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Re: FW: Discuss on draft-ietf-dhc-agentopt-radius-06.txt (fwd)
---------- Forwarded message ----------
Date: Wed, 23 Jun 2004 11:13:20 -0400
From: Ralph Droms <rdroms@cisco.com>
To: Thomas Narten <narten@us.ibm.com>
Cc: aboba@internaut.com, Margaret Wasserman <margaret@thingmagic.com>,
aaa-doctors@ops.ietf.org, "Wijnen, Bert (Bert)" <bwijnen@lucent.com>,
jschnizl@cisco.com
Subject: Re: FW: Discuss on draft-ietf-dhc-agentopt-radius-06.txt
Thomas (copying Bernard, as I think the aaa-doctors list is filtered),
In fact, John and I have already responded to the comments from the
aaa-doctors (see attached document). We sent this response to Bert about
three weeks ago. As all of the substantive issues in the comments from the
aaa-doctors have been discussed in the past and are answered in our
response, there's probably no need for a discussion on the dhc WG mailing list.
- Ralph
At 10:51 AM 6/23/2004 -0400, Thomas Narten wrote:
Bernard (or some other volunteer):
Bert is out, but I don't think this can wait.
I think it is great that AAA doctors has been created. But we also
need to make it work for document authors/WGs that get feedback.
In this case, a number of comments (some overlapping) were returned,
but there is frustration at the lack of clear process for resolving
the issues in a constructive fashion. In partcular...
We need to get discussion going out on the DHC mailing list or some
other open forum.
It is unclear whether the individual comments are supposed to be
anonymous or not. It is OK (if not ideal) for them to be anonymous,
but then we need someone who will act as a representative that will
deal with getting them resolved to mutual satisfaction.
It is unclear whether the comments have been registered in ID Tracker
properly (I would argue that if they do not show up as "discuss"
comments, they are not properly recorded). In any case, they are hard
to find...
So, could the AAA doctors please
- assign a person (or persons) to act as a shepherd for dealing with
the issues for the document
- can we agree to a plan for getting the technical discussion on the
issues moved to the DHC mailng list so we can get some sort of
closure?
Thanks,
Thomas
> - There was a review of revision 5 back in ..
> I checkedith the reviewer of back then and aksed if the issues
> raised against rev 5 were addressed. This is the response:
> Yes, I did check and the comments that related to updates to
> RFC 2865 were not addressed.
> The AAA-doctors picked up on most of those issues (and found
> a lot more things, too).
> I have attached below (at the very bottom of this email),
> the review copmments from Bernar on rev 5.
We addressed Bernard's comments on rev 5 in an earlier e-mail to you.
> - Comments on revision 6 (again from Bernard):
> Draft -06 states:
>
> " The NAS truncates the RADIUS attributes to fit in the RADIUS
> Attributes sub-option. For predictable behavior, the RADIUS
> server should be configured to return few than 255 octets of
> RADIUS attributes."
>
> In RADIUS, a single attribute (such as User-Name) can be 253 octets,
> and packets may be up to 4096 octets in length. Since the draft does
> not provide a way for the NAS to tell the RADIUS server that this
> specification is implemented, it would seem like a RADIUS server
> would always have to configured to return no more than 255 octets
> of RADIUS attributes in order to function correctly.
>
> That's a pretty major constaint on RADIUS server implementations.
> I'm not sure why this is necessary, since RFC 3396 enables encoding
> of long options in DHCPv4.
>
> The draft also imposes other constraints on RADIUS implementations
> using normative language. Given that it is not possible for the RADIUS
> server to know if this specification is being implemented, the effect
> is to update RFC 2865. This seems inappropriate to me.
Because the RADIUS server and NAS are in the same administrative
domain, the RADIUS server can be configured to return the appropriate
attributes to the NAS, sized to fit in the RADIUS Attributes
sub-option, and the RADIUS server can be aware of whether the NAS is
configured to use the RADIUS Attributes sub-option.
The last paragraph in section 5 of the draft is:
The relay agent SHOULD include the User-Name and Class attributes
in the RADIUS Attributes sub-option if available, and MAY include
other attributes.
which specifies how the relay agent should behave if the attributes
from the RADIUS server will not fit in the RADIUS Attributes
sub-option.
While RFC 3396 enables encoding of long options in DHCP, it does not
specify the encoding of long relay agent option sub-options.
Therefore, the RADIUS Attributes sub-option is still contrained to
carry 253 octets of data.
We don't believe the RADIUS Attributes sub-option specifies any
updates to RFC 2865, because the RADIUS server can be configured to
send the appropriate attributes.
> - Comments from various other AAA doctors below.
>
> Bert
>
> -----Original Message-----
> From: Nelson, David [mailto:dnelson@enterasys.com]
> Sent: woensdag 28 april 2004 19:59
> To: aaa-doctors@ops.ietf.org
> Subject: RE: Pls review/comment on:
> draft-ietf-dhc-agentopt-radius-06.txt
>
>
>
> Top of page 4:
>
> "...in RFC 2865, in octets b1...bN." In the corresponding figure, the
> octets are labeled as o1...oN.
Typo to be fixed...
> I concur with Bernard's comments about the attributes truncation
> issue.
We've addressed the truncation issue above.
> On page 4:
>
> " ...a RADIUS server SHOULD send only those
> attributes for which the relay agent can ensure that either the
> relay agent or the DHCP server will provide the associated
> service."
>
> How would the RADIUS Server know? RADIUS provides for the use of "hint"
> attributes in Access-Request messages, which the RADIUS Server may use
> to determine an appropriate set of atttributes for includion in an
> Access-Accept message. Does this ID anticiapte the use of hints for
> conveying the capabiliites of the DHCP realy agent and/or server to the
> RADIUS Server? Otherwise, it is not clear how this "SHOULD" requirement
> might be met.
Because the RADIUS server, the relay agent and the DHCP server are all
presumed to be in the same administrative domain, the RADIUS server can
be configured to provide only the appropriate attributes.
> Why does the DHCP Server want to see the RADIUS User-Name attribute? Is
> there an intent for the DHCP server to make AAA-like policy decisions
> based on user identity? It should be mentioned that in some RADIUS use
> cases, the User-Name attribute is only a "billing identity" or an
> "anonymous" identity, with the acutal authenticated user identity only
> available to the peers of an EAP session.
Many DHCP services make use of different pieces of information about
the DHCP client to determine the exact configuration information,
including the assigned IP address, to be returned to the client. The
intent of this sub-option is to make attributes supplied by the RADIUS
server, such as the User-Name, available to the DHCP server as part of
the information available about the DHCP client.
> I agree with Bernard's comment that any normative text that would modify
> the behavior described in the base RADIUS RFCs would seem to be
> inappropriate in this document.
We agree that modifying the RADIUS specifications would be
inappropriate and we don't believe the RADIUS Attributes sub-option
specification does so.
> It seems to me that some guidance should be provided in this document,
> specifically addressing the static IP address assignment attribute of
> RADIUS, and the incompatibility of that attribute with DHCP. While
> static IP address assignment via RADIUS is little (not ?) used today, it
> probably ought to be discusssed.
The table in section 4 lists the attributes that can be used with the
RADIUS Attributes sub-option, and specifically does not include
Framed-IP-Address. Is that sufficient? Also, wouldn't
Framed-IP-Address typically not be used with 802.1X authentication?
> If the DHCP Server is using the contents of the sub-option as advisory
> material (i.e. "hints") as to how to provision DHCP information for the
> DHCP client, then the security model as stated is probably sufficient.
> This document specifies a transitive trust relationship. The NAS and
> RADIUS Server establish trust by means of a shared secret, and
> optionally by use of IPsec protections. The NAS (and therefore the DHCP
> Relay Agent) and the DHCP Server establish trust by the measns
> referenced in the Security Consideration section. I haven't taken any
> time to give serious consideration to whether this provides the
> opportunity for any form of attack based on a compromised NAS, that
> would not already be covered by the Security Considerations section of
> the base RADIUS RFC(s).
>
> -- Dave
The summary above does reflect our intention for security model in
this specification. Yes, it involves transitive trust. It is beyond
the scope of security in DHCP to deal with the situation in which a
NAS has been compromised.
> -----Original Message-----
> From: Greg Weber [mailto:gdweber@cisco.com]
> Sent: donderdag 29 april 2004 3:23
> To: bwijnen@lucent.com
> Cc: aaa-doctors@ops.ietf.org
> Subject: Re: Pls review/comment on:
> draft-ietf-dhc-agentopt-radius-06.txt
>
>
>
> Some additional comments,
>
> The table in Section 4 lists RADIUS attributes which MAY be
> returned by the server to the NAS, but most of these are
> precluded from inclusion in Access-Accepts by RFC 2865 Section
> 5.44. E.g. Calling-Session-Id (attr.30) is not returned by
> the server- it is sent to the server. If the intent is that
> the NAS supplies these directly to the DHCP relay, that
> conflicts with Section 5 of the draft:
> "The RADIUS Attributes sub-option MUST only contain the
> attributes provided in the RADIUS Access/Accept message."
The table in section 4 was derived from the list in RFC 3580. We will
remove attributes 4, 30, 31, 32, 44, 50 and 87, which appear not to be
valid in Access-Accept messages. There is no intent for the NAS to
supply any attributes to the DHCP relay, so there is no conflict with
the text quoted from Section 5.
> Guidance is needed on the lifetime of the authorization
> data. DHCP is independent of 802.1x; there is no guarantee
> that any DHCP packets will immediately (or ever) follow
> the authentication process. How long is the NAS supposed to
> hang on to this authorziation data hoping to insert it into
> DHCP requests? Are the data inserted into all subsequent
> DHCP requests- until what point?
We assumed that the NAS would only insert the attributes received
from the RADIUS for the duration of the 802.1X session. We can
clarify if necessary. While we recognize there is no guarantee that
DHCP will be used with 802.1X, we expect that there will be many cases
in which the DHCP will be used through an 802.1X authorized port.
> Guidance may be needed on how the NAS is supposed to correlate
> DHCP requests with the previous RADIUS requests. Is this based
> on MAC address? Does that pose a spoofing threat specific to
> the proposed functionality which should be covered in the
> Security Considerations section?
>
> Greg
No guidance is needed because 802.1X enables forwarding on a single
port switch port for a single host. Any DHCP messages received on
that port then presumably were sent by the just authorized host.
> -----Original Message-----
> From: Ashwin Palekar [mailto:ashwinp@windows.microsoft.com]
> Sent: donderdag 29 april 2004 6:02
> To: list,
> Cc: bwijnen@lucent.com; Bernard Aboba
> Subject: Comments on dhc-agentopt-radius-06.txt
>
>
> I've looked at draft-ietf-dhc-agentopt-radius-06.txt.
>
> Here's my review:
>
> 1. "a RADIUS server SHOULD send only those
> attributes for which the relay agent can ensure that either the
> relay agent or the DHCP server will provide the associated
> service. "
>
> Comment 1: How does the RADIUS server know that the DHCP server will
> provide the associated service?
The RADIUS server and the DHCP server are presumed to be in the same
administrative domain so the RADIUS server can be configured with the
knowledge of the DHCP server's behavior.
> 2. "The RADIUS server that implements this specification MUST be
> configured to return the User-Name and Class attributes to the
> NAS, and MAY return other attributes."
>
>
> # Attribute
> --- ---------
> 1 User-Name (RFC 2865 [3])
> 4 NAS-IP-Address (RFC 2865)
> 6 Service-Type (RFC 2865)
> 25 Class (RFC 2865)
> 26 Vendor-Specific (RFC 2865)
> 27 Session-Timeout (RFC 2865)
> 30 Called-Station-Id (RFC 2865)
> 31 Calling-Station-Id (RFC 2865)
> 32 NAS-Identifier (RFC 2865)
> 44 Acct-Session-Id (RFC 2866 [5])
> 50 Acct-Multi-Session-Id (RFC 2866)
> 87 NAS-Port-Id (RFC 2869 [6])
> 88 Framed-Pool (RFC 2869)
> 100 Framed-IPv6-Pool (RFC 3162 [8])
>
> Comment 2: Newer EAP authentication protocols allow the RADIUS
> server to authenticate multiple identities. What if the RADIUS
> server is authenticating multiple identities user and machine? Which
> identity should it return?
RFC number please?
> Comment 3: The para possibly imposes a major constraint on RADIUS
> implementations by requiring them to return User-Name attribute. Not
> all RADIUS servers return this attribute.
The network operator who want to use the RADIUS Attributes sub-option
would be wise to obtain a RADIUS server that can return the attributes
to be used by the DHCP server.
> Comment 4: The para requires the DHCP relay agent to send many
> RADIUS attributes to the DHCP server. In addition, the
> interoperability impact is unclear if the DHCP relay does not
> forward certain attributes (like Vendor-Specific). Instead, can't we
> achieve the same thing if the RADIUS server returns one additional
> RADIUS attribute (DHCP-user-class) specifically designed to be
> handled by DHCP relay-agents. The DHCP relay agent only forwards the
> single attribute DHCP-user-class to the DHCP server. The DHCP server
> can then assign configuration options based on this option.
Yes, the use of a new DHCP-user-class attribute would be an
alternative design. The first draft of this specification included a
specific options for the User-Name and a new User-Class attribute. We
were given wise counsel by the dhc WG to avoid the requirement of
establishing a new registry. Therefore, the current design reuses the
existing attributes for simplicity.
> Regards,
>
> Ashwin
>
> -----Original Message-----
> From: Paul Funk [mailto:paul@funk.com]
> Sent: donderdag 29 april 2004 11:29
> To: Wijnen, Bert (Bert); aaa-doctors@ops.ietf.org
> Subject: Re: Pls review/comment on:
> draft-ietf-dhc-agentopt-radius-06.txt
>
>
> Yet another approach would be to have a RADIUS attribute
> that encapsulated a DHCP option. Then, the RADIUS server
> could be configured to allow arbitrary options to be added to
> a DHCP request. They could be vendor-specific as well.
>
> Paul
This approach appears to be the beginning of a design that is not yet
compatible with the current DHCP specification. Relay agents can only
add relay agent information sub-options and cannot add arbitrary DHCP
options to a DHCP message. Why require the RADIUS server to pack, for
example, the User-Name attribute into a new DHCP option that would
require a new registry to be estalished by IANA.
> ----------------------
>
> Bert,
>
> I read the draft quickly, but it appears that the all the RADIUS
> attributes are encoded into a single DHCP option. Since that
> is limited to 255 bytes, I'm not sure how useful or reliable this
> will be. RADIUS servers cannot be configured to limit the
> total length of their attributes to 255 bytes and remain usable
> for their main purpose, which is authentication and authorization.
See above...
> I'm not sure if it is legal in DHCP to repeat an option. If it is,
> RADIUS attributes could be encoded one per option.
There is no standard for encoding long relay agent option sub-options
(the equivalent of RFC 3396 for sub-options).
> An alternate approach would be to configure the RADIUS server
> as to which attributes should be forwarded to the DHCP server.
> There might be a new or VS attribute that would be useful in
> some application to forward to the DHCP server, and it is always
> more convenient to configure such things in the RADIUS server
> rather than in the NAS. The NAS is best left to blindly do what
> it is told rather than to make decisions like which attribute to
> forward as a DHCP option.
Yes, the assumption is that the RADIUS server will have knowledge of
which requests will be forwarded to the DHCP server, and, therefore,
will be configured to send the appropriate attributes. This is not an
alternative approach but is exactly what is defined in the RADIUS
Attributes sub-option specification - the RADIUS server is configured
with the attributes to be sent and the NAS simply forwards all of the
attributes to the DHCP server.
> The basic problem is the sheer volume of information produced.
> In such cases, maybe the best thing is to just to provide a method
> of indirection, like a policy name or something. For example, to
> configure a packet filter, you can send the name of the filter rather
> than a list of rules. So maybe what's really required is a RADIUS
> "DHCP-Policy" attribute, jointly configured at RADIUS server and
> DHCP server, and the NAS simply forwards between the two.
>
> Paul
Why invent a new attribute with all of the baggage associated with
"policy" when all that's required is to let the DHCP server know the
outcome of a AAA decision? As indicated in the previous paragraph,
the volume of information is configurable at the RADIUS server and
can therefore be managed by the network administrator.