How to prevent SSH from disconnecting if it's been idle for a while

To make it clear: I am looking for a solution that I start once after logging in, and then I want to use that terminal, walk away, come back two hours later and continue working, without typing anything before walking away.

The problem is that there is something (usually a firewall or load-balancer), which is dropping idle sessions. If you configure session keepalives, the keepalives will prevent network devices from considering the session as idle.

Linux / Unix / Cygwin OpenSSH fix:
The simplest fix is to enable ssh client keepalives; this example will send an ssh keepalive every 60 seconds:

ssh -o "ServerAliveInterval 60" <SERVER_ADDRESS>

If you want to enable this on all your sessions, put this in your /etc/ssh/ssh_config or ~/.ssh/config:

ServerAliveInterval 60

For more information, see the ssh_config manpage

Putty Fix:

Save this to your PuTTY "Default Settings"...

• Click on Connection
• Type 60 into "Seconds between keepalives"

In addition to Mike Pennigton's answer, I would like to make you aware of ServerAliveCountMax too.

• The ServerAliveInterval will send a keepalive every x seconds (default is 0, which disables this feature if not set to something else).
• This will be done ServerAliveCountMax times if no response is received. The default value of ServerAliveCountMax is 3 (see manpage ssh_config).

Example: If you set ServerAliveInterval to 60 and leave ServerAliveCountMax as it is, this means the keepalive will only wait for 3 * 60 = 180 seconds = 3 minutes before quiting.

To increase this to e.g. 2 hours of trying to keep the connection alive, you can do:

Per command:

Therefore you should consider to set

ssh -o "ServerAliveInterval 60" -o "ServerAliveCountMax 120" <SERVER_ADDRESS>

Persistent:

To make it persistent write it to /etc/ssh/ssh_config (will apply system-wide) or ~/.ssh/config (will apply user-only):

ServerAliveInterval 60
ServerAliveCountMax 120

Note

As dislick correctly pointed out, this might not what you want, depending on your situation:

• If you would like to quickly terminate the session as soon as the server does not respond anymore, you should choose a low value for ServerAliveCountMax.
• If you are more interested in keeping an already established connection (e.g. you go by train and have a high latency), you should choose a higher value for ServerAliveCountMax to allow ssh to keep trying to reestablish the connection.

See also:

• unix.stackexchange.com - What is the default idle timeout for OpenSSH?

I am using Mobaxterm and have also met with this problem. Mobaxterm also ships with an option to keep the client alive when the client is idle. Go to Settings -> Configuration -> SSH. There is section titled SSH settings, check the option SSH keepalive. Then it the problem should disappear.