Issue with DHCP with Wireless card

Hugh McLenaghan hughmcl at
Fri Mar 14 16:37:36 CET 2014

If you do a packet trace you will see that it does send to the destination MAC address.  Remember MAC addresses are layer 2, and IP addresses are layer 3.  IP addresses just get the packet to the desired network.  Once on the desired network, the packets are actually sent to the destination MAC (after arp to decode destination IP to destination MAC).

So once the DHCP server assigns the IP address it sends it from itself using the IP address of the interface, and sends it to the device that requested the DHCP address using a unicast to the MAC address also supplying it's IP address.  But before it sends it, the DHCP server adds the MAC -> IP address to the ARP table.

Hope that helps :)


> Date: Fri, 14 Mar 2014 17:47:02 +0400
> From: iperegudov at
> To: freeradius-users at
> Subject: Re: Issue with DHCP with Wireless card
> On 13.03.2014 15:34, Arran Cudbard-Bell wrote:
>> Out of interest have you tried setting:
>> listen {
>> type = dhcp
>> broadcast = no
>> }
> I think server must use broadcast when sending DHCPOFFER to directly
> connected client.
> What happens when you send DHCPOFFER to directly connected client using
> unicast? DHCPOFFER is sent to destination IP address your-ip-address.
> Unicast packet should be sent to specific destination MAC address. To
> determine this destination MAC address server host uses ARP table to
> resolve your-ip-address to MAC address. ARP entry for your-ip-address is
> missing and server host have to send ARP request for your-ip-address.
> But client host is not yet received DHCPOFFER and cannot respond to this
> ARP request. ARP request timeout leads to DHCPOFFER dropped by the
> server host with destination unreachable error.
> -
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