Subject: Re: [e2e] Re: Question on "identification" field of IP header

From: Ramesh Shankar <[email-masked]>
Date: Sat, 14 Dec 2002 14:06:53 -0700

Thanks for the pointer. I have heard that the IP ID field is used for 
covert channel by hackers. Apparently a rogue SSL implementation was 
leaking session keys in the IP ID field. While not foolproof or the 
ultimate defense, if I don't need to use the IP ID field for IP 
datagrams with the don't fragment bit set (mostly TCP), then it may be 
useful as an intrusion detection technique.

Thanks,

S.R.

Felix Hernandez-Campos wrote:

> Ramesh Shankar wrote:
>
>> If the "Don't fragment bit" is set in the IP header, what purpose 
>> does the "identification" field serve? Why can't I simply put 0 for 
>> this field in such a case? I remember coming across some e-mail chain 
>> in one of the mailing lists (TCP-IMPL, e2e, TSVWG) about this issue 
>> and the interaction with NAT. But I am not sure what came out of that 
>> discussion.
>
>
> You may want to have a look at Steve Bellovin's "A Technique for 
> Counting NATed Hosts", presented at IMW 2002. The paper discusses how 
> the IP header's ID field can be used to infer the number of hosts 
> behind a NAT box.
>
> Regards,
> Felix.
>

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