Best practices for securing SSH server

cpghost cpghost at
Wed Jun 24 14:02:25 UTC 2009

On Wed, Jun 24, 2009 at 03:53:15PM +0200, Erik Norgaard wrote:
> RW wrote:
> > On Tue, 23 Jun 2009 22:37:12 +0200
> > Erik Norgaard <norgaard at> wrote:
> > 
> >> You're right, as long as port-knocking as a first pass authentication 
> >> scheme is not in wide spread use, then any attackers will not waste
> >> time port-knocking. If ever port-knocking becomes common, attackers
> >> will adapt and start knocking.
> > 
> > It would be fairly straightforward to prevent that by having a
> > combination of knocking ports and secret guard ports. When a guard port
> > gets hit the sequence is broken, and the source IP gets blocked for a
> > while.
> Great: Wouldn't that be the same as monitoring failed login attempts and 
> temporarily blacklisting ips that repeatedly connect through standard 
> methods?

Hmmm..., you're right on this point.

But port knocking can be useful and provide more security *if* you
modify the kocking sequence algorithmically and make it, e.g. a
function of time, source IP/range (and other factors). This could
prevent a whole class of replay-attacks.

Of course, you can modify the keys/passwords algorithmically and
make them a function of time, source IP etc. as well... ;-)

And while we're at it: how about real OPIE? Or combining SSH keys,
OPIE, and port knocking?

> Erik N?rgaard
> Ph: +34.666334818/+34.915211157        


Cordula's Web.

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