Using confirmation of key usage per-host?

Carson Gaspar carson at
Tue Feb 24 07:02:47 AEDT 2015

On 2/23/15 11:45 AM, Johannes Kastl wrote:
> Assume there is a workstation, which connects to multiple machines,
> one of which is considered potentially unsafe. So, it would be nice to
> have agent forwarding to that machine combined with the confirmation
> option of ssh-add (-c). If the 'forwarded key' is used on this
> machine, the user is prompted on the workstation. An intruder cannot
> use the authentication information without the user knowing (at least
> that is how I understood the idea of agent confirmation).
> Using ssh-add -c on the workstation together with setting
> 'ForwardAgent=yes' in the .ssh/config achieves the desired behaviour.
> Unfortunately, this means the user is asked for confirmation, each
> time the keys is used. Even if it is just to connect to a safe machine
> or without agent forwarding.
> Question:
> Is it possible to just get asked for confirmation, when the key is
> used on a machine, to which agent forwarding is used? Can this be set
> on a per-host-basis, like enabling/disabling agent forwarding in
> .ssh/config?

You'll need to run 2 agents if you want different agent behaviour. Sadly 
I don't know of any way to select which agent gets used in ssh_config - 
you'd also have to wrap ssh to flip the SSH_AUTH_SOCK env var.


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