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Re: DHCP Flood on inside network. HELP!!
From: Gregory Gilliss <ggilliss () netpublishing com>
Date: Mon, 11 Oct 2004 23:53:18 -0700

Sounds like someone discovered the DHCP discover flood trick and set it
to work on you. A little packet filtering kung fu on your part ought to be 
sufficient to prevent it happening again.

-- Greg

On or about 2004.10.11 22:00:07 +0000, Eddie  (EddieS () softhome net) said:

I don't have much information on this yet, I am driving down to the office now to pull an all nighter. I figured I 
would toss this out to the list and see if anyone has any 
idea.  This is just info from what I can get from talking to people and what little time I can get on the network 
before it goes down. 

Starting 2 days ago, I discovered the PIX 515 was locked hard.  It seems to be random, but around every 15-30 minutes 
something floods the network hard for a few 
minutes. Broadcast flood too. This is a small network with 30 workstations and 5 servers (Linux and SCO, no Wins). It 
overloads the Extreme switches and I see pdu (or 
something like that, not udp tho) errors on about every port. 
The Pix 515  overloads and is having issues, but I did see it say something about ARP problems when I could get to 
the syslog for more info. I looked up the error 
number and it said it could be ARP poisoning. Not sure what would do that. 

In the syslog of the DHCP server, I see thousands of DHCP DISCOVER request(and the REPLAY request from the server, a 
Linux box).  It looks like one client on the 
network (I have seen this both from XP and Win98) will send 100+  DISCOVER request a second swamping the network. Not 
always DISCOVER too. 
That will go on for a few minutes, then all is well. Then another computer will do the same thing. 

This is quickly overloading things and I am getting IRQ busy and overload errors on some of the servers. 

What should I look for. I have never seen something like this before. 


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Gregory A. Gilliss, CISSP                              E-mail: greg () gilliss com
Computer Security                             WWW: http://www.gilliss.com/greg/
PGP Key fingerprint 2F 0B 70 AE 5F 8E 71 7A 2D 86 52 BA B7 83 D9 B4 14 0E 8C A3

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