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<p>Hi all,</p>
<p>I agree with Kari; it could be useful to have an option which
permits to ignore the DHCP DECLINE messages like the one present
in ISC dhcpd ("declines" keyword in config file: <a
href="https://www.isc.org/wp-content/uploads/2018/02/dhcp44.html">https://www.isc.org/wp-content/uploads/2018/02/dhcp44.html</a>).<br>
Another option it could be to implement on server side a DHCP
DECLINE per source MAC rate limiting (or a kind of Fail2ban for
DECLINE messages) because usually the L2 switch support DHCP rate
limiting accordint to the switch port.</p>
<p>Thanks,<br>
Alberto<br>
</p>
<div class="moz-cite-prefix">Il 18/04/2019 09:08, Mohammed Khallaf
ha scritto:<br>
</div>
<blockquote type="cite"
cite="mid:CAHzwxC9u6ph5+K0yM=0p9wYOKcAiPcn-AoCdcVPn8jkq5Q6i-w@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">
<div>Hello Kari,</div>
<div><br>
</div>
<div>I'm not sure about Kea, Kea hooks, or if someone is going
to write a Kea hook for that, but there is no DHCP server that
I know about that implements this outside-of-the-box.
Actually, most or all effective solutions in
network-originating layer 2 attacks are basically built on
networking devices software and/or network monitoring
software, or the least: manual troubleshooting.</div>
<div><br>
</div>
<div>If your switching equipment has a feature to help, then use
it. If not, you can set a network monitoring software that
analyzes DHCP DISCOVER messages and alert you if the rate from
a specific MAC is abnormal, or the general rate on the network
is abnormal. SolarWinds and PRTG come to mind.</div>
<div><br>
</div>
<div>--</div>
<div>MK<br>
</div>
<div><br>
</div>
</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Wed, Apr 17, 2019 at 2:56
PM Kari Karvonen <<a href="mailto:kari.karvonen@kasenet.fi"
moz-do-not-send="true">kari.karvonen@kasenet.fi</a>>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">Hello<br>
<br>
If there is faulty DHCP-client on a network that keeps
requesting IP's<br>
and after receiveing IP-offer client sends DHCPDECLINE and
DHCP-server<br>
marks IP-address as declined for 24 hours. If client keeps
repeating<br>
this, address after address will be marked as declined and
soon entire<br>
DHCP-pool is exhausted.<br>
<br>
I looked Kea 1.5.0 user guide and found that it is possible to
shorted<br>
decline time<br>
<br>
"decline-probation-period": 3600<br>
<br>
But is there something else on dhcp-server side to prevent
this kind of<br>
behaviour?<br>
<br>
-- <br>
Kari Karvonen<br>
Network specialist<br>
+358445557360<br>
<a href="http://www.kasenet.fi" rel="noreferrer"
target="_blank" moz-do-not-send="true">www.kasenet.fi</a><br>
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</blockquote>
</div>
<br>
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