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RE: Port-Knocking vulnerabilities?
From: "Craig Wright" <Craig.Wright () bdo com au>
Date: Tue, 1 Jan 2008 08:32:08 +1100

First, you have mentioned SPA, and true this offers more then port knocking, you have also mixed port knocking and SPA 
up in a couple of your comments.

Now you make the comment that SPA can "encrypt" and store in the IP ID. IP ID is a 16 bit flag. That is there are a max 
of 65535 values. Even with 4 IP packets it is functionally equivilant to a 3 character password. Given a standard ADSL 
line and 68 byte packets I can send all combinations in just under 7.5 seconds (and this is not doing any analysis on 
the IPID).

That is SPA and not port knocking. That is the MORE secure of the two options.

Yes this way will make a log entry, but are you sitting at the server 24x7 and monitoring ALL scans. Will you stop me 
in less then 8 seconds?

When did people consider a 3 character password safe?

Dr Craig Wright (GSE-Compliance)

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From: listbounce () securityfocus com [listbounce () securityfocus com] On Behalf Of Goldstein101 [goldstein101 () 
gmail com]
Sent: Tuesday, 1 January 2008 6:40 AM
To: Robert Inder
Cc: Ansgar -59cobalt- Wiechers; security-basics () securityfocus com
Subject: Re: Port-Knocking vulnerabilities?

I guess most of you haven't bothered to check what port knocking
really is capable of 'cause I'm reading a lot of things that are not

First of all, Port Knocking is just another layer of security. Think
of it as the door of a room that contains a safe. You first have to
break the port knocking daemon and then the safe. Not that easy,
believe me.

Second, Who said port knocking is like transmitting a password in
cleartext? Most modern PK systems are encryption based. An attacker
can sniff port numbers but packets usually contain other information
that is used for authentication.

For example I use Aldaba Knocking Suite (aldabaknocking.com) which
provides Port Knocking and Single Packet Authorization.

In Port knocking mode, basically the client sends this: [IP
Address][Port Number][Open/Close Flag][Checksum].

That information is encrypted and sent encoded in the IP-Id field of 4
TCP-SYN packets. This way you have 2 forms of authentication: The
first is simple: you need to know the exact port numbers to use when
sending those TCP-Syn Packets. Second: you need to know a secret (the
encryption key) used to encrypt the information. If you don't have it,
you can send random data but when decrypted, it won't verify the

However, Port Knocking has some disadvantages and vulnerabilities. A
better and more elegant approach is SPA. Check it out. There are some
papers out there.


On Dec 31, 2007 7:27 PM, Robert Inder <robertinder () googlemail com> wrote:
On 29/12/2007, Ansgar -59cobalt- Wiechers <bugtraq () planetcobalt net> wrote:
On 2007-12-28 Jay wrote:
Portknocking is a security mechanism as it is a type of
authentication. "Something you know" in this case the sequence of
ports to knock before a unstarted service or daemon begins listening
for connections.

Since everything is transmitted in the clear port-knocking is as much of
a security mechanism as cleartext passwords. Technically: maybe
(depending on your definition). Realistically: no.

I think your dismissal of port knocking (and, indeed, plain text
passwords) is unrealistic.

If you can intercept my interaction with some remote server, you can
steal the relevant secrets (the password or the sequence of ports).

But isn't that quite a substantial "if"?

How are you going to do it?  Aren't you going to have to compromise
some other machine, either where I am, or where the server is (or, I
guess, where the relevant DNS records are), and then plant software to
deliberately wait and watch until a relevant interaction takes place?

I'm not saying that's impossible.  But it would take considerable
knowledge, planning and effort.

Why doesn't that make it a substantial defence against most kinds of
casual attack?


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in Scotland
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