We have several remote workers connecting through a VPN and using telnet to open a window to one of our servers. As we have upgraded them from fedora systems to ubuntu, I am told that after appx 5 minutes of inactivity, the telnet window closes and they have to start a new one.
Before ubuntu, they did not have a problem.
Since nothing else has changed except for the machine upgrades to ubuntu, is there a setting that can control the timeouts for these users?

Going from Fedora to Ubuntu is NOT an upgrade.

Seriously though there are various timeouts that can be set. The first one I'd look for is TMOUT shell variable. Login as a user and then type "echo $TMOUT". If set you'll see a value which is a number of seconds. 1800 = 1/2 hour, 3600 = hour etc... The number of things that might set timeouts are too numerous to list. I do know one VPN I used previously did disconnects if it saw no activity (e.g. no keystroke press or screen updates) for a finite period of time.

Also you really ought to NOT be using "telnet" as it is mostly obsolete and is insecure. Even though you're doing vpn to get into your network the traffic between machines inside the network is not encrypted. You should look into use of ssh instead.

I ran the 'echo $TMOUT' command and it returned nothing.

We use telnet because we are connected over vpn, providing security we need.

Even with VPN you should get in the habit of using ssh. VPN *may* protect traffic from outside your network into it but it does NOT protect traffic between machines inside your network. Having the telnetd daemon running is a security hole you should avoid.

Thank you for your original post and suggestion. But this post is not related to whether telnet is secure or not. I need help determining what is causing the timeout issue.