I have a ssh connection to a machine which gets disconnected by that machine after 30 minutes of no user input. However, if I start something like top, the connection stays alive. Since this is a client's machine, I can not reconfigure that machine's SSH server. So I am looking for a way to automatically detect idleness and start something like top. Kind of a "screensaver" for Bash.

I know that I can do that with screen, but unfortunately screen is not installed, and I can not install software. So I need to use what Bash offers.

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. Also, I am not looking to tunnel stuff (for that I recommend the great tool sshuttle)

Any ideas?

  • Just a quick note to make clear screen and Keepalive are not the same thing: for instance, if either ssh client or server gest disconnected, your ssh session will be terminated even if you have KeepAlive on, while a screen shell wouldn't be terminated. – MariusMatutiae Jan 10 '14 at 11:50
  • Possible duplicate of How to reliably keep an SSH tunnel open? – Murmel Jan 15 at 22:30
up vote 58 down vote accepted

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"

putty_screenshot

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).
  • BUT this will only be done ServerAliveCountMax times. 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 keep the connection alive for 3 * 60 = 180 seconds = 3 minutes.

To increase this to e.g. 2 hours of keeping 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

See also:

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.

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