Filtering ICMP (Was Re: SMURF amplifier block list)

D'Arcy J.M. Cain darcy at
Tue Apr 21 16:26:10 UTC 1998

Thus spake Alex P. Rudnev
>  deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm echo-request log
> to prevent smurf originating, or
>  deny icmp any aaa.bbb.ccc.ddd www.ccc.nnn.mmm echo-reply
> to prevent smurf flooding into your network.
> No important ICMP are affected this case.

Depends what you (or your users) consider important.  Consider that
users think that they understand networking because they know how
to ping or traceroute and your support lines will be busy explaining
that you aren't really down just because they can't traceroute to you.

We have a little script that looks at network usage and when it sees
a spike in traffic it temporarily blocks echo-reply in.  It isn't
perfect but it helps.  We know what our normal traffic is and when
it goes much higher we kick the filter into place.  If the script
makes a mistake and blocks when it isn't really an attack then we
haven't actually cut anyone off but we don't flood our downstreams
when there is an actual attack.

D'Arcy J.M. Cain <[email protected]{druid|vex}.net>   |  Democracy is three wolves                |  and a sheep voting on
+1 416 424 2871     (DoD#0082)    (eNTP)   |  what's for dinner.

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