[En-Nut-Discussion] DHCP on multiple interfaces?

Ole Reinhardt ole.reinhardt at embedded-it.de
Fri Dec 11 14:05:59 CET 2009

Hi Harald,

let me make an educated guess, based on consolidated half-knowledge.

> but it doesn't exactly describe, what this means in case of multiple
> interfaces. Today's IP stacks seem to handle this in different ways.
> Often one packet is sent to the "main interface" only, whatever that
> means. Many stacks allow to set a route to Several
> stacks seem to behave completely insane.

On linux a packet to won't be routed by default. So a
DHCP request is always assigned to one interface. Will mean: DHCP client
sends it broadcast only to the interface which shall be configured.
Everything else does not make much sense, right? The DHCP offer also
only contains the answer for the one interface the request was send
over. Last but not least the dhcp offer is based on the mac address of
the used interface (where I have to admit that I don't know if the DHCP
request contains an own copy of the mac address or if the server get's
the info from the IP header of the packet.)

> RFC 2131 specifies
> "3.6 Use of DHCP in clients with multiple interfaces
> A client with multiple network interfaces must use DHCP through each
> interface independently to obtain configuration information parameters
> for those separate interfaces."

That's exactly the behaviour I described above, isn't it?




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