Subject: [dhcwg] should the client id be unique across subnets?

From: B Naveen <[email-masked]>
Date: Tue, 11 Feb 2003 17:45:45 +0530

RFC 2132 says," the client identifier value is expected
to be unique for all clients in an administrative domain "
(section 9.14).

Also under the same section, we have  "for correct
identification of clients, each client's client-identifier MUST be
unique among the client-identifiers used on the subnet to which
the client is attached".

Say a client is plugged to subnet A. After a while, this machine
is moved to subnet B. At this time, it is desirable for the dhcp
server to release the previous IP address, and grant a new IP
address to the machine in the new subnet. The identification of
the client is through the client-identifier. Therefore, if the
client-identifier is unique among all the dhcp clients, then we
can implement this feature.

But unfortunately, if there are 2 clients with the same client
identifier but different subnet, then there may be a problem
with the above mentioned feature.

I am not very clear on this part of client-identifier. Can
anybody throw some light on this issue?

Thanks and Regards,
Naveen.

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From: Robert Elz <[email-masked]>
Date: Tue, 11 Feb 2003 20:11:48 +0700

Date:        Tue, 11 Feb 2003 17:45:45 +0530
    From:        B Naveen <[email-masked]>
    Message-ID:  <[email-masked]>

  | Say a client is plugged to subnet A. After a while, this machine
  | is moved to subnet B. At this time, it is desirable for the dhcp
  | server to release the previous IP address,

No, it shouldn't do that (until the lease expires anyway).

  | and grant a new IP address to the machine in the new subnet.

Obviously...

If the client then moves back to subnet A, while its lease on that
subnet is still valid (and assuming it wasn't released) then the
client can (after attempting to communicate with the DHCP server,
and perhaps failing) simply use the old lease.

If the server has released it, just because the client turned up
somewhere else (and worse, reassigned it to someone else) that fails.

Lease times MUST be respected by servers (excepting only when they
receive a release request).

I think this makes the rest of your question moot.   That is, it
simply doesn't matter whether client ids are supposed to be unique
everywhere, or only unique on the net (about the only place where
it might matter would be if the client wanted to release an old
address after it has moved to a new subnet).

kre

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From: "Vasu, Vallabhaneni" <[email-masked]>
Date: Tue, 11 Feb 2003 07:52:31 -0600

> with the above mentioned feature.

Yup you are right about this. I have seen the dhcp servers they look for
the client identifier to be unique for the administrative domain of the
dhcp server and I think this is the correct behaviour.

I think clients should generate their client identifier as they defined
in IPv6 dhcp RFC.