[Dnsmasq-discuss] dnsmasq - dhcp unicast or multicast

[Simon Röblreiter]

Bonsoir, Albert!

I should have taken a closer look at
https://tools.ietf.org/html/rfc2131.

It is explained well there.
My problem was just that i did not know what to look for.

RouterOS indeed sets the broadcast flag in each message.

>> Any suggestions?
>
> What do you need a suggestion for?

I want to reduce broadcast traffic as much as possible to keep the
network clean, but as your answer implies, this behaviour is
RFC-conform and the only point I can work on is the client.

I guess even IF there was a way to force unicast replies,
it would not be a sane decision because you never can tell
how some clients react.

I will therefore try to dig into the DHCP-Client of RouterOS and see,
what I can do.

Merci beaucoup!


Am 26.10.2016 um 22:11 schrieb 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,
>