Filtering ICMP (Was Re: SMURF amplifier block list)

Richard Irving rirving at
Fri Apr 24 16:50:12 UTC 1998

Ok. You know how I always ask the obvious... So, here I go again..

This is only slightly off topic.. If you have no amplifiers
greater than 2x-4x, is there really a need to turn off ip directed

  And if this is true, doesn't designing your network with minimized
amplifier space sort of negate all this ?

Enlighten me ....


Pete Ashdown wrote:
> Jason Lixfeld said once upon a time:
> >Seriously.. what do you recommend?  I'm totally open.  I'm using deny icmp
> >to protect myself.  I'm up to an alternative.
> >:> You could always "deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm log" on
> There apparently is a bit of misunderstanding when it comes to how a smurf
> attack works.  To understand a smurf attack you need to understand a
> standard ping request.
> Say we have a remote ping destination, named "target" and a originator of
> the ping request named "source".  In the first step of a ping request,
> "source" sends an ICMP request of "echo" to "target":
>         "source"  --- ICMP echo ---> "target"
> When "target" receives the ICMP echo, it sends back an ICMP echo-reply to
> "source"
>         "source"  <--- ICMP echo-reply --- "target"
> Upon reception of the "echo-reply" "source" realizes a good ping and coughs
> you back the statistics on how long the whole interaction was.
> With a smurf attack you have a perpetrator forging the "source" address,
> which in this case could also be known as victim.  The perp takes advantage
> of open directed-broadcast networks to get lots of addresses responding
> back to the "source" (victim) with "echo-reply".  Thus the original request
> looks like this:
>     perp (forged "source") --- ICMP echo ---> "target" (directed-broadcast)
> and the reply looks like this:
>     "source" (victim) <==== ICMP echo-reply x "target" addresses listening to
>                                               the broadcast request for
>                                               ping echo
> You can easily see how the broadcast size of "target" and whether it is
> open to "directed-broadcast" is the fundamental exploit in the smurf
> attack.  The larger the subnet, the better.  However, it is also easy to
> see that by blocking just "echo-reply" to certain addresses (IRC servers,
> Quake servers, etc), you can at least minimize the effects of the attack.
> The sad part is, the en masse echo-replies will still travel over your pipe
> to get to your filter and will still consume a significant portion of your
> bandwidth.
> Note, my understanding of the function of "directed-broadcast" is limited
> by the fact that I've never used it in a useful function.

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