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Re: Filtering ICMP (Was Re: SMURF amplifier block list)
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 broadcasts? And if this is true, doesn't designing your network with minimized amplifier space sort of negate all this ? Enlighten me .... Richard 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.