[Dnsmasq-discuss] ARP ignores DHCP ACKs !

Simon Kelley simon at thekelleys.org.uk
Thu Dec 26 17:05:05 GMT 2013

On 25/12/13 11:07, Nikita N. wrote:
>> The important point for your situation is that it's perfectly normal to
>> see a DHCP client make an ARP request as part of the address-aquisition
>> process, and for that ARP request to go un-answered.
> Hi Simon :)
> Yes, that I understood very clearly, such unanswered ARPs are normal,
> everybody told me that.. :))
> As matter of fact, such ARPs appear in XP and Vista too, but 2/3 frames
> at most..
> On Win7 they appear in number of 50/100 frames, multiple times, that
> raised my suspects..
> Still, would like to ask you about ICMP frames, if I can.. ;)
> because you say client uses ARP, the GW uses ICMP.. right?

The DHCP server uses ICMP. That's because the DHCP server may be on a 
different subnet to the client, if DHCP-relay is in use, so a level-3 
routable protocol is needed.
> Well, I cant see that.. the only ICMP I see are from the client to GW,
> sent after DNS answers from dnsmasq (dnsmasq runs on GW)..

To send the ICMP echo, the gateway on the local net will need to do ARP 
first. If the ARP is never answered, the ICMP echo requests will never 
make it onto the wire.

> As I wrote, I set to drop all in/out ICMP frames on GW (iptables), and
> still connection keeps alive allright on Vista, XP and Linux..
> But on Win7 cant see the light.. lots of those unanswered ARPs, few ARP
> req about who has GW, few Netbios, ipv6 frames and other useless
> protocol frames, stop.. Win7 gives up, red X on connection, finish.. any
> hint comes in your mind about that?
> About ICMP in general, since Im serving only local pages to client, is
> it ok in my local network to set drop all ICMP on GW?
> Thanks :)

Dropping ICMP is generally not a good idea. I'd be removing firewall 
rules to debug things, not adding them.

Also, try setting the dnsmasq dhcp-broadcast option.



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