On Sat, May 05, 2001 at 11:21:55PM -0700, Ofir Arkin wrote:
> RFC 791 gives a description about the IP Identification field.
...
> The first ICMP Echo request sent from the Microsoft NT 4 based machine was
> sent with IP ID of 28416. The second ICMP Echo request was sent with IP ID
> value of 28672. Simple calculation will show a gap of 256 between the IP ID
> field values.
>
> Looking at the replies the LINUX based machine produced, we see a gap of 1
> between one IP ID to the next.

This is know since a long time that Microsoft switched (or forgot to) bytes
in its IPID, look at the -W option in hping2
<http://www.kyuzz.org/antirez/hping.html>

> How Can We Use This?
> We can use this information as another parameter for Active OS
> fingerprinting and for Passive OS fingerprinting.

And a lot of crackers do use it to actively/passively fingerprinting
systems.

Another important use is to count the number of packets sents by a remote
system : send a packet per second and you know how many... This permit a
much more important use : to scan remote systems by spoofing its address.
Again look at the hping documentation and the bugtraq archive to know how.

Now some systems protects against been used to spoof-scan :
 . OpenBSD and IPFilter(*) : IPID are random
 . Linux 2.4.x : IPID is null if the packet is small enought to be carried
   unfragmented in which case the DF (don't fragment) bit is set
 . others perhaps ?

(*) Only IPID generated by IPFilter are random which correspond to reset
    packets and icmp unreachable messages, other packets are generated by
    the underlying TCP/IP stack.

Regards,

Denis Ducamp.

--
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