Outlook 2016 querying Add-in root route

Question

So I have a question about some odd behavior I'm seeing in my Outlook Add-in logs and finally found the culprit. The client is Outlook 2016 and the user is backed by an on prem exchange server.

Whenever I open Outlook-2016 that has a user with the Add-in, Outlook seems to query the Add-in host for the root route https://<$host>/. This is odd because in my manifest I have not specified this path.

This is an easy enough fix to just add a root route, but my main question is why is it querying this route? Is it working as some sort of health check and if so can I specify another route that I already have a health check setup on?

Answer 1

If anyone else runs into this issue, Outlook 2016 when opened performs an HTTP OPTION on the root route. In nodejs with express, the solution was

  router.options("/*", (req, res) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Methods', 'GET,POST,OPTIONS');
    res.header('Access-Control-Allow-Headers', 'Content-Type, Authorization, Content-Length, X-Requested-With');
    res.sendStatus(200);
  });