[Dnsmasq-discuss] Re: DHCP: different IPs for same same MAC

Jan 'RedBully' Seiffert redbully at cc.fh-luh.de
Mon May 5 02:57:06 BST 2008

Sven Köhler wrote:
>>>  here's the leases file from a currently running dnsmasq installation:
>>>  86400 00:12:f0:a1:81:7d bert 01:00:12:f0:a1:81:7d
>>>  86400 00:12:f0:a1:81:7d *
>>> ff:65:74:68:31:00:01:00:01:0e:91:06:82:00:00:f0:7a:14:04
>>>  How can that be? Same MAC-addresses, but different IPs?
>>>  Well, if i boot Linux on this comuter, it get's the
>>>  If i boot Windows, then it get's the Why is that?
>>>  (BTW: if i delete the leases file, restart dnsmasq, and boot windows 
>>> first,
>>> then windows get's the 22 and linux get's the 233).
>> One is sending a client ID and one is not.  The fact that switching
>> them reverses it seems odd to me, since I would have a possible
>> explanation if it didn't...
> My guess is, that the two DHCP-requests by the two OS differ - and 
> dnsmasq compares them, and then says: oh, different DHCP request, so 
> let's give him a different IP (which seems completely beyond the idea of 
> DHCP to me, since leases-file should only be a MAC->IP mapping).

In dnsmasq, client-ID overrides MAC-Address, and thats the difference 
between the two requests.
I was bitten by this when my linux-distro updated the dhcpc, now sending 
a random id, when sending none before...

so you want to change in your dnsmasq.conf:
dhcp-host=00:00:F0:7A:14:04  , #bert
dhcp-host=00:00:F0:7A:14:04  , id:* , #bert


The Theorem Theorem: If If, Then Then.

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