DHCPOFFER sent to wrong MAC?

[Jason Lixfeld]

On 18-Aug-06, at 4:28 PM, George C. Kaplan wrote:

> Jason Lixfeld wrote:
>> On 18-Aug-06, at 12:18 PM, Simon Hobson wrote:
>>
>>
>>> Jason Lixfeld wrote:
>>>
>>>> I have isc-dhcpd-3.0.4 installed via FreeBSD ports on a 6.1- 
>>>> RELEASE-
>>>> p3 machine with a custom kernel.
>>>>
>>>> My dhcp server is at 00:50:04:d1:c9:f0
>>>> My dhcp client is at 00:11:a0:02:67:36
>>>>
>>>> The dhcp server logs show constant discover/offer pairs, but the
>>>> client does not respond with a request, etc...
>>>>
>>>> Aug 17 14:35:59 bubbles dhcpd: DHCPDISCOVER from 00:11:a0:02:67:36
>>>> via xl0
>>>> Aug 17 14:35:59 bubbles dhcpd: DHCPOFFER on 192.168.100.249 to
>>>> 00:11:a0:02:67:36 via xl0
>>>> Aug 17 14:36:01 bubbles dhcpd: DHCPDISCOVER from 00:11:a0:02:67:36
>>>> via xl0
>>>> Aug 17 14:36:01 bubbles dhcpd: DHCPOFFER on 192.168.100.249 to
>>>> 00:11:a0:02:67:36 via xl0
>>>> Aug 17 14:36:03 bubbles dhcpd: DHCPDISCOVER from 00:11:a0:02:67:36
>>>> via xl0
>>>> Aug 17 14:36:03 bubbles dhcpd: DHCPOFFER on 192.168.100.249 to
>>>> 00:11:a0:02:67:36 via xl0
>>>>
>>>> I span'd the port on my switch which the dhcp server is  
>>>> connected to,
>>>> and ran ethereal on it and noticed this:
>>>>
>>>> # Discover
>>>> <snip>
>>>> Ethernet II, Src: VtechEng_02:67:36 (00:11:a0:02:67:36), Dst:
>>>> Broadcast (ff:ff:ff:ff:ff:ff)
>>>>     Destination: Broadcast (ff:ff:ff:ff:ff:ff)
>>>>     Source: VtechEng_02:67:36 (00:11:a0:02:67:36)
>>>> </snip>
>>>>
>>>> # Offer
>>>> <snip>
>>>> Ethernet II, Src: 3com_d1:c9:f0 (00:50:04:d1:c9:f0), Dst:
>>>> Cisco_bb:c6:a1 (00:05:9b:bb:c6:a1)
>>>>     Destination: Cisco_bb:c6:a1 (00:05:9b:bb:c6:a1)
>>>>     Source: 3com_d1:c9:f0 (00:50:04:d1:c9:f0)
>>>> </snip>
>>>>
>>>> It's sending the offer to the wrong MAC address.  00:05:9b:bb:c6:a1
>>>> is the MAC address of my default gateway router.
>>>>
>>>> Is this a configuration glitch on my end or is something else  
>>>> wrong?
>>>> All clients appear to be affected similarly.
>>>
>>> Just a stab in the dark, do you have a spurious routing table entry
>>> directing 255.255.255.255 via the router ?
>>
>>
>> No, no entry for 255.255.255.255:
>
> A couple of things to check on the router:
>
> - Is the netmask correct?

Netmask is indeed correct.  One of the first things I verified,  
albeit if the netmask was wrong, I would probably have seen other  
problems much sooner :)

> - Is proxy-ARP enabled?

It was, and I disabled it, but that didn't seem to make a difference.

What DID seem to fix it was to configure a helper address.  This  
seems strange to me because I shouldn't have to configure the helper  
address on an interface if the dhcp server is on the same network as  
the interface the helper is configured for.

I wonder if I have some buggy IOS.

> -- 
> George C. Kaplan                            gckaplan at ack.berkeley.edu
> IST - Infrastructure Services               510-643-0496
> University of California at Berkeley
>
>