RE: [dhcwg] Re: DHCID and the use of MD5 [Re: Last Call:'Resolution of FQDN Conflicts among DHCP Clients' to Proposed Standard]

[Jeffrey Hutzelman <jhutz at cmu.edu>]

On Sunday, December 04, 2005 11:58:25 PM -0500 "Bernie Volz (volz)" 
<volz at cisco.com> wrote:

> If you're going to have multiple DHCP servers, such as failover pairs,
> doing the DNS updates, you need to have those servers agree on how they
> will identify the clients. This is not JUST for DNS updates. Failover
> partners need to use the same identifiers for clients.

The document I read identifies several possible situations in which DHCID 
records are used to coordinate between updaters which are not DHCP failover 
partners.  It discusses a scenario in which multiple clients may attempt to 
issue updates for the same name (and, presumably, in which more than one 
client is authorized to issue such an update; otherwise, there would be no 
problem), and one in which a client moves between subnets served by 
different DHCP servers, both of which are authorized issue updates for the 
client's FQDN.

You can plausibly argue that the two DHCP servers in the second scenario, 
while not failover partners, are nonetheless part of the same 
administrative domain and require coordination.  Such an argument seems a 
little weak to me, but if that were the only issue, I could live with it.

I suppose you can also argue that two clients configured to use the same 
name will (by design) not produce the same DHCID RR's even if they use the 
same type, and therefore there's not a problem if they use different types. 
That I'll definitely buy.

However, what about a scenario where both a client and the DHCP server on 
its home network are authorized to do the updates.  When the client is at 
home, it lets the server do the update.  When it is off-site at an IETF 
meeting, the IETF DHCP server has no authorization to update the client's 
fqdn, so the client must do so itself.  Now, if the client and its home 
DHCP server disagree on which type to use, then the update may fail.


> The rules are pretty clearly described in the RFC:
>
> For DHCPv4:
> 1. Use the DUID if the client identifier option is provided by the
> client and it is a DUID and the server supports it. This is a new RFC
> that is in the RFC-editor queue so no clients and servers yet support
> this.
> 2. Otherwise, use the client identifier option if provided by the
> client,
> 3. Otherwise, use htype and chaddr.

The rules are clear, but require that all possible updaters have the same 
view of the world.  Consider the client I described above, which identifies 
itself with a DUID.  So, as far as the client knows, it should follow rule 
1 and use the DUID to form a DHCID record.  Unfortunately, the client's 
home DHCP server doesn't support DUID's, so it skips rule 1 and follows 
rule 2, using the client identifier.

It gets worse when you start adding new types after DHCID support is widely 
deployed.  In fact, you arguably have that problem already, since a client 
supporting this option doesn't know whether its home server even supports 
the DHCID RR, as opposed to using the TXT record method.


If you think my example involving both a client and a server is contrived, 
consider a client which always does its own updates.  When such a client is 
updated, it may begin supporting new DHCID record types.  It may begin 
supporting DUID's, or even be required to switch from non-DUID client 
identifiers to DUID's because it now supports IPv6.  In each of these 
cases, the client will fail to perform an update because its new DHCID 
value is different from the old one.  You can require the client to give up 
its lease and remove the record prior to shutting down, but such a 
requirement is fragile because a client may shut down unexpectedly, with no 
chance to send a DDNS update first.



In short, as long as the only authorized updaters are a set of carefully 
coordinated DHCP servers which never receive configuration changes or 
software upgrades, everything works.  Once you introduce clients (which by 
their nature are unpredictable) or support for new DHCID types, the 
negotiation problem becomes an issue.  It's possible to work around by 
performing an extra query to determine what DHCID type is in use, but you 
seem to want to avoid that.


-- Jeff

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