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Comment by justinsaccount | Hacker News Reader

eavesdropping, maybe. There are tools like https://github.com/mrash/fwknop that are not vulnerable to that or brute-forcing.

But brute forcing in general? not a chance. There are 18446744073709551616 4 port sequences.

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And there are roughly 1267650600228229401496703205376 port 22's in the IPv6 space - I've substracted a few for reserved and unassigned spaces, but at this scale a few orders of magnitude hardly matter.

Here, for comparison:

  281474976710656 - total ports in IPv4 space
  18446744073709551616 - 4 port combinations
  1267650600228229401496703205376 - my estimation for 22 in IPv6

And if you don't block the knocking when receiving traffic on another port, brute forcing gets quite a bit easier. I mean, it's still unreasonable. But my point is, when we accept 0,001% chance as possible, I don't think we can say that 0,000001% is impossible - just a lot less possible ;)

The actual chances for guessing the 4 port combination are closer to 0.000000000000000000001%, about as likely as winning the lottery three times in a row. If you're trying to brute-force me with those odds, I'll take my chances.

18446744073709551616 is 65 bits. Let’s simplify, you're trying to guess a number in 2^64 You can’t guess in parallel. Reasonable constraints on the server side (i.e. limit tries on the combination/per hour before suspending ssh for a while) may have been implemented.

I’d say cracking that is… Unfeasible.

That also assumes you know the existence of a server on which there is ssh under an unknown combination of port knocking of length 4.

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