Subject: Re: DHCP query/reply using IP directed-broadcast address

From: "C. M. Heard" <[email-masked]>
Date: Wed, 9 Oct 2002 10:45:43 -0700 (PDT)

>>>>> Ramkumar Sankar wrote:
RS> is there any server implementation that replies to client requests using the
RS> 'subnet directed-broadcast' rather than the limited ip broadcast (i.e all
RS> 1s)? ...

>>>>> Joe Touch replied:
JT> What would be the utility in doing so, e.g., given the fact that they're
JT> no more likely to traverse a router than all-1's (see rfc2644)?

That's actually not true ... forwarding of limited broadcasts is categorically
forbidden, while forwarding of network-directed broadcasts is permitted but
must default to OFF unless specifically allowed.

That said, there are other problems with using a network-directed broadcast
with DHCP (or BOOTP), namely that a client that does not yet have a subnet
mask configured cannot tell the difference between a network-directed
broadcast address and a unicast address that happens to have a string of
1's at the tail end.  A network-directed broadcast, however, will be sent
as a link level broadcast when it arrives at the destination subnet, and
according to RFC 1122 Section 3.3.6 should be discarded:

         A host SHOULD silently discard a datagram that is received via
         a link-layer broadcast (see Section 2.4) but does not specify
         an IP multicast or broadcast destination address.

Fortunately, DHCP servers do not in general transmit replies to clients to
a broadcast address (see the discussion of the BROADCAST flag in RFC 2131
for exceptions) and when they do it's always to a client on an attached
subnet (a BOOTP relay agent to speak to clients on a remote subnet).  So
there is never any reason for a DHCP server to use a network-directed
broadcast in preference to all-1s.

//cmh