How Somebody Helped Kill dhcpd on Our Network

Martin McCormick martin at
Mon Jul 31 13:03:20 UTC 2006

	We recently had our dhcp V3.0.3 system crash or, I should
say, crashed by a denial-of-service scheme in which the miscreant
ran something on his/her computer that behaved like a dhcp server
enough to send DHCPNAK's to every dhcp request it saw on the
VLAN.  This made every client send more traffic to the real dhcp
server and apparently caused the 1-gig processor with 1 gig of
RAM to consume all available memory.  For 3 minutes, it generated
about 2,500 "out of memory" messages and then dhcpd finally
exited with a 11) SIGSEG and a core dump.  Good old FreeBSD unix
is pretty bullet-proof, but this was more than the box could
handle.  The platform continued to operate properly afterwards,
but dhcpd had to be restarted again.

	We are in the process of instituting dhcp failover with a
second server although I suspect that this situation would have
added maybe another few minutes to the mayhem before it, to,
succumbed to the hammering since failover isn't going to protect
against external attacks that hit both servers equally.

	We are also installing switches with port snooping
capabilities as funds permit to kill off anybody who aspires to
run dhcpd on anything other than the proper dhcp server.  This
brings me to my question.

	Is there any particular configuration parameter I can use
in dhcpd to make it rate-limit itself since it would have been a
much better outcome for it to come up for air, so to speak,
rather than crash and have to be restarted.

	This particular event occurred on a Sunday afternoon on
the weekend that classes finished for the Summer semester and
campus activity was as near to dead as it ever gets around here
so we didn't even know anything was wrong for a period of time.

	Our dhcp server gives service to around 10,000 clients of
several types and the platform it is on hardly breaks a sweat
even on the busiest days, but this was too much to handle.

Martin McCormick WB5AGZ  Stillwater, OK 
Systems Engineer
OSU Information Technology Department Network Operations Group

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