Umm, I think this has already been hashed out. This is not the only netmask on the planet, and you don't know what other networks netmasks are under CIDR. Trying to guess the netmask just leads to breakage. All you want to do is stop packets coming in to your broadcast address. For example, for your network x.y.z/n (n=24) with your broadcast address of x.y.z.255: (I presume everyone can translate between CIDR notation and dotted decimal ;-) deny ip any x.y.z.255 255.255.255.255 no ip directed broadcast basically puts in the same rule, but it does it automatically by looking at the netmasks on the interfaces. --Dean
Why don't use the filter
deny icmp any 0.0.0.255 255.255.255.0 echo-request
on the incoming lines? It just block 99.999% of this smurf amplifiers; and I hardly think someone eve sence this restriction for the real PING tests.
???
On Fri, 17 Apr 1998, Dean Anderson wrote:
Date: Fri, 17 Apr 1998 18:09:08 -0400 From: Dean Anderson <dean@av8.com> To: jlixfeld@idirect.ca Cc: nanog@merit.edu Subject: Re: SMURF amplifier block list
Does no ip directed broadcast really work?
Yes. It works.
And it works for whatever your particular netmask or broadcast address happens to be, which is what's important.
The only time you shouldn't do it globally is when some other network really needs to see broadcasts. For example, If we manage a client's network with HP OpenView over the internet, we need to be able to send them directed broadcasts, so that OpenView host discovery will work. Patrol works the same way, as do other products. In this case you can't use the "no ip directed broadcast" switch, but you can still set up access rules which do the same thing except for the permitted network.
Bottom line is that you should protect your network from people who would either abuse it via smurfing, or simply have no business looking for hosts on your network. You have the tools to do it.
--Dean
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