I agree that port knocking is not the christ child. It will not solve world hunger nor will it allow the wolves to lay down with the lambs.

But it's something.

It's the ROT13 of defense mechanisms.

ROT13 would stop and attacker peeking over everybody's documents, one doc per second, in search for something interesting to actually devote time and resources on.

You have to decide if the tradeoff between a thousand bot's scans and a sole eavesdropper noticing the knocking pattern and marking your host as "(incompetently) trying to hide" is really worth it.

One commenter on a previous iteration of this argument said that moving the port (not port knocking per se) made the access logs quieter, and that was reason enough for their use.

Edit: heh, same thing is going on a few comments down :)

What would you recommend?

SSH key with public key auth over VPN.

Restrict SSH access inbound to trusted networks only. Then you don't have to worry about all this hoopla.

> It's the ROT13 of defense mechanisms.

Not true. Modern port knocking is provably secure and requires network layer attacks to bypass.

What software is this "simple encrypted tunnel"?

Any of them. Try spiped.

More attack surface, not less.

Picture time!

More surface (both spiped and ssh are accepting connections):

     |
  →  |ssh
     |
  →  |spiped

Less surface (spipe accepts connection, ssh is behind it):

                 |
  →  | spiped  ↝ | ssh
                 |

With proper configuration, spiped sits in front of ssh, authenticating and decrypting traffic and passing it to ssh.

spiped is a tiny program, uses symmetric crypto (small surface). SSH isn't a tiny program, uses more complex asymmetric crypto (larger surface).

To learn more: http://www.daemonology.net/blog/2011-07-04-spiped-secure-pip...

To learn how to put it in front of SSH, see https://github.com/Tarsnap/spiped#example-usage

There are some similarities between this and SPA:

-> | SPA / fwknop | ssh

There are important differences too though (beyond the fact that SPA is not encrypting/decrypting traffic for SSH itself). SPA is a UDP authenticator so it cannot be scanned.

Design goals: http://www.cipherdyne.org/fwknop/docs/fwknop-tutorial.html#d...

No, SPA is the first picture: both SPA and OpenSSH are directly responsive to attacker communications. I don't think "attack surface" is the dispositive argument here (the fact that SPA doesn't protect OpenSSH connections at all is), but either way: SPA is inferior to spiped.

Not exactly. OpenSSH gated by SPA can only be interacted with by an attacker that can either hijack an SPA-authenticated connection, or is on the same network as the SPA client if the client must go through a NAT. This is a fairly limited set of possible attackers. For those not in this set, how can they interact with OpenSSH without first breaking SPA?