How to Prevent Concurrent User Logins in PHP/MySQL Site?

Question

I am developing the user management portion of a website that will host a webcast. The goal is to prrevent the same user nam (email address) from being used concurrently.

Answer 1

The problem here is detecting the user is logged in (i.e. whether he didn't logout).

One possible way is to register in the database the time of his last activity and the time of his explicit logout. You could then deny a login if it this was attempted less than say 5 minutes ago relatively to his latest activity and if he didn't login in between.

You could force "activity" by having the website pages periodically poll the server with Javascript.

Answer 2

It's easy to determine when someone logs in. It's much harder to determine when someone logs out. If you have a mechanism of killing the webcast streaming to a particular user quickly, you might want to have something which pops up asking the user if they want to kill their other session if you think there might be one active.

Answer 3

How are you doing user sessions on the server? If you store them in the db, you could query the active sessions anytime someone attempts to log in and see if they're already in there. Of course you'd probably also have to check some kind of timestamp since you're not guaranteed that sessions will disappear at session.gc_maxlifetime.

Answer 4

You might want to consider making a global variable in php to store a hash array with login status. This has the benefit that if the application has to be restarted for some reason, the user isn't stuck in the wrong state in the database.

You can store a mapping from user ID to IP or session cookie and redirect requests that come with different information to the login page. If the user logs in, the other session would be invalidated and further requests in the last session forward to the login page.

Answer 5

Without rolling your own session handler, you could do a little parallel tracking. When a user logs in, you can store the user's session ID and login time in the database (maybe inside the user information table). The login script could then check for the existence if this sessionID and allow/deny login based on the presence of the session ID. If the ID's null/blank, then the user logs in. If there's a session ID present, and it's more than X minutes old, allow the login. Otherwise deny them.

Of course, you'd probably want to roll your own session cleanup handler at that point, so that when stale session files get deleted, you can remove the associated IDs from the database at the same time.

Answer 6

If it is OK to logout an already logged in user if someone else logs in with the same credentials then you could do the following: when a user logs in generate a random ID in your database for that user and the same in a cookie session. The two must match to authenticate.