Is there any solution, or even work on, limiting which keys gets forwarded where?

Philip Hands phil at
Tue Oct 20 18:23:06 AEDT 2015

Phil Pennock <phil.pennock at> writes:

> On 2015-10-16 at 12:47 +0200, hubert depesz lubaczewski wrote:
>> But the flip side is that using agent opens access to all keys in it
>> from any connected host :(
> I scripted a setup which generates different `config` and `known_hosts`
> files in ~/.ssh and has a wrapper around invoking ssh which uses those
> files.  The wrapper looks in an index file (also generated) to decide
> which ssh-keys need to be loaded for which destinations and starts a
> dedicated ssh-agent before connecting.
> This works well for our use-case.  Persistent connection to a jump-host;
> SSH key to reach the jump-host doesn't need to be (and isn't) the key
> which is loaded into the agent (so that the key passed around for use
> remotely does not grant access to the perimeter).  It's not the cleanest
> and requires strict hygiene to protect against people helpfully setting
> the internal key to grant access to the perimeter class of machines.
> The core boils down to (with some renaming):
>     eval $(ssh-agent -s)
>     #... stuff including loading the correct keys
>     if ssh-add -l >|/dev/null 2>&1 ; then
>       true # all is good
>     else
>       eval $(ssh-agent -k)
>       exit 1
>     fi
>     set +e
>     ${SSH_CMD:-ssh} -F "${YourSpecialConfigFile:?}" "$@"
>     exit_status=$?
>     set -e
>     eval $(ssh-agent -k)
>     exit $exit_status
> where the stanzas in the config file should include:
>     IdentityFile path/to/perimeter/access
>     IdentitiesOnly yes
>     UserKnownHostsFile ~/.ssh/your_special_config_file
>     StrictHostKeyChecking yes
>     ForwardAgent yes

If you used the ProxyCommand ssh -W trick with that, you would not need
the ForwardAgent, at which point each internal server would never see
the jumphost key but also the jumphost would never see any of the
internal keys, because they'd be going over the still-encrypted links
that are being forwarded to the internal servers.

This also means that the hostkey checking on the internal servers is
done at your end, and not on the jumphost, so one doesn't need to have
any real trust in the jumphost as it's just acting as a router really.

Cheers, Phil.
|)|  Philip Hands  [+44 (0)20 8530 9560]  HANDS.COM Ltd.
|(|  Hugo-Klemm-Strasse 34,   21075 Hamburg,    GERMANY
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