SSH timeouts as a result of inactivity can be quite irritating. This usually compels you to reinitiate the connection and start all over again.
Thankfully, you can easily increase the SSH timeout limit and keep your SSH session alive even after some inactivity. This happens when either the server or the client sends null packets to the other system to keep the session alive.
Related Read: How to Secure and Harden OpenSSH Server
Let’s now explore how you can increase the SSH connection timeout in Linux.
Increase SSH Connection Timeout
On the server, head over to the /etc/ssh/sshd_config configuration file.
$ sudo vi /etc/ssh/sshd_config
Scroll and locate the following parameters:
#ClientAliveInterval #ClientAliveCountMax
The ClientAliveInterval parameter specifies the time in seconds that the server will wait before sending a null packet to the client system to keep the connection alive.
On the other hand, the ClientAliveCountMax parameter defines the number of client alive messages which are sent without getting any messages from the client. If this limit is reached while the messages are being sent, the sshd daemon will drop the session, effectively terminating the ssh session.
The timeout value is given by the product of the above parameters i.e.
Timeout value = ClientAliveInterval * ClientAliveCountMax
For example, let’s say you have defined your parameters as shown:
ClientAliveInterval 1200 ClientAliveCountMax 3
The Timeout value will be 1200 seconds * 3 = 3600 seconds. This is an equivalent of 1 hour, which implies that your ssh session will remain alive for idle time of 1 hour without dropping.
Alternatively, you can achieve the same result by specifying the ClientAliveInterval parameter alone.
ClientAliveInterval 3600
Once done, reload the OpenSSH daemon for the changes to come into effect.
$ sudo systemctl reload sshd
Conclusion
As an SSH security measure, it’s always advisable not to set the SSH timeout value to a huge value. This is to prevent someone from walking by and hijacking your session when you are away for an extended period of time. And that’s it for this topic.
At least in Ubuntu for OpenSSH, it looks like this “ClientAliveInterval” setting was a hack and no longer supported as a connection timeout setting according to this August 2020 post: https://askubuntu.com/a/1265510
Hi,
I am looking for a parameter that can handle the server timeout request for an SSH connection. In other words, if a user is trying to establish a connection with an SSH Server (which doesn’t exist), then how long can we keep the connection request alive, using that parameter.
Kindly suggest the same.
Regards
Good article, but I think the variable is ‘ClientAliveInterval’, not ‘ClientIntervalAlive’.
@Rich,
Thanks for notifying, corrected the variable in the article.