title: "Copy this file to your authentication server" - Firebase Custom auth
body: <p>My goal is to prevent users of multiple login in. I do not want this to be client-side, with like the onDisconnect and onConnect values, but with a server check. I came across this answer:</p>
  
  <p><a href="https://stackoverflow.com/questions/21321234/how-to-prevent-simultaneous-logins-of-the-same-user-with-firebase">How to prevent simultaneous logins of the same user with Firebase?</a></p>
  
  <p>Which tells me to create a custom auth system. When I am following the docs (<a href="https://firebase.google.com/docs/auth/ios/custom-auth" rel="nofollow noreferrer">https://firebase.google.com/docs/auth/ios/custom-auth</a>) I need to "Copy this file to your authentication server" (3c). How would I do this? I am just using Firebase and my little iOS app. I would like to manage everything on these 2 things, no server in between, is this possible? Or can this file only be uploaded through another server?</p>
  
  <p>If above things are not possible, how can I server check if the user really signed in? I am using Cloud Functions, but I did not came across a trigger for a user signing in. Please no answers with onDisconnect/onConnect, I want it server side. A user may NOT login if he is already logged in. Thanks :)</p>