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Re: ICMP unreachable question
From: "Crist J. Clark" <cristjc () earthlink net>
Date: Fri, 26 Oct 2001 19:50:51 -0700

On Fri, Oct 26, 2001 at 11:05:24AM +0100, Steve Culligan wrote:
I'm interested in a particular ICMP packet which seems to change the client 
/ servers MTU size.
The scenario is like this
client----------->Router-vpn-vpn-vpn-vpn-vpn-Router --------------->Firewall 
- Client initiates a connection with the server and starts to transmit data.
- Router places its ESP header on the packets coming from the server which 
brings the MTU over the maximum size
- Router sends the following packet back to the server
      icmp: 172.*.*.*  unreachable - need to frag (mtu 1454)
- ICMP packet from the router gets blocked by the firewall and the 
connection is eventually lost as the router cannot handle this MTU size.


If the Firewall permits the ICMP packet from the router through to the 
server, the server will lower its MTU and continue the connection.

So my question is , Can this be used as a denial of service attack to 
continually send these ICMP packets to a server to confuse it or bring it 
Anybody had any experience with this or know any tools which can generate 
these ICMP reachable packets ?

It is unlikely that you could actually bring down a server with these
packets. The worst you can probably do is degrade service. In order to
do this, the hostile party would have to be able to sniff the data
stream. There are any number of potential attacks someone can perform
if they can sniff your data. This particular attack is one of the less
devistating and more complicated ones so it is unlikely to be used.

I am not aware of a specific tool that builds ICMP "destination
unreachable, fragmentation required and DF-bit set"-messages. You can
use a tool like hping to make your own at the command line. Building a
quick C program to build them is trivial.

IIRC, there was a recent thread about the potential for someone to
flood networks by causing very small packets to be sent. I do not
think the attack would work. You could degrade the performance of the
connection attacked, and possibly the machine being attacked, but not
the whole network (at least not without combining some other attacks
with it). 
Crist J. Clark                           cjclark () alum mit edu

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