I confess that I don't see the problem. The updater would do a DNS
query for DHCID RRs; it would be given all of the stored
records.
That's not how the current update algorithm works. Sure, we could do
almost anything but we'll be debating this for the next 100 years. It
has already gone on for almost 10 years!!!
Can we get serious about this and really ask what are we trying to
protect.
And where were you folks when IPv6 was designed to use the mac address
as the interface identifier. Come on.
We're trying to make it NON-TRIVIAL, not impossible.
This technique has been in use for years by implementations using TXT
records because we've been unable to get the DHCID RR approved.
- Bernie
-----Original Message-----
From: dhcwg-bounces(_at_)ietf(_dot_)org
[mailto:dhcwg-bounces(_at_)ietf(_dot_)org]
On Behalf Of Steven M. Bellovin
Sent: Monday, November 28, 2005 11:49 AM
To: Ted Lemon
Cc: iesg(_at_)ietf(_dot_)org; dhcwg(_at_)ietf(_dot_)org;
namedroppers(_at_)ops(_dot_)ietf(_dot_)org;
Pekka Savola; ietf(_at_)ietf(_dot_)org
Subject: [dhcwg] Re: DHCID and the use of MD5 [Re: Last Call:
'Resolution ofFQDN Conflicts among DHCP Clients' to Proposed
Standard]
In message <200511261243(_dot_)21694(_dot_)mellon(_at_)fugue(_dot_)com>, Ted
Lemon writes:
Making a hash function interchangeable in DHCID makes the
conflict detection
algorithm hugely more complicated, and possibly not workable
at all. Think
about how that would work.
I confess that I don't see the problem. The updater would do a DNS
query for DHCID RRs; it would be given all of the stored
records. The
updater would then use local policy -- that is, an ordered list of
preferred hash functions -- until it found one that was in the
response. That one would be used. If no locally-known hash
functions
are in the list, it should behave as if there were no DHCID records
present for that name. DNSSEC could protect against
downgrade attacks.
(Speaking of which -- were I still AD, I'd ding this document for an
inadequate Security Considerations section -- apart from the
lack of discussion of brute force attacks, you should cite
3833 for DNS
attacks and explain what the risks are if someone can crack the hash
function by any means, including brute force or eavesdropping on the
wire or (perhaps) a misbehaving updater.)
If you don't agree, I'd strongly suggest using SHA-256
instead of MD5.
Yes, it's more expensive, but I doubt that that's a major hit on
overall system performance here. It would also be useful to
include in
the document some discussion of upgrade strategy -- how would we ever
switch to a new hash function? That's non-trivial even for protocols
designed for agility, as Eric Rescorla and I have shown. No
matter how
it's done, this one is among the very hardest, since DNS
servers would
have to supply DHCID records for several hashes for a number of years.
--Steven M. Bellovin, http://www.cs.columbia.edu/~smb
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