Persistent SSH sessions

[Mukund Sivaraman]

Hi all

I have an ADSL modem which reboots when there is a power cut and the
inverter (UPS) kicks in.  Internet access is down for a duration of 1
to 2 minutes while the modem boots.

I have many SSH tunnels and shells active.  Due to the default
"TCPKeepAlive On" setting, these sessions are terminated almost
immediately.

I tried the following configuration:

sshd_config on server:
  TCPKeepAlive no
  ClientAliveInterval 90
  ClientAliveCountMax 6

~/.ssh/config:
  Host *
    Protocol 2
    Compression yes
    TCPKeepAlive no
    ServerAliveInterval 90
    ServerAliveCountMax 6

But I guess the ssh client doesn't try to re-establish the session for
the ServerAlive messages to work.  The shells remain blocked after the
modem reboots, and after approximately 90*6 seconds, ssh aborts
complaining of timeout with the remote server.

I want to know if there is any way I can get ssh to try to renegotiate
the active sessions to remote servers, without disconnecting them.

Note: Please don't bother suggesting workarounds such as the use of
screen, autossh, etc.  I am looking for a specific answer about how to
keep a session alive, or the impossibility of doing that.  For example,
autossh restarts ssh.  I want existing sessions to continue as if
nothing happened, as long as net access is not down for longer than
some timeout.

FWIW, I'm using OpenSSH_5.4p1, OpenSSL 1.0.0a-fips 1 Jun 2010.

		Mukund