Re: Questions about server's use of giaddr

[Shawn Mamros <[e-mail]>]

>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."

I don't know that I'd go that far...  Maybe it would be easier just to
say, "A relay agent MUST NOT use yiaddr to determine the interface on
which to relay a message from a DHCP or BOOTP server to a client; it MUST
use giaddr solely instead.  yiaddr MUST only be used as the destination
IP address when relaying a message from a server to a client (when the
broadcast bit is not set).  A relay agent MUST NOT send server responses
through a router, even if yiaddr and giaddr appear to be on different
logical networks, because the server could have knowledge about the
topology of the network that is unknown to the relay agent (e.g., the
logical networks indicated by yiaddr and giaddr may both reside on the
same physical network)."

Even if a relay agent were to have addresses configured on every logical
subnet, it still might get "confused" in the event of seeing a server
response that has yiaddr and giaddr on different networks.  I think my
language above would make it clear how to avoid such confusion.  giaddr
selects the interface, yiaddr specifies the unicast destination, and
leave the router(s) out of it...

-Shawn Mamros
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