_opts) do
    MyApp.Endpoint.config(:url)[:host]
  end

  def call(%Plug.Conn{host: host} = conn, root_host) do
    case extract_subdomain(host, root_host) do
      subdomain when byte_size(subdomain) > 0 ->
        put_private(conn, :subdomain, subdomain)
      _ ->
        conn
    end
  end

  defp extract_subdomain(host, root_host) do
    String.replace(host, ~r/.?#{root_host}/, "") # not the most efficient way
  end
end

and then some plug down the line looks up the subdomain in conn.private[:subdomain] in your database and assigns it to a user.

defmodule MyApp.Plugs.SubdomainMatcher do
  @moduledoc "Matches a subdomain in `conn.private` to a user"
  @behaviour Plug
  import Plug.Conn, only: [assign: 3]

  def init(opts), do: opts

  def call(%Plug.Conn{private: %{subdomain: subdomain}}}, _opts) do
    case MyDB.Accounts.lookup_for_subdomain(subdomain) do
      %Account{user_id: user_id, id: account_id} ->
        conn
        |> assign(:user_id, user_id)
        |> assign(:account_id, account_id)

      _not_found ->
        conn # or return 403
    end
  end
  def call(conn, _opts) do
    conn
  end
end

That is, if you need this functionality.

3 Likes
3

Q1) how can I assign subdomain to a user from elixir code ( say when he want
to create a different instance of an app)?

An actual “subdomain” with a resolvable address? You need to create a
DNS entry for that.

Q2) How can I query whether a particular subdomain exists or not?

:inet.gethostbyname('something.example.com', :inet) would work,
though if you have access to the DNS provider for the main domain you
should be able to see the existing entries directly.

I would probably cache them locally for convenience and speed but that
could require handling synchronization if other processes have write
access.

1 Like
4

An actual “subdomain” with a resolvable address? You need to create a
DNS entry for that.

If all subdomains point to the same server, a wildcard DNS record should probably be enough.

2 Likes
5

Thanks for the description sir, I’ll try this and let u if I will be able to make it work or if got any more doubt.:sweat_smile:
Thanks again☺️