I think you are looking for the _url helpers instead of the _path helpers. _path will be a relative path whereas _url will be the full URL.

def index(conn, _params) do
  #...
  |> render(to: Routes.dashboard_url(conn, :index))
end

Note that your code uses the non-put-router-url’d connection when call dashboard_url. So you need to rebind it, otherwise it is ignoring the previous plug calls:

def index(conn, _params) do
  url = %URI{host: conn.assigns.subdomain <> "." <> conn.host}
  conn = put_router_url(conn)
  redirect(conn, to: Routes.dashboard_url(conn, :index))
end

Thanks, Chris. Changing that has me a bit further. I am sure you knew I would run into problems once I started redirecting. I hope you might be able to help a little bit more.

As I mentioned above I am trying to add a subdomain after a user logs in (As they may be able to access multiple subdomains). By default I am directing them to the first subdomain.

I have a Plug.check_session to see if there is a user_id in the session cookie. If there is an there is no subdomain on the host I add in default subdomain and redirect to the new url.

If there is a subdomain I pass it through adding the user as the current_user.

defmodule BookingCentralWeb.Plugs.CheckSession do
import Plug.Conn

alias BookingCentral.Accounts

def init(opts), do: opts

def call(conn, _opts) do
    user_id = get_session(conn, :user_id)        
    case user_id && Accounts.get_user_by_id(user_id) do
        nil  -> 
            assign(conn, :current_user, nil)
        user ->
            orgs = Enum.map(user.organisations, fn (x) -> x.subdomain end)
            case get_subdomain(conn.host) do
                "" -> 
                    subdomain = get_first_subdomain(orgs)
                    url = %URI{host: subdomain <> "." <> conn.host, port: 4000}
                    conn = Phoenix.Controller.put_router_url(conn, url)
                    conn = assign(conn, :current_user, user) 
                    Phoenix.Controller.redirect(conn, external: BookingCentralWeb.Router.Helpers.dashboard_url(conn, :index))
                _  -> 
                    assign(conn, :current_user, user)
            end
   end

end

defp get_subdomain(host) do
    root_host = BookingCentralWeb.Endpoint.config(:url)[:host]
    String.replace(host, ~r/.?#{root_host}/, "")
end

defp get_first_subdomain([head | _tail]) do
    head
end

I changed my index action on the Dashboard Controller on the basis that the host will include the subdomain by this stage:

def index(conn, _params) do
           render(conn, "index.html")
end

Running this when I authenticate it adds the subdomain and redirects but then I end up at the login page, presumably because I get a new session for eth subdomain. I modified the config in my endpoint.ex:

plug Plug.Session,
    store: :cookie,
    key: "_booking_central_key",
    signing_salt: "H4nzWhe9",
    allow_hosts: [".localhost"],
    domain: ".localhost" 

But now I get a CSRF error:

invalid CSRF (Cross Site Request Forgery) token, make sure all requests include a valid '_csrf_token' param or 'x-csrf-token' header

Any direction or examples will be greatly appreciated.