[Kea-users] Duplicate Addresses, address pool exhaustion with DHCPDECLINE flood
Alberto Pollastro
alberto.pollastro at mobimesh.it
Thu Apr 18 13:16:29 UTC 2019
Hi all,
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:
https://www.isc.org/wp-content/uploads/2018/02/dhcp44.html).
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.
Thanks,
Alberto
Il 18/04/2019 09:08, Mohammed Khallaf ha scritto:
> Hello Kari,
>
> 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.
>
> 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.
>
> --
> MK
>
>
> On Wed, Apr 17, 2019 at 2:56 PM Kari Karvonen
> <kari.karvonen at kasenet.fi <mailto:kari.karvonen at kasenet.fi>> wrote:
>
> Hello
>
> If there is faulty DHCP-client on a network that keeps requesting IP's
> and after receiveing IP-offer client sends DHCPDECLINE and DHCP-server
> marks IP-address as declined for 24 hours. If client keeps repeating
> this, address after address will be marked as declined and soon entire
> DHCP-pool is exhausted.
>
> I looked Kea 1.5.0 user guide and found that it is possible to shorted
> decline time
>
> "decline-probation-period": 3600
>
> But is there something else on dhcp-server side to prevent this
> kind of
> behaviour?
>
> --
> Kari Karvonen
> Network specialist
> +358445557360
> www.kasenet.fi <http://www.kasenet.fi>
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