Re: spoofed packets to RFC 1918 addresses

From: Daniel Polombo (
Date: 06/27/02

Date: Thu, 27 Jun 2002 08:42:08 +0200
From: Daniel Polombo <>
To: Dirk Koopman <>

Dirk Koopman wrote:

> a) how the attackers are able to "guess" correct (ie existing) rfc1918
> addresses as, AFAIK, these are not being leaked thru the firewall.

There are at least two possibilies that spring to mind :

- if you are using a web proxy for your protected network(s), the proxy
may be adding an X-Forwarded-For field containing the rfc1918 address.
Other protocols might provide the same kind of information as well.

- in some cases, the firewall may leak information about the protected
network if there is some DNAT set up (and in particular, the recent
advisory named "Linux Netfilter NAT/ICMP code information leak" by
Philippe Biondi).

> b) how these packets are getting to me in the first place as they don't
> seem to be source routed.

That's the real catch. I think a number ISPs don't filter rfc1918
addresses within their domains, letting BGP4 make sure they don't get
routed outside instead. So, theoretically, a spoofed packet could make
its way to a target not too far away (eg, within the same AS).

I don't know of any automated tools who would do that, but building one
using antirez's hping, for instance, shouldn't be too hard.



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