Subject: Re: broadcast ping response
To: John Nemeth <>
From: Steven M. Bellovin <>
List: tech-net
Date: 01/22/2005 20:05:48
In message <>, John Nemeth write
>On Jun 14,  1:00pm, Eric Haszlakiewicz wrote:
>} On Sat, Jan 22, 2005 at 03:57:21PM -0800, John Nemeth wrote:
>} >      Why is NetBSD 2.0 responding to broadcast ICMP ECHO REQUEST (ping)
>} > packets?  Is there any way to stop it.  Because this is a well known
>} > DOS most modern OSes don't respond, so I'm surprised that current
>} > versions of NetBSD do.
>}       DoS?  How so?  I would think that responding to a ping takes
>} considerably less resources than, say, responding to a connection attempt.
>     It is a traffic amplification attack.  Picture a network with 50+
>machines, which respond to broadcast packets.  You send one ping packet
>to the broadcast address and get 50 back.  A great way to flood a
>network with very little effort.  Send a continuous stream of packets
>and even if you don't have a very high speed network, due to the
>amplification effect you can completely saturate a remote network thus
>making it useless.  An even better trick is to fake the source address
>(since ICMP is a connectionless protocol this is easy) and you can get
>some sucker to flood the crap out of a third party.  Tracing packets
>with faked source addresses is not easy.

That's called a smurf attack.  The usual defense is to make sure that 
your routers don't send out broadcasts to a local LAN when the packet 
originated somewhere else.  Those are known as directed broadcasts, and 
they've been known to be a bad idea since 1998 -- see .  It's only an issue if 
the attacker is on your own LAN, and those are relatively easy to trace.

		--Prof. Steven M. Bellovin,