The Linux ssh server never disconnects ssh session from server side by default, check switch/router/firewall configuration to avoid the issue without changing the configuration of ssh servers/clients. If the ssh session is getting disconnected due to inactivity and one wants to stop this then please follow the below steps either on client or server side as described below:
Client Side Settings
To enable the keep alive system-wide (root access required), edit the configuration file /etc/ssh/ssh_config. Similarly, to set the settings for just a specific user, edit ~/.ssh/config (create the file if it doesn’t exist). Insert the following:
Host * ServerAliveInterval 300 ServerAliveCountMax 2
These settings will make the SSH client or server send a null packet to the other side every 300 seconds (5 minutes), and give up if it doesn’t receive any response after 2 tries, at which point the connection is likely to have been discarded anyway.
From the ssh_config man page:
Sets the number of server alive messages (see below) which may be sent without ssh(1) receiving any messages back from the server. If this threshold is reached while server alive messages are being sent, ssh will disconnect from the server, terminating the session. It is important to note that the use of server alive messages is very different from TCPKeepAlive (below). The server alive messages are sent through the encrypted channel and therefore will not be spoofable. The TCP keepalive option enabled by TCPKeepAlive is spoofable. The server alive mechanism is valuable when the client or server depend on knowing when a connection has become inactive.
The default value is 3. If, for example, ServerAliveInterval (see below) is set to 15 and ServerAliveCountMax is left at the default, if the server becomes unresponsive, ssh will disconnect after approximately 45 seconds. This option applies to protocol version 2 only; in protocol version 1 there is no mechanism to request a response from the server to the server alive messages, so disconnection is the responsibility of the TCP stack.
Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server. The default is 0, indicating that these messages will not be sent to the server, or 300 if the BatchMode option is set. This option applies to protocol version 2 only. ProtocolKeepAlives and SetupTimeOut are Debian-specific compatibility aliases for this option.
Server Side Setting
If you have administrator access to the server, you can configure the ClientAliveInterval, TCPKeepAlive and ClientAliveCountMax options in the SSHd configuration file. The file’s path is /etc/ssh/sshd_config
# vi /etc/ssh/sshd_config ClientAliveInterval 30 TCPKeepAlive yes ClientAliveCountMax 99999
You will need to restart the SSH server for the changes to take effect.
# service sshd restart