unknown UDP caused by dhclient

Kimmo Paasiala kpaasial at gmail.com
Thu Jun 4 19:01:03 UTC 2015

On Thu, Jun 4, 2015 at 9:20 PM, Matthias Apitz <guru at unixarea.de> wrote:
> El día Thursday, June 04, 2015 a las 07:54:35PM +0300, Kimmo Paasiala escribió:
>> That is how a DHCP client ask for lease renewal from the DHCP server,
>> you should allow the traffic if the interface in question is
>> configured to use DHCP.
> Thanks for your kind answer. I was wondering why I only see this on the
> ue0 interface (which is to my Ubuntu mobile phone when I'm in the
> fields) and not on the Wifi wlan0. But, perhaps this is due to the very
> short renewal interval of 1800 secs:
> DHCPREQUEST on ue0 to port 67
> DHCPACK from
> bound to -- renewal in 1800 seconds.
> I will let pass this traffic from now.
>         matthias
> --
> Matthias Apitz, guru at unixarea.de, http://www.unixarea.de/ +49-170-4527211    +49-176-38902045
> "Wenn der Mensch von den Umständen gebildet wird, so muß man die Umstände menschlich bilden."
> "Si el hombre es formado por las circunstancias entonces es necesario formar humanamente
> las circunstancias", Karl Marx in Die heilige Familie / La sagrada familia (MEW 2, 138)

What you saw there was the most specific way to ask for lease renewal
using the last known address of the DHCP server. If that fails the
client falls back to broadcasting to because the DHCP
server might have relocated to a new address in the subnet. If even
that fails the client will start over from zero broadcasting to DHCP is a bit complicated case for stateful
filtering, that's why you should allow all outgoing UDP traffic to
port 67 regardless of addresses.


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