Last Call Review of draft-ietf-dhc-forcerenew-nonce-
review-ietf-dhc-forcerenew-nonce-genart-lc-campbell-2012-02-06-00

Hi,

This is a followup on my Gen-ART review of draft-ietf-dhc-forcerenew-nonce-04,
based on my previous review of version 03. In summary, this version is
improved, but I still don't think it's ready for publication.

On Feb 6, 2012, at 5:17 PM, Ben Campbell wrote:

> I am the assigned Gen-ART reviewer for this draft. For background on Gen-ART,
please see the FAQ at
<http://wiki.tools.ietf.org/area/gen/trac/wiki/GenArtfaq>. > > Please resolve
these comments along with any other Last Call comments you may receive. > >
Document: draft-ietf-dhc-forcerenew-nonce-03 > Reviewer: Ben Campbell > Review
Date: 2012-02-06 > IETF LC End Date: 2012-02-06 > > Summary:This draft is not
quite ready for publication as a proposed standard. There are some potentially
significant issues that should be addressed first. > > [Note: Hopefully this
draft has had or will have a SecDir review, since it seems ripe for significant
security implications.] >

I didn't see a comment about whether this happened. I still recommend a SecDir
review if one has not yet occurred.

>
> *** Major issues:
>
> -- I admit to not being a DHCP expert, but If I understand this draft
correctly, it proposes to send what is effectively a secret-key in a DHCPACK
request, then use that key to authenticate a force renew message. It seems like
any eavesdropper could sniff that key, and use it to spoof force renew
requests. The introduction mentions that there may be some environments where
the use of RFC3118 authentication could be relaxed, and offers an example of
such an environment. But nowhere does this draft appear to be limited in scope
to such environments. > > I think some additional text in (perhaps in the
security considerations) is needed to explain either why the vulnerability to
eavesdroppers is either okay in general, or limits the mechanism's use to
environment where it is okay. It also seems like that, in the best case, this
mechanism proves only that a Forcerenew request comes from the same DHCP server
as in the original transaction, but otherwise does not prove anything about the
identity of that server. If so, it would be worth mentioning it. >

This is improved by some changes to the introduction and the security
considerations. But I still don't see a clear, normative statement about what
environments this can be safely used in. I know Ted objected to Roberta's
specific proposal, but I think to move forward this needs _some_ clear,
concrete guidance about when to use or not to use it, or when 3118
authentication is more appropriate. Don't leave it up to the reader to draw the
right conclusion.

> -- The mechanism appears to be limited to HMAC-MD5, and there does not appear
to be any way of selecting other algorithms. Is HMAC-MD5 really sufficient as
the only choice? Is there some expectation that stronger mechanisms or
algorithm extensibility would be too expensive? (Perhaps the extensibility
method would be to specify another mechanism that's identical except for a
different HMAC algorithm. But if that's the intent it should be mentioned.)

There's a statement in the security considerations that "... security of the
nonce in the case of on-link attacks isn't relevant." It would help drive the
point home to add something to the effect that "Therefore HMAC-MD5 is by
definition adequate for the purpose, and there is no need for an extensible
HMAC mechanism."

>
> *** Minor issues:
>
> -- Section 1 "
> In such environments the mandatory coupling between FORCERENEW and DHCP
Authentication [RFC3118] can be relaxed." > > It's not clear to me what
connection that assertion has with this draft. Is there an intent that the
proposed mechanisms be used only in such environments? I don't find any
language scoping this proposal to any particular environment.

See comment under the first "major issue". I still think we need normative,
concrete guidance about when to use or not to use this mechanism.

[...]

>
> -- section 3.1.3, 2nd paragraph: "The server SHOULD NOT include this option
unless the client has indicated its capability in a DHCP Discovery message." >
> Why not; what harm would it do? And on the other hand, if you want to
discourage it, why not go all the way to "MUST NOT"? >

I don't think my comment has been addressed by the discussion so far. I
understand that the server should not insert a nonce unless the client has
advertised support--but is it the author's intent that the server should not
_advertise_ support unless the client has first advertised support?

Additionally, this update added the following text to the section:

> The
> server SHOULD NOT include the nonce in an ACK when responding to
>    a renew unless a nonce was generated.  This minimizes the number of
>    times the nonce is sent over the wire.

I don't understand the first sentence. Don't include a nonce unless you
generated it first? That seems like a tautology. Do you mean to say unless the
nonce was sent in a previous message? Or unless a _new_ nonce was generated?

[...]

>
> -- section 3.1.4, 1st paragraph: "DHCP servers that support Forcerenew nonce
Protocol authentication MUST include the DHCP Forcerenew Nonce protocol
authentication option…" > > Only if the client advertised it, right? Otherwise,
this seems to conflict with the previous SHOULD NOT.

The text is updated, but it still conflicts with the previous statement in
3.1.3, paragraph 2 that says the server SHOULD NOT insert the option unless the
client first inserted it. (See comment above).

[...]

>
> -- section 6:
>
> You mention this mechanism is vulnerable to MiTM attacks. Why is this okay?
Are there some environments where it is good enough and others where it is not?
(Also, do they really need to be MitM attackers? Seems like any eavesdropper
could learn the nonce.)

The text still refers to a MiTM attacker. Do you mean "on-link" attacker?

Otherwise, you've added text to motivate the mechanism for a specific use case.
But I'd still like to see concrete guidance about when this draft is
appropriate to use.

>
> *** Nits/editorial comments:
>
> -- Abstract, second sentence:
>
> I have trouble parsing this sentence. Are the propositions correct?

Partially fixed. Should "on the initial ACK" be "in the initial ACK"?

>
> -- Section 3.1.1,4th paragraph: "… configured to require …"
>
> Do you mean configured to "require", or configured to "use"? I would normally
take "requires" to mean that the server would not work with clients that don't
advertise support for the mechanism.

You changed the text from "configured to require" to "requires". I'm still
confused on whether "requires" means that the server will refuse to service
clients that don't advertise support. This is in contrast to "use", which would
mean to me the server would invoke the option if the client supports it, but
still work with clients that don't support it.

[...]