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From a Linux SSH shell, type /etc/init.d/network restart to restart the network service.
I expect my SSH connection to die since the network service goes down. But it doesn't. Very cool. But how does Linux achieve this? How does it keep my SSH connection alive across the service restart?
It does this by doing nothing special. The network restarts in less time than the TCP connection takes to time out, so the TCP connection survives the "outage" the same way it would survive any transient network outage.
The only reason Windows doesn't do the same thing is because Windows specifically resets TCP connections when a network interface goes down. This is, at least arguably, a pretty boneheaded thing to do because TCP was specifically designed to survive transient network outages.
SSHD forks a child process on connection. This child process will not die if either SSHD or the whole network is restarted. This is the reason why you can update ssh and/or its configuration, do a service sshd restart and still keep connected to your old ssh-session with the old settings. Apart from that ssh recovers well from small network outages.
sshd does to avoid going down on service sshd restart. If you update sshd (security update etc), the child processes would still be running on the original code (pre-update), but would continue to provide service, rather than being interrupted. I guess the availability of the listening socket for the main sshd process is very briefly interrupted, however, when the first child process ends and the network interface momentarily indicates the port as closed.
Dec 14, 2013 at 5:03