[Dnsmasq-discuss] interface+macvlan on same network confuses dnsmasq v2.66rc2

Gui Iribarren gui at altermundi.net
Tue Oct 22 17:17:40 BST 2013

On 10/22/2013 05:45 PM, Simon Kelley wrote:
> On 21/10/13 20:31, Gui Iribarren wrote:
>> Hello Simon!

> My hunch is that this is something to do with ARP.

indeed! i'm sorry i didn't provide tcpdumps
you're right, instead of the expected DHCPOFFER, all i got on the client 
were endless ARP requests for the would-be-offered IP, coming from the 

so, what you're saying is that dnsmasq is prepopulating the ARP cache 
pointing to the wrong interface?

# arp -a
IP address       HW type     Flags       HW address            Mask 
Device    0x1         0x2         20:16:d8:65:4d:29     * 

that's on the router, just after asking for a lease from the client,
with dnsmasq.conf containing no-dhcp-interface=br-lan

Then, do you think there's any chance of getting that arp trick point to 

i'll try dhcp-broadcast later today
what would be the downsides / cons of having dhcp-broadcast permanently 
enabled? maybe some broken clients?

thanks a lot!


> For an unconfigured
> client (ie doing DHCPDISCOVER) dnsmasq ends up knowing the MAC address
> of the client and its IP address, but the client _doesn't_ know that IP
> address, and so can't respond to a ARP request. To get round this,
> dnsmasq populates the ARP table on the server directly with the IP
> address, MAC address and interface. Then it send the OFFER packet to the
> IP address and the kernel knows how to route it. I think that  you
> "interesting" setup may be confusing this process. In the second
> instance, I suspect that the crucial thing that makes it work is not the
> presence of a lease in the lease-file, but a correct ARP-table entry
> from the previous activity.
> Certainly, a few more experiments, and looking at the output of
> arp -a
> would be instructive.
> Looking again - there are more clues:
>   ### the OFFER it's sending is a funny mix:
>   ### it is sent *from* br-lan MAC and IP (
> That may be the MAC address thing I talked about, or it may be something
> else. Another experiment: try setting
> --dhcp-broadcast
> and see what happens then.
> Cheers,
> Simon
> Cheers,
> Simon.
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