Re: Questions about server's use of giaddr

[Ralph Droms <[e-mail]>]

Seems like this discussion has split into two parts - one concerning
implementation issues (how to choose among multiple subnets on a wire) and
one concerning relay agent issues (how does a relay agent send responses to
a client on a subnet for which the relay agent doesn't have an address).
I've sent my response to the former issue to the dhcp-impl list...

At 12:45 PM 1/31/96, Shawn Mamros wrote:
>Picture a relay agent that sits on top of a potentially multi-homed
>stack.  On one of its interfaces, it has an address of a.b.c.d with
>a subnet mask of 255.255.255.0.  It has been configured to relay
>broadcast DHCP or BOOTP requests received on this interface to some
>server on another net.  Unknown to the relay agent (for whatever reason),
>network x.y.z.0 (same subnet mask) shares the same wire as a.b.c.0.  The
>server is aware of this, and will assign x.y.z addresses when it runs out
>of a.b.c addresses.  Let's further assume that the relay agent does not
>have an x.y.z address on any of its interfaces.
>
>When the server sends back its reply to the relay agent, the destination
>IP address (in the IP header) and the giaddr will be a.b.c.d, but yiaddr
>will be x.y.z.something.  Unless it knows that a.b.c and x.y.z are on
>the same physical net, will it know how to properly relay the reply?

Ouch.  This sounds like an ugly problem.  I think you're right - relay
agents, especially those not running in a router - could very easily
receive server responses for which the relay agnent has no corresponding
interface.

Perhaps the easiest way to deal with this pthga case would be to add a few
words to RFC 1542 to the effect of: "A relay agent with an interface to a
network with multiple IP subnets SHOULD be configured with an address on
each of the IP subnets.  Otherwise, the relay agent may receive responses
from the DHCP server that it will be unable to forward to the client."

Anybody have a better suggestions?

- Ralph