[Dnsmasq-discuss] dnsmasq - dhcp unicast or multicast

[Albert ARIBAUD]

Hi Simon,

Le Wed, 26 Oct 2016 19:06:40 +0200
Simon Röblreiter <simon.roeblreiter at gasslfeld.at> a écrit:

> Hello, dnsmasq-community!
> 
> 
> I ran into a strange situation today.
> I was configuring a virtual mikrotik router (routerOS)
> to request a specific IP from my provider.
> 
> I did a series of tests in my local network.
> When I checked the traces I was capturing, I noticed,
> that routerOS was sending multiple dhcp-discover packets
> in a burst-like manner and dnsmasq responded multiple times.
> I suggest that is intended behaviour in order to ensure
> the fastest possible completion of the handshake.
>
> What puzzled me, was the fact, that each frame of the
> communication was a broadcast.
> Shouldn't each device send unicast frames as soon as
> it knows the mac-address of the other device?

A DHCP client can set a flag in its requests to ask that replies be
broadcast. Probably routerOS requests broadcasts.

> To be able to compare against something, i also traced
> the dhcp-handshake of my smartphone (Galaxy S2 / cyanogenmod13)
> to the same server.
> 
> You can view screenshots of the two traces through the following link:
> (I wanted to attach them to the mail, but there's a size limit of
> 40KB. @mod: Sorry for the repost. Didn't know that.)

Maybe you could have captured to file and attached the files? Plus,
this would have allowed me (or anyone else) to check whether the
client frames had the broadcast request flag set. :)

> https://www.gasslfeld.at/showcase/dnsmasq/
> 
> The cyanogenmod-handshake is doubled because I switched
> the wifi on and off two times.
> Both traces were captured with tshark directly on the machine
> running dnsmasq (OS=ubilinux).

DHCP clients are allowed to request broadcast replies.

> Any suggestions?

What do you need a suggestion for?

> Thanks in advance!
> Simon Röblreiter
> 
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Amicalement,
-- 
Albert.