Re: an anternative to port-knoking using the OpenBSD pf only



If an attacker has targetted your system, port-knocking and this alternative are totally useless. Knokcing sequences (and os fingerprints) are not encrypted so their content can be easly sniffed out. Port knocking increases sealthness (so it can probably be considered a security measure) but cannot be used to make a system safe.
I've noted that many people misunderstand the port-knocking's meaning, it's not a security layer and doesn't prevent security holes, it can only make your system quite harder to hack.
Anyway the use of port-knocking to make a system safer can be a bad solution, because it's just like wrapping a vault with cellophane. You can use it to hide a computer in internet, or to keep your logfiles clean as you can use cellophane to protect iron against rain.


Gimeshell, it's not right to consider tcp options size unlimited, and I think the real port-knoking is harder to brutefoce. Authentication through os fingerprints cannot use all tcp options (p0f doesn't analyze them all) and, anyway, it must fit tcp options structure and respect the protocol rules.

poplix


On 17 Feb 2006, at 9:52 AM, gimeshell@xxxxxx wrote:

On Mon, 23 Jan 2006 22:44:52 +0100
poplix <poplix@xxxxxxxxxxxx> wrote:

OUT (tcp.flags='syn')
set(
tcp.win='8192',ip.ttl='128',ip.flags.df='0',
tcp.options='mss=3884+noop+wss=0+noop+noop+ts=TS'
);

This rule tells tripp to rewrite all outgoing tcp packets with
the SYN
flag set only allowing my first handshake packet to match "poplix"
fingerprint.

Does an attacker who wants to attack a server include aspect that
this maybe a system utilizing port knocking in this way?
Perhaps if he has focused at this system?

Am i right to say, your invented os fingerprint is random and unique
and because of unlimited IP option field's length there's no
chance left for attacker to brute force ports with different option
field values et cetera to get correct option field + window size + ttl +
flag?

regards

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