Remote Authentication Dial-In User Service (RADIUS) Attributes Suboption for the Dynamic Host Configuration Protocol (DHCP) Relay Agent Information Option
draft-ietf-dhc-agentopt-radius-08

[Ballot discuss]
The -08 version of the draft still contains the following text:

      If the relay agent
      forwards RADIUS attributes not included in the table in Section 4,
      the DHCP server SHOULD ignore them.

I asked previously why this was not a MUST, and I'm afraid I still don't
understand the answer.  It seems like interoperability of servers and
relay agents is going to be much easier if this is simply ruled out;
is there a use case in which the DHCP server NOT ignoring them
is useful enought to override that?  If there is an example, could
it be included in the text?

If I've missed the point, and this matches some other behavior,
please let me know; taken alone, though, this doesn't quite
make sense to me.

Meeting held at IETF60 in San Diego.  Agreement was reached to make changes to this MIB to satisfy Bert's discuss issues (based on AAA Doctor review).

[Ballot discuss]
From the original 9 issues, revision 07 has addressed quite a bit.
These are the remaining open issues.

1. Truncation issue.
    The truncation issue raises objections from several reviewers,
    and other objections related to the inability of the RADIUS
    server to determine that the lower MTU was in effect.
    As a result, it seems that the truncation issue is the
    fundamental problem here.

    Eliminating the 253 byte limitation is necessary in order
    for this mechanism to be robust.

    See detailed issues 1 and 2 from the AAA-doctor review
    summary (review of revision 07).

    There are a number of ways in which this can be handled.
    Paul Funk has made a suggestion about enabling long options
    and is probably the most general way.
    But the DHC WG may also have other ideas on how to fix this.

    If I understand the discussion correctly then the original
    purpose of this specification was to allow a RADIUS server
    to send a Framed-Pool or Framed-IPv6-Pool attribute to
    802.11 APs, much as it does today in dialup networking
    situations.  The idea was to allow existing RADIUS server
    functionality to be used with new media without the
    needs for changes to RADIUS servers, proxies, etc.

    Restricting RADIUS servers to 253 octets of attributes without
    a means of informing them of the restriction, or limiting
    inter-domain use of RADIUS (something which is widely deployed
    today) in order to use an existing RADIUS server facility
    takes us fairly far from the original design goals.

    In particular, in large networks with many RADIUS clients,
    it can be quite difficult to keep track of the capabilities
    of each client.  Frequently the capabilities can vary by the
    characteristics of the client, the firmware load, etc.
    In that kind of environment, requiring the RADIUS server to
    keep track of whether the client implements the DHCP options
    draft, and therefore whether the MTU needs to be lowered can
    be a large administrative burden.

2. Normative language updating or contradicting RFC 2865.

    Section 4 of -07 states:

      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.

    If it is desired that the RADIUS server change its behavior
    based on the NAS implementation of this specification, then
    it is necessary for the NAS to inform the RADIUS server that
    the specification has been implemented (see Issue #1).

    The issue can be resolved if the above sentence gets deleted,
    since Section 5 states:

      The relay agent SHOULD include the User-Name and Class
      attributes in the RADIUS Attributes sub-option if available,
      and MAY include other attributes."

A detailed summary has also been sent to DHC WG chair

---- original DISCUSS on 2004-04-29 -- now replaced by above
Quite a set of comments and concerns from the AAA-doctors.
Need to merge them into one set of DISCUSS and COMMENT statements.

Added on 2004-05-24
It is a pretty long set of comments. So I have send the raw data to the
WG chair (Ralph Droms) and recorded it separately in the ID-tracker.

[Ballot discuss]
Original DISCUSS on 2004-04-29
Quite a set of comments and concerns from the AAA-doctors.
Need to merge them into one set of DISCUSS and COMMENT statements.

Added on 2004-05-24
It is a pretty long set of comments. So I have send the raw data to the
WG chair (Ralph Droms) and recorded it separately in the ID-tracker.

[Ballot discuss]
Quite a set of comments and concerns from the AAA-doctors.
Need to merge them into one set of DISCUSS and COMMENT statements.

It is a pretty long set of comments. So I have send the raw data to the
WG chair (Ralph Droms) and recorded it separately in the ID-tracker.

-----Original Message-----
From: Wijnen, Bert (Bert)
Sent: maandag 24 mei 2004 14:08
To: 'Ralph Droms'
Cc: jschnizl@cisco.com; Margaret Wasserman
Subject: RE: Discuss on draft-ietf-dhc-agentopt-radius-06.txt


> Bert - draft-ietf-dhc-agentopt-radius-06.txt is part of the
> PROTO team pilot in which the WG chair (in this case, me) takes
> on the responsibility to follow up on discuss points published
> against a document that has been submitted to the IESG for
> publication.
>
> I see that you have a discuss point against
> draft-ietf-dhc-agentopt-radius-06.txt:
>
>    Discuss:
>    Quite a set of comments and concerns from the AAA-doctors.
>    Need to merge them into one set of DISCUSS and COMMENT statements.
>
Sorry for not having done so earlier.

> Would you please forward the comments and concerns from the AAA-doctors so
> we (John Schnizlein and I) can respond and revise the doc as necessary?
> Thanks...

OK, here we go. Did not yet take the time to merge them. So just
frowarding raw comments so you have it right now. Can you let me know
if you need more info from me?

- 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.

- 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.

- 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.

I concur with Bernard's comments about the attributes truncation issue.

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.

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.

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.

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.

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 DPCH
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

-----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."

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?

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

-----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?

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?

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.

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.

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

----------------------

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.

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.

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.

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

---------- Forwarded message ----------
Date: Wed, 7 Apr 2004 09:47:51 -0700 (PDT)
From: Bernard Aboba
To: Margaret Wasserman
Subject: Re: Fwd: draft-ietf-dhc-agentopt-radius-05.txt

Here's my review:

Review of:
http://www.ietf.org/internet-drafts/draft-ietf-dhc-agentopt-radius-05.txt

There should be an informative reference to the IEEE 802.1X standard:

[IEEE-802.1X]
              Institute of Electrical and Electronics Engineers, "Local
              and Metropolitan Area Networks: Port-Based Network Access
              Control", IEEE Standard 802.1X, September 2001.

Section 1

In Figure 1, change "Authentication confirm/deny" to
"Access-Accept/Reject"

Change "RADIUS Service" to "RADIUS Server"

"  The RADIUS Attributes sub-option for the DHCP Relay Agent option
  provides a way in which network elements can pass information
  obtained through layer 2 authentication to a DHCP server [2]."

Actually, the information is obtained via a RADIUS exchange, and
may not relate to layer 2 authentication (e.g. a VPN authentication
would still work). Suggest rephrasing to:

"  The RADIUS Attributes sub-option for the DHCP Relay Agent option
  provides a way in which a NAS can pass selected attributes
  obtained from a RADIUS server to a DHCP server [2]."

"The NAS then supplies these
credentials to a RADIUS server, which either confirms or denies the
identity of the user of the device requesting network access. "

Actually, the RADIUS server accepts or rejects access.  This could
be due to failed authentication or it could be for other reasons
(insufficient authorization).  Suggest rephrasing to:

"The NAS then supplies these credentials to the RADIUS server,
which eventually sends either an Access-Accept or an Access-Reject in
response to an Access-Request."

Section 2.2

It would probably be safest to copy definitions from other
documents, such as from [RFC2865] Section 1:

  RADIUS Server
      A RADIUS server is responsible for receiving user connection
      requests, authenticating the user, and then returning all
      configuration information necessary for the client to deliver
      service to the user.

  Network Access Server
      A Network Access Server (NAS) provides access to the network and
      operates as a client of RADIUS.  The client is responsible for
      passing user information to designated RADIUS servers, and then
      acting on the response which is returned.

  Attribute
      A  Type-Length-Value tuple encapsulating data elements as
      defined in [RFC2865].

Section 3

You might say a few words about what happens if the Length of the enclosed
RADIUS attributes exceeds the potential length of the sub-option.

Section 4

"  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.

  To avoid dependencies between the address allocation and other state
  information between the RADIUS server and the DHCP server, only the
  attributes in the table below SHOULD be included in this sub-option.
  Because RADIUS servers rely on the directive in section 1.1 or RFC
  2865 that "A NAS MUST treat a RADIUS access-accept authorizing an
  unavailable service as an access-reject instead.", 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.  The following table, based on the analysis
  in RFC 3580 [9], lists attributes that MAY be included:"

Since there is no way for a RADIUS server to know that the NAS supports
this specification, I'm not sure how the directives in this paragraph
can be carried out.  The implication here seems to be that the User-Name
and Class attributes are required to determine the IP configuration of the
host, even where the Framed-Pool or Framed-IPv6-Prefix attributes are
included.

Since RFC 2865 specifies that the inclusion of the User-Name and Class
attributes in an Access-Accept is optional,  the statement above
effectively implies an udpate to the attribute table in RFC 2865 Section
5.44.

Also, I think the issue is not what a RADIUS server sends, but what a
DHCP relay encapsulates in the option or what the DHCP server
looks at or ignores, because the RADIUS server has no way to know
whether the NAS supports this specification or not.

I'd prefer that these paragraphs be rephrased as follows:

"  The table below, based on the analysis in RFC 3580 [9], lists
  attributes that MAY be included in the sub-option.
  In order to avoid potential side effects that would change the
  meaning of existing RADIUS attributes, a relay agent SHOULD only
  encapsulate the attributes included in the table below.
  The DHCP server MUST use the attributes that are included to provide
  the service implied by those attributes and MUST NOT introduce
  unintended side effects.

  Inclusion of the User-Name and Class attributes may be useful for
  diagnostic purposes as well as to assist the DHCP server in
  determining the appropriate configuration.  However, since
  this specification does not provide a mechanism for a RADIUS
  server to determine whether the NAS supports this specification,
  a RADIUS server cannot know that the User-Name and Class
  attributes are expected.

  As indicated in [RFC2865] Section 5.44, the inclusion
  of the User-Name and Class attributes within an Access-Accept is
  optional.  A DHCP server therefore cannot depend on these attributes
  being present, and where they are ommitted, the DHCP server MUST
  provide IP configuration as best it can, based on the contents of
  the DHCP packet and the other RADIUS attributes present,
  such as Framed-Pool and Framed-IPv6-Prefix.

  However, since inclusion of the User-Name and Class attributes
  in an Access-Accept may be useful for other reasons (such as where
  Identity privacy or tunneled EAP methods are supported), for
  optimal diagnosability and interoperability, it is suggested
  that RADIUS servers be configured to return the User-Name and
  Class attributes in the Access-Accept whenever EAP authentication
  is used."

Section 5

"  The RADIUS Attributes sub-option MUST only
  contain the attributes provided in the RADIUS Access/Accept message.
  The DHCP relay agent MUST NOT add more than one RADIUS Attributes
  sub-option in a message."

This seems to imply that all the attributes are included. Suggest
rephrasing to:

"  The RADIUS Attributes sub-option MUST only contain a subset of
  attributes provided in the RADIUS Access-Accept message.
  The DHCP relay agent MUST NOT add more than one RADIUS Attributes
  sub-option in a message."

Section 6

"  When the DHCP server receives a message from a relay agent containing
  a RADIUS Attributes sub-option, it extracts the contents of the
  sub-option and uses that information in selecting configuration
  parameters for the client.  Even if the relay agent forwards other
  RADIUS attributes from the RADIUS server, the DHCP server SHOULD
  ignore any attributes it receives for which it cannot ensure that the
  associated service will be provided either by the DHCP server or the
  relay agent.  If the DHCP server uses attributes not specified here,
  it might result in side effects not anticipated in the existing
  RADIUS specifications."

Suggest rephrasing to:


"  When the DHCP server receives a message from a relay agent containing
  a RADIUS Attributes sub-option, it extracts the contents of the
  sub-option and uses that information in selecting configuration
  parameters for the client.  If the relay agent forwards RADIUS
  attributes not included in the table in Section 4, the DHCP server
  SHOULD ignore them.  If the DHCP server uses attributes not specified
  here, this may result in side effects not anticipated in the existing
  RADIUS specifications."

[Ballot comment]
Please ensure that "IEEE" appears before "802.1X" throughout the document.

  In section 8, please add "[1]" following the reference to RFC 2131.

  In section 8, please add "[5]" following the reference to RFC 3046.

  I find the following alternative to Figure 1 easier to read.  If you
  agree, use it.

        +-----------------+
        |Device requesting|
        | network access  |
        +-----------------+
            |        ^
            |        |
          (1)        |            (1) Request for access
            |        |
            |        (4)          (2) Request for authentication
            v        |
        +-----------------+      (3) Access-Accept/Reject
        |      NAS      |
        |(802.1X and DHCP |      (4) Success/Failure
        |  relay agent)  |
        +-----------------+
            |        ^
            |        |
          (2)        |
            |        |
            |        (3)
            v        |
        +-----------------+
        |    RADIUS      |
        |    Server      |
        +-----------------+

[Ballot discuss]
These two statements seem to be at odds:

      The RADIUS Attributes sub-option described in
      this document is not limited to use in conjunction with IEEE
      802.1X and can be used to carry RADIUS attributes obtained by the
      relay agent for any reason.

and

      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.  The following table, based on the analysis in RFC 3580
      [10], lists attributes that MAY be included

That is, there seems to be a functional contradiction in the second
statement to the first statement--you SHOULD limit the use of
this sub-option to those cases where the relay agent can
ensure that the service will be provided.  Perhaps it might
be simpler to adjust the first to say "but it is not limited to
use in 802.11x contexts".

I'm also a bit confused as to why the second is a SHOULD.  This section
raised a similar question:

      If the relay agent
      forwards RADIUS attributes not included in the table in Section 4,
      the DHCP server SHOULD ignore them.

Why are these SHOULDs rather than MUSTs?  For example, could someone
use something like draft-adrangi-radiusext-location-information or a
Vendor specific attribute that did something similar?

[Ballot discuss]
These two statements seem to be at odds:

      The RADIUS Attributes sub-option described in
      this document is not limited to use in conjunction with IEEE
      802.1X and can be used to carry RADIUS attributes obtained by the
      relay agent for any reason.

and

      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.  The following table, based on the analysis in RFC 3580
      [10], lists attributes that MAY be included

That is, there seems to be a functional contradiction in the second
statement to the first statement--you SHOULD limit the use of
this sub-option ot those cases where the relay agent can
ensure that the service will be provided.  Perhaps it might
be simpler to adjust the first to say "but it is not limited to
use in 802.11x contexts".

I'm also a bit confused as to why the second is a SHOULD.  This section
raised a similar question:

      If the relay agent
      forwards RADIUS attributes not included in the table in Section 4,
      the DHCP server SHOULD ignore them.

Why are these SHOULDs rather than MUSTs?  For example, could someone
use something like draft-adrangi-radiusext-location-information or a
Vendor specific attribute that did something similar?