Best practices inquiry: tracking SSH host keys

Shumon Huque shuque at
Sun Jul 9 18:47:13 UTC 2006

On Thu, Jul 06, 2006 at 04:52:52PM -0400, Steven M. Bellovin wrote:
> SSH is a distributed single point of failure, just like the old thick
> yellow Ethernet.  Remember how reliable and easy to debug that was?
> More seriously, the original virtue of SSH was that it could be deployed
> without centralized infrastructure.  That's great for many purposes; it's
> exactly what you don't want if you're an ISP managing a lot of servers and
> network elements.  You really do want a PKI, complete with CRLs.  I know
> that (most) SSH implementations don't do that -- complain to your vendor.
> (Note: the CAs are also single points of failure.  However, they can be
> kept offline or nearly so, booted from a FooLive CD that logs to a
> multi-session CD or via a write-only network port through a tight
> firewall, etc.  Yes, you have to worry about procedures, physical access,
> and people, but you *always* have to worry about those.
> 		--Steven M. Bellovin,

The problem is how do you ensure that you've distributed the most 
current CRLs to all your SSH clients. You might need to deploy
a redundant highly available set of OCSP responders. Which means
that at least a part of your centralized infrastructure is now
online and inline :-) Admittedly not the part that necessarily 
needs access to the CA's private key, so not terrible from a 
security paranoia point of view.

We already have a deployed key management infrastructure at our 
site (Kerberos). If it were (practically) possible to authenticate 
login sessions to routers with it, we'd definitely use it. I can't 
see us deploying a PKI just to authenticate SSH host keys.

There is the general chicken-and-egg concern about using network 
based authentication services to access critical network hardware. 
But I think many (most?) of us have other means to access routers 
during catastrophic failures or unavailability of the former. We 
have an out of band ethernet connected to the router consoles, which 
can be dialed into (needs authentication with a hardware token).


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