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Re: [PATCH] TCP Idle Scan in IPv6
From: Luis MartinGarcia <luis.mgarc () gmail com>
Date: Mon, 3 Jun 2013 19:19:04 +0200

Hi Mathias,

I haven't tested your patch, but if what you describe is true, it looks
like an interesting discovery, congratulations!

You might be aware of it already, but just in case you are not, Fernando
Gont just published a draft RFC on the security implications of predictable
fragment fields in IPv6. You may find it interesting.

http://tools.ietf.org/id/draft-ietf-6man-predictable-fragment-id-00.txt

Regards,

Luis MartinGarcia.


On Mon, Jun 3, 2013 at 5:59 PM, Mathias Morbitzer <m.morbitzer () student ru nl
wrote:

Hi everybody,

I managed to port the TCP Idle Scan to IPv6!

My masterthesis as well as a shorter paper on the details will come soon,
but meanwhile let me sum up the details here:

In IPv6, we don't have an IPID in the header. But, there is an extension
header for fragmentation, which provides an IPID. So, all we need to do is
forcing the idle host to append this extension header for fragmentation
each time he is sending a packet.

RFC 1981 says if an ICMPv6 Packet Too Big message is received, and an MTU
smaller than the IPv6 minimum MTU is announced within, the receiving host
should simply append a fragmentation header to each IPv6 packet on the path.

So we can achieve the TCP Idle Scan in IPv6 by first sending a ping with a
lot of data to the idle host. When the idle host replies, we tell it in an
ICMPv6 packet Too Big message that the reply is to huge, we only support a
maximum MTU of less than 1280 bytes, which is the IPv6 minimum MTU. From
now on, all IPv6 packets being sent from the idle host to us will have an
extension header for fragmentation, which contains an IPID.

Now we execute the same step for the path from the idle host to the
target. We spoof a ping from the target to the idle host, and after the
idle host sent the answer, we send an ICMPv6 packet Too Big message that
the MTU of the target is smaller than 1280 bytes, so from now on the idle
host will also append the fragmentation header there.

Afterwards, the TCP Idle Scan in IPv6 works the same way as in IPv4 - just
that the IPID is not directly in the IPv6 header, but in the extension
header for fragmentation.

Additional cool stuff: Compared to IPv4, the IPID is not used (and
incremented) for every IPv6 packet sent, but only for those which use the
extension header for fragmentation. This means that our idle host actually
does not need to be idle, it just shouldn't send fragmented packages!


I hope my explanation is not too short and understandable :)


However, to show that it really works, I also tried to implement the scan
in Nmap. To do so, I hacked idle_scan.cc, and used most of the stuff which
was already there. What I had to add was the sending of the pings and the
ICMPv6 packet too big messages for the initialization, and I changed the
parts where the IPID is accessed, so that it works for IPv4 and IPv6.

The usage is the same as using the scan in IPv4: -sI <idlehost:probeport>
for the idlescan, plus add the -6 switch for IPv6.

I tested my patch with Windows 7 Ultimate, and Linux 3.8 (but there is
does not work, the IPIDs are on a per-host-base).

The patch is not perfect yet. There are still some things which need to be
improved, but I wanted to get a first feedback to know if i can continue
working on it this way. Also, my C/C++ knowledge is not the best, so let me
know if I made bigger mistakes.


Cheers,
Moe


PS: I'm on a conference from Wednesday to Friday, so I won't be able to
read my mails.







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