[Ubuntu] SSH: Staying connected to the server

1. PuTTY

In the menu of the PuTTY,
go to Connection > Seconds between keep-alives (0 to turn off),
and enter the number of seconds, which will
makes PuTTY to send a null packet to keep session alive.

2. ssh on Ubuntu

http://ubuntu.wordpress.com/2006/02/03/keeping-ssh-sessions-alive/

In sum, in /etc/ssh/ssh_config

add ServerAliveInterval 5