dhcpd sending on the same IP it receives on

Simon Hobson dhcp1 at thehobsons.co.uk
Wed Dec 12 17:41:58 UTC 2007

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.

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