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Re: Network Operators and smurf
From: Jason Lixfeld <jlixfeld () idirect ca>
Date: Sun, 26 Apr 1998 12:29:19 -0400 (EDT)

What config options/access-lists are you using.  If Martin says his
routers choked, and you say yours don't, I'd like to know (as would
anyone, I think) what you are running and how you are running it.  I
assume you are using access-lists.  Are you using NetFlow?  On which
interfaces?  Are you blocking inbound echo-reply?  To where?  Everywhere
or just your core network?  The problem I'm faced with, is I'm currently
blocking ICMP to my core networks, NOT to customers or dialup ports.  They
will REALLY bitch if I start doing that.  I'm going to change it to block
echo-reply to those networks only. 

Currently, the measures "I" am taking is:

o outbound access-list permitting ONLY networks which source on our

o inbound access-list denying ICMP (which will be modifiec to block ICMP 
echo-reply only) to core networks along with .0 and .255 address blocks.

o no ip directed-broadcast on ALL interfaces

o ip route-cache flow (so I can get some information on these *ssholes)

What else do I need?!  What am I doing wrong with the above (if anything)


On Sun, 26 Apr 1998, Karl Denninger wrote:

:Since I have started blacklisting people, the list has grown to more than 40
:However, only ONE, and that one was very short (< 5 minutes) smurf has taken
:down a customer circuit or our IRC server since the last edits were made to
:the amplifier list over a week ago.
:I'm going to try to post a web page on this tonight.  What we're doing here
:WORKS.  It inconveniences a few people (those who amplify smurfs), but it
:WORKS and it STOPS the smurf attacks from burying your connections.
:Our core routers don't even get mildly bothered doing the discards.
:Karl Denninger (karl () MCS Net)| MCSNet - Serving Chicagoland and Wisconsin
:http://www.mcs.net/          | T1's from $600 monthly / All Lines K56Flex/DOV
:                            | NEW! Corporate ISDN Prices dropped by up to 50%!
:Fax:   [+1 312 803-4929]     | *SPAMBLOCK* Technology now included at no cost
:On Sun, Apr 26, 1998 at 02:08:04AM -0400, Martin, Christian wrote:
:> All,
:> Given the contributions to this thread have been mostly theoretical in
:> nature, I'd like to share an experience of mine that in some ways
:> negates some of the propsed solutions to smurf attacks in the context of
:> smaller ISPs.
:> Recently, one of our downstream customers was subject to a smurf attack,
:> and we placed an access-list on our egress interface to the customers
:> network.  The customer hangs of an SMDS cloud - our link to the cloud is
:> at 34 Mbps, his T1.  We were blocking echo replies destined for his
:> network.  We are connected upstream at 45Mbps.  As the attack
:> intensified, router CPU Utilization jumped to 99%, and the input queue
:> on our inbound HSSI was at 75/75.  We started dropping packets at a rate
:> of about 7000/sec.  The attacks were coming in from all over the world.
:> The NetFlow cache was growing at an alarming rate, and, after a while
:> the HSSI just DIED.  As the HSSI bounced, our BGP session bounced with
:> it, causing some mild route flapping (Not vaccilating enough to be
:> damped, but enough nonetheless).  Eventually the attack subsided, and
:> all went back to normal, but for a period of time, say 10 minutes, we
:> had a 7507, 64megs, RSP2 WITH the HSSI on a VIP2 on its knees.
:> We decided that parsing NetFlow logs would give us a better idea of who
:> was amplifying the attacks, and with a simple shell script, we were able
:> to build a database of ASNs, with admin contacts from RADB/RIPE/ARIN.
:> We are planning on sending emails to these customers to ask them to stop
:> amplifying smurfs (script does that too), because this is unacceptable.
:> My point, then is this:  Filtering echo replies is/can be a futile
:> attempt at preventing this type of attack.  I watched a 7507 die
:> defending against one attack.  More recently, we got hit so hard that
:> the router was screaming without ANY access-lists blocking ICMP
:> echo-replies, perhaps because there was no real fast-switching taking
:> place (each source is different, so the first packet is process
:> switched.  Our NetFlow cache went from 3900 flows to 27000 flows in
:> about 4 mins.)  And, as we were not amplifying the smurfs, source
:> address verification is a moot point.  I am all for allowing these
:> netblocks time to implement this type of filtering (layer3 to layer2
:> broadcast translation prevention), but not for very long.  It appears
:> the best way to light a fire under someones rear end is to publicly
:> shame them into acting.  For those who don't act quickly enough, there
:> can be no quarter.
:> If I appear hostile, I am...
:> PS
:> If anyone has similar experiences, please share them, as there has
:> already been enough rhetoric filling this thread, and it is clear that
:> everyone knows the solution lies at edge and beyond, not in the core.
:> -Christian Martin


Jason A. Lixfeld             jlixfeld () idirect ca
iDirect Network Operations   jlixfeld () torontointernetxchange net

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