dhcpd sending on the same IP it receives on

Phil Mayers p.mayers at imperial.ac.uk
Wed Dec 12 23:41:03 UTC 2007

On Wed, Dec 12, 2007 at 02:47:59PM -0800, Scott Baker wrote:
>Simon Hobson wrote:
>> Phil Mayers wrote:
>>> I have tried to use dhcpd in various policy-routed ways in the past,
>>> and it is hard. It would be nice if dhcpd behaved in a similar manner
>>> to bind/named and ntpd, and bound a file descriptor to each IP
>>> (re-discovering the IPs every N minutes) and always replied via the
>>> same file descriptor the request came in on.
>> Can it always do that ?
>> bind and ntpd have the luxury of only ever dealing with unicast packets,
>> dhcpd must deal with broadcast packets.
>> Also, back to the original problem, I would suggest that the devices
>> being fussy are broken ! It is valid and legal to set the server address
>> to a broadcast address and hence have the packets 'delivered' to
>> multiple servers. Since it is (in the general case) not possible* to
>> determine whether an address is a broadcast address they should not
>> assume or expect replies to return from the address they sent the
>> request to.
>> * Well you can infer that a zero lsb of the address means that it can't
>> be a broadcast. And you can infer that if the next bit is zero then it's
>> highly unlikely that it's a broadcast address. But that's about it.
>In my case our DHCP server is ONLY acting as a DHCP-Relay receiver.


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