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

[Harald Kipp]

Hi all,

while trying to figure out how to handle broadcasts the right way, I got
stuck. May be someone else can help.

There is a lot of discussion about how to transmit IPv4 packets to
255.255.255.255. RFC 919 specifies

 broadcast to all of its immediate neighbors

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 255.255.255.255. Several
stacks seem to behave completely insane.

http://serverfault.com/questions/72112/how-to-fix-the-global-broadcast-address-255-255-255-255-behavior-on-windows

In general we do not need to care much about this, because an
application can use directed broadcast to each individual net.

The real trouble I have is with DHCP discovery. The client must use
255.255.255.255 for the discovery telegram.

Let's assume, we have 2 Ethernet interfaces connected to 2 different
physical networks.

If 255.255.255.255 is always routed to the same "main interface", we
will never reach the DHCP server on the second network.

If we follow RFC 919, sending out one broadcast packet on each
interface, we will receive 4 offers, 2 from each interface providing 2
different IP adresses for each MAC address.

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."



Mh!

Harald