openssh server and tun devices

Alex Bligh alex at
Mon Sep 21 04:38:51 EST 2009

If an ssh server receives a successful inbound ssh connection with 'ssh -w'
without a tunnel number specified (i.e. in 'any' mode), it allocates the
next tunnel device available on the server. The next thing the server
needs to do is to set up the tunnel device. How does the server know
which tunnel device was set up by the current connection? I'd really
like something race-condition free here.

Similarly (but less pressingly), if a client executes a ssh -w and uses
the next available local tunnel device, is there some way of reading
the tunnel device the child ssh process allocated?

Alex Bligh

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