Today’s article can be used by anyone that has started using SSH on their computers, as we learn to disable inactive SSH sessions. It’s not going to be a very complicated article. This is easy enough for anybody to do, and there aren’t too many ways to permanently mess things up.
What is SSH?
My regular readers will know that I’ve written quite a few articles about SSH. Basically, SSH stands for “Secure Shell” and is a tool you use to remotely control other devices through the terminal. The tool is ancient but still valuable. I use SSH regularly, and that’s not even counting the stuff I do online.
Indeed, I’ve written some SSH articles before:
Install SSH to Remotely Control Your Linux Computers
How To: Restart SSH
Show Failed SSH Login Attempts
(And many more.)
By default, your server/device may not close inactive or idle sessions. It will let you maintain your connection until you tell it to exit. This can be a security issue and it may be worth setting your SSH to disable inactive SSH services.
In this article, we’ll be using Nano and you’ll need to be able to use an SSH connection. If you don’t default to using Nano, you can install Nano or just use the default text editor that you’re more accustomed to. In theory, if you’re just practicing, you could set this up on your computer and then tell SSH to connect to ‘user@localhost’.
Disable Inactive SSH Sessions:
Up above I said, “SSH stands for “Secure Shell” and is a tool you use to remotely control other devices through the terminal.” If you were paying attention, you’d notice the last word is “terminal”. So, you’ll need an open terminal. For most distros, you can open a terminal easily, you press
With your terminal now open, you’ll need to connect to the device that’s running the SSH server and enter the following command:
1 | sudo nano /etc/ssh/ssh_config |
Next, you’ll add a couple of new lines. The syntax is straightforward and easy enough for anyone to understand. There’s a little bit of math, but you can do that math in your head. The syntax looks like this:
1 2 | ClientAliveInterval <seconds> ClientAliveCountMax <number_of_checks> |
The first entry is how long you want to wait to check for an idle connection.
The second entry is how many times you want (set by the first entry) to check for an idle connection.
For example, look at this:
1 2 | ClientAliveInterval 300 ClientAliveCountMax 5 |
With the above, it’d check for an idle state every 300 seconds (five minutes). It will perform this check 5 times. If the connection is idle for all of those checks, the system will disconnect the SSH user. That’s allowing for 25 minutes of idle time before disconnecting the inactive user. That seems reasonable to me.
You can set those values to anything you’d like, perhaps shorter for an open office and longer if you’re using your home computer and connected to your own devices. The math doesn’t change. It’s just the number of seconds between text multiplied by the number of times the system will perform those checks. If you regularly have many users connected via SSH, you can save some resources by disconnecting them after a reasonable period of inactivity.
As we edited this with Nano, you’ll need to save the file. To do that, you simply press
After you have saved the file, you’ll need to restart SSH for the changes to take effect. That’s easy enough. Assuming you’re using Systemd, you restart SSH with this command:
1 | systemctl restart sshd |
If you’re still using SysV, the command would be this:
1 | service sshd restart |
(I don’t bother with that often, I just assume you’re using Systemd. After all, according to the stats I can get, most of us are in fact using distros that use Systemd.)
Closure:
So, if you want to disable inactive SSH sessions, you can use the above as a template. You’ll need to figure out how long you want to wait between checks and you’ll want to decide how many times you’re going to make those checks. It’s simple math that anyone can do. It’s also probably not a bad step to take if you’re dealing with something public or sensitive.
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