How to Increase SSH Connection Timeout in Linux

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
Increase SSH Timeout
Increase SSH Timeout

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.

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