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Hiding open ports with shimmer

Hiding open ports with shimmer

Posted Jan 10, 2008 4:17 UTC (Thu) by jimparis (subscriber, #38647)
Parent article: Hiding open ports with shimmer

To me, shimmer and port knocking both seem ridiculous.  In the case of shimmer, from what I
understand, you're essentially using a shared secret to create somewhere between 4 and 16 bits
of data (= "the port number"), and sending that to the server as your unencrypted
"authentication".  If you want to secure a service, why not just add a real authentication
step?  Make a TCP connection, authenticate yourself using the shared key, and then be granted
access.  No flashy tricks...


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Hiding open ports with shimmer

Posted Jan 10, 2008 5:12 UTC (Thu) by wahern (subscriber, #37304) [Link]

The argument is that using Shimmer reduces the exposure of any bugs in, say, OpenSSH's
authentication code.

But given that OpenSSH uses privilege separation during the authentication phase, and that
using Shimmer adds more code to the application stack, it's possible (probable?) that Shimmer
could increase susceptibility and exposure to attack. Just because Shimmer doesn't exchange
messages over the network doesn't mean its immune to bug exploitation.

It may prove in this case that "less is more" is a more apposite cliche than "defense in
depth".

As for the argument that Shimmer is just obsfuscation, I agree. At best it adds only a few
bits of potential entropy to the access key. As regards better passwords or public/private
keys it hardly compares favorably.

On OpenBSD I just use PF rate-limiting to keep the bots from filling my logs. It's the only
use I have for a packet filter (or traditional "firewall"). Most anything else just adds
additional work for no appreciable gain.


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