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flumignan
Asked: 2011-04-28 15:03:50 +0800 CST

Know of a trick to sustain quiet ssh connections from firewall auto-disconnects?

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Our enterprise firewall is configured to disconnect ssh connections after 30 minutes of of inactivity. Sometimes, I need to perform an operation on a server that requires a sustained connection exceeding this period, but which my shell appears inactive (at least to the firewall).

It seems that opening a second ssh connection with continuous activity (say, running top) isn't enough to sustain the shell. I'd prefer not to set the process to the background and then foreground something more interactive, because any alerts or warnings might escape my attention. I could periodically, repeatedly hit the return key in the shell window every x-minutes, but that's undesirable.

Some programs, like sweep (from Sophos) have a cursor that rotates in the shell, sustaining the connection (that is, it cycles through \ | / – and so on). That works nicely.

Is there some trick, perhaps associated with bash, that could serve this purpose?

firewall command-line-interface ssh

2 Answers

  1. Best Answer

    In your .ssh/config on your client, try adding something like this.

    Host *
      ServerAliveInterval 60

    man ssh_config

     ServerAliveInterval
         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...

    From putty, you would want to adjust the Connection\Seconds between keepalives to a non-zero value.

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  2. If running within a screen session - you could add "system load" counters to the bottom of the screen (or system date/time - adding something like caption always "%d.%m.%y %0c%{=b kW}" to your .screenrc could do that), which will non-regularly generate some minor output (redrawing load numbers or date/time).

    This is how it looks - lower left corner, system load

    This is how it looks - lower left corner, system load

    An even less straight :) solution (with keepalives being the straightest) would be to have some shell script running in background and generating some output to stderr every 30-60 seconds... E.g. while true; do echo "ding" > /dev/stderr; sleep 30; done &.

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