Re: [Captive-portals] Benjamin Kaduk's Discuss on draft-ietf-capport-architecture-08: (with DISCUSS and COMMENT)

[Kyle Larose <[e-mail]>]

Hi Benjamin,

Thanks for the thorough review!

Responses inline

On Tue, 9 Jun 2020 at 01:30, Benjamin Kaduk via Datatracker
<[e-mail]> wrote:
>
> Benjamin Kaduk has entered the following ballot position for
> draft-ietf-capport-architecture-08: Discuss
>
> ----------------------------------------------------------------------
> DISCUSS:
> ----------------------------------------------------------------------
>
> (1) and (2) should be easy to fix; (3) may well be "fixed" by telling me
> I'm too naive :)
>
> (1) Given that section 1 describes other options, the abstract should not
> limit to just DHCP and RA as options for provisioning the API URL.
>

Should be simple enough to open up the wording. E.g.

"Network provisioning protocols such as DHCP or Router Advertisements,
an optional signaling protocol..."


> (2) Section 4.1 says that:
>
>    5.  The Captive Portal API server indicates to the Enforcement Device
>        that the User Equipment is allowed to access the external
>        network.
>
> but I believe this should be the "Captive Portal Server" (or, as the
> previous point has it, the "web portal").
>

You're right. We'll change it up.

> (3) Probably a "discuss discuss", but ... in Section 1 we have:
>
>    *  Solutions SHOULD NOT require the forging of responses from DNS or
>       HTTP servers, or any other protocol.  In particular, solutions
>       SHOULD NOT require man-in-the-middle proxy of TLS traffic.
>
> I'd like to understand the motivation for this one a little better.
> Naively, it seems like we could get away with "MUST NOT require" while
> still allowing it to be done.  Am I missing something obvious?
>
>

I'll continue the discussion here by replying to Michael's reply.

> ----------------------------------------------------------------------
> COMMENT:
> ----------------------------------------------------------------------
>
> I'd like to see some more discussion of which signals are authenticated
> and how, and what kind of authorization checks are possible.  In
> well-run networks DHCP and RA signals should be relatively trustworthy,
> but clients don't always have a good indicator for whether a given
> network falls into that category.  Are there (other) mechanisms that can
> be used to give trust in the authenticity of a given Captive Portal API
> URI and that that API is authorthorized to provide unconstrained access
> for the network in question?  We require TLS for accessing the API
> server, but (as I note inline) there are more details that can be given
> about this TLS usage.  What can be done to authenticate and authorize
> the Captive Portal Server?  Most importantly (and most appropriately for
> an architecture document), which of these properties are strictly
> required vs. merely optional?  These are not Discuss-level points
> because an architecture does not strictly-speaking need to specify all
> of them, but having some indication of how we plan to achieve them would
> give greater confidence that this architecture will be a useful one.
>
> I'm happy to see the response to the genart reviewer's comment regarding
> "a" vs. "the" capport architecture; thanks!
>
> Abstract
>
>    This document describes a CAPPORT architecture.  DHCP or Router
>    Advertisements, an optional signaling protocol, and an HTTP API are
>    used to provide the solution.  The role of Provisioning Domains
>
> nit: there's perhaps a bit of a lack of parallelism in the list
> structure, where we talk about specific mechanisms for provisioning
> without describing the more abstract concept of provisioning, and list
> that alongside an abstract mention of "a signaling protocol" and the
> both-abstract-and-concrete "HTTP API".

I  think my proposal in response to your Discuss for this section
addresses this. Does it?

>
> Section 1
>
>    Implementations generally require a web server, some method to allow/
>    block traffic, and some method to alert the user.  Common methods of
>
> nit: I'd suggest clarifying that this is "implementations of captive
> portals" (or is it "captive networks"?).

Will do


>
>    alerting the user involve modifying HTTP or DNS traffic.
>
> nit: perhaps "at present" or "prior to this work"?  If I understand
> correctly one of the goals of this work is to shift the balance of
> captive portals away from these practices (while acknowledging that
> fully eliminating them is not feasible in the near future).
>

Fair point. I'd hope that readers of this statement 10 years from now would
be confused unless we change it!


>    *  Solutions MAY allow a device to be alerted that it is in a captive
>       network when attempting to use any application on the network.
>
> I'm also not sure I understand this one, especially in light of the
> following (paraphrased) "SHOULD allow learning of captivity before
> application attempts to use the network".  What's the alternative to
> "MAY allow", not-allowing such detection at all?

In short, the SHOULD case is covered by using network provisioning/the
API to discover
whether you're captive, and the MAY case is covered by using the
Captive Portal Signal.

Perhaps rephrasing "MAY allow" to "MAY implement a signal to alert a
device that ..."
 would clarify the meaning of the requirement? Similarly, rephrase
"SHOULD allow" to
 "SHOULD implement a mechanism whereby a device can learn ..."

>
>    *  The architecture MUST provide a path of incremental migration,
>       acknowledging a huge variety of portals and end-user device
>       implementations and software versions.
>
> nit: "preexisting" or similar would go a long way here.

Will change. e.g. "...a huge variety of preexisting portals..."

>
>    *  Network provisioning protocols provide end-user devices with a
>
> side note: using the word "provisioning" to describe things like DHCP
> and RA feels odd to me, presumably due to my background and what I
> expect provisioning to be.  I can see why it makes sense to use the term
> for this purpose, though.  Perhaps an additional adjective could help
> clarify what is meant, though I don't have a suggestion at hand.

I'm not really an expert in this area. Does anyone have any alternatives, or
reasons why we should stick with "network provisioning"?

>
>       for this purpose are available in [RFC7710bis].  Other protocols
>       (such as RADIUS), Provisioning Domains [I-D.pfister-capport-pvd],
>       or static configuration may also be used.  A device MAY query this
>
> side note: personally, I'd expand to "may also be used to convey this
> API URI", though it's probably not required for clarity.

Your suggestion reads better. We'll make the change.

>
>       The device MAY take immediate action to satisfy the portal
>       (according to its configuration/policy).
>
> side note: it's not entirely clear to me that we need a normative MAY
> for this.

I think you're right. Anyone else have an opinion on this?


>
> Section 2.1
>
>    have Internet access).  The User Equipment communication is typically
>    restricted by the Enforcement Device, described in Section 2.4, until
>    site-specific requirements have been met.
>
> It seems like these "site-specific requirements" must be the "Captive
> Portal Conditions" that we just defined.
>
>    *  SHOULD have a mechanism for notifying the user of the Captive
>       Portal
>
> It is pretty important that this mechanism be non-spoofable by, e.g.,
> untrusted websites.  I think we should mention something about
> "non-spoofable" here.

To clarify, an example spoofed notification would be a browser notification
sent from one of these untrusted sites? Seems reasonable. How much detail
do you think we need to get into about what it means to be non-spoofable?
Is it sufficient to just say "SHOULD have a non-spoofable mechanism..."?

>
>    *  MAY prevent applications from using networks that do not grant
>       full network access.  E.g., a device connected to a mobile network
>       may be connecting to a captive WiFi network; the operating system
>       MAY avoid updating the default route until network access
>       restrictions have been lifted (excepting access to the Captive
>
> nit: maybe say in which direction the update would go and/or something
> about why the move to wifi is desirable?

How about "... the operating system MAY avoid updating the default route
to a device on the captive WiFi network until ..."

>
>    None of the above requirements are mandatory because (a) we do not
>    wish to say users or devices must seek full access to the captive
>    network, (b) the requirements may be fulfilled by manually visiting
>    the captive portal web application, and (c) legacy devices must
>    continue to be supported.
>
> side note: in my opinion, it's possible to support legacy devices in
> practice without baking their limitations into the spec.
>
>    If User Equipment supports the Captive Portal API, it MUST validate
>    the API server's TLS certificate (see [RFC2818]).  An Enforcement
>
> We should probably cite RFC 6125 here and say something about how the UE
> gets a name to validate the server's certificate against (and what name
> type to use).

We can do this.

>
>    [I-D.ietf-capport-api] for more information.  If certificate
>    validation fails, User Equipment MUST NOT proceed with any of the
>    behavior described above.
>
> I'm not sure which behavior the "behavior described above" is.
> "[accessing...] OCSP responders, CRLs, and NTP servers" doesn't seem
> quite right since that's *how* you determine that certificate validation
> fails, but the bits further up about "navigate [to] the Captive Portal
> user interface" do not seem to clearly call out a single behavior or set
> of behaviors by the UE.

Yeah, this is a bit ambiguous. It's meant to basically say: "Stop
interacting with the API". We can
probably rephrase to "If certificate validation fails, the User
Equipment MUST NOT make any calls to
the API Server".



>
> Section 2.2.2
>
>    Although still a work in progress, [I-D.pfister-capport-pvd] proposes
>    a mechanism for User Equipment to be provided with PvD Bootstrap
>    Information containing the URI for the JSON-based API described in
>    Section 2.3.
>
> I don't think "JSON-based" is supported by the text of § 2.3 (and isn't
> really appropriate for an architecture doc in most cases, anyway).
>

I agree. We'll drop "JSON-based".

> Section 2.3
>
>    The purpose of a Captive Portal API is to permit a query of Captive
>    Portal state without interrupting the user.  This API thereby removes
>    the need for User Equipment to perform clear-text "canary" HTTP
>    queries to check for response tampering.
>
> nit: probably don't need to be specific about HTTP, here.

I agree. Will remove HTTP.

>
>    At minimum, the API MUST provide: (1) the state of captivity and (2)
>    a URI for the Captive Portal Server.
>
> Is there anything useful to say about the URI scheme for the captive
> portal server URI?  I guess I could probably (grudgingly) come up with a
> case where http-not-s would be tolerable, but given that we admit the
> possibility of "payment" as a captive portal condition, I don't want us
> to encourage sending payment or other sensitive information over schemes
> inappropriate for such information.\

I think that's reasonable. Does anyone have an objection to this? I
would at least
like to recommend that it's TLS, so it's considered.

>
>    A caller to the API needs to be presented with evidence that the
>    content it is receiving is for a version of the API that it supports.
>
> What about evidence that the content it is receiving is intended to be
> used with, and authorized to speak for, the network it is joining?
>

I'm not quite sure how we'd achieve this.  The only information the network
has given to the User Equipment is the URI for the Captive Portal API, which,
I think, it can assume covers those cases. This particular point was
simply intended
to ensure that the API had a method of ensuring compatibility.

>    When User Equipment receives Captive Portal Signals, the User
>    Equipment MAY query the API to check the state.  The User Equipment
>
> nit: we seem to use "the state of its captivity" most places.

Will update.

>
>    The API MUST use TLS to ensure server authentication.  The
>    implementation of the API MUST ensure both confidentiality and
>    integrity of any information provided by or required by it.
>
> It's a little weird to split the TLS requirements between here and
> Section 2.1, though I guess if we're splitting things by role it's
> probably unavoidable.  (I made my RFC 6125 comment in Section 2.1 and it
> probably doesn't need to appear in both places.)
>
> Section 2.4
>
>    *  May signal User Equipment using the Captive Portal Signaling
>       protocol if certain traffic is blocked.
>
> nit: I think that "optionally signals" might be a better fit for the
> list structure as used in the other bullet points.
>

Will change.

> Section 2.5
>
>    When User Equipment first connects to a network, or when there are
>    changes in status, the Enforcement Device could generate a signal
>    toward the User Equipment.  This signal indicates that the User
>    Equipment might need to contact the API Server to receive updated
>    information.  For instance, this signal might be generated when the
>    end of a session is imminent, or when network access was denied.
>
> Would this signal also be used when the UE has successfully met the
> Captive Portal Conditions?

The intention was for it to indicate when there are problems (similar
to say a destination
unreachable ICMP message). But, the language isn't precise about
that... I'm not sure that's a
problem. Does anyone else have some thoughts on this? Should we
clarify it either way (make
explicit that it's for any state transition to 'access denied'?)

>
> Section 2.6
>
>    *  The User Equipment queries the API to learn of its state of
>       captivity.  If captive, the User Equipment presents the portal
>       user interface from the Web Portal Server to the user.
>
> [we previously discussed this UE behavior as optional.  I don't mind
> having the text be descriptive like this, since it's describing the
> diagram, and the diagram is not binding on all UEs, but it seemed worth
> noting just in case.]
>

We could probably preface this section with a comment indicating that
it's covering the case
where the API provides a web portal uri

> Section 3.1
>
>    An Identifier is a characteristic of the User Equipment used by the
>    components of a Captive Portal to uniquely determine which specific
>    User Equipment is interacting with them.  An Identifier MAY be a
>
> Do we want to say anything about what scope within which the uniqueness
> must hold?  ("No" is probably fine.)

I think we're probably fine here. Anyone else agree/disagree?

>
> Section 3.2.1
>
>    Each instance of User Equipment interacting with the Captive Network
>    MUST be given an identifier that is unique among User Equipment
>    interacting at that time.
>
> side note: "MUST be given" gets a knee-jerk "by whom?" response from me.
> It's probably okay for this document to not specify, though, as it may
> depend on the nature of the Captive Network.
>

What if we say "by the Captive Network"? I think that makes it clear
that it's the responsibility of
the network, though that does perhaps complicate things if the system
chooses to use the MAC address, for example, which isn't "given" by
the network. In that case, the network would use the MAC that it has
inferred from its communication with the UE to construct the identity
that it then gives to the UE. I guess it makes sense when put that
way.

But, perhaps "given" is the wrong term here? Should we rephrase to
"The Captive Network MUST associate the User Equipment with an
identifier that is unique among User Equipment that is interacting at
that time?"

>    Over time, the User Equipment assigned to an identifier value MAY
>    change.  Allowing the identified device to change over time ensures
>    that the space of possible identifying values need not be overly
>    large.
>
> Is the identifier assigned to a given UE on the same network expected to
> be able to change as well?  This may have some privacy considerations...
>

I think so. E.g. if a DHCP lease expires, or the UE reattaches
multiple times. What are the
privacy considerations you're thinking of?

> Section 3.2.2
>
>    are active at the same time.  This property is particularly important
>    when the User Equipment is extended externally to devices such as
>    billing systems, or where the identity of the User Equipment could
>    imply liability.
>
> nit(?): is it the UE that is extended externally or the identifier
> thereof?

The identifier. Will update.

>
> Section 3.2.4
>
>    In some situations, the User Equipment may have multiple IP
>    addresses, while still satisfying all of the recommended properties.
>
> nit: as written, "while still satisfying all of the recommended
> properties" is describing the UE, but the context of Section 3.4
> suggests that we want to be talking about the recommended properties for
> identifiers.
>

Will rephrase.

> Section 3.5
>
>    Accessing the API MAY depend on contextual information.  However, the
>    URIs provided in the API SHOULD be unique to the UE and not dependent
>    on contextual information to function correctly.
>
> Should the per-UE APIs and/or the mapping between UE and per-UE API be
> unguessable?  (Do we want to reference Capability URLs
> [https://www.w3.org/TR/capability-urls/]?)
>

That's not a bad idea. Does anyone else have an opinion on this? I'm currently
planning on taking the suggestion of mentioning that they should be
unguessable and
reference that document.

> Section 4
>
> I might consider explicitly saying "non-normative" somewhere in here.

Will do.


>
> Section 4.1
>
>    4.  If necessary, the User navigates the web portal to gain access to
>        the external network.
>
> nit: "navigates to"

Thanks. Will fix.

>
> Section 4.2
>
>    3.  The User Equipment's UI indicates that the length of time left
>        for its access has fallen below a threshold
>
>    4.  The User Equipment visits the API again to validate the expiry
>        time
>
> side note: I feel like there's implicitly some User action in here,
> though I don't know that we need to actually say anything about it.
> (Otherwise we wouldn't have the UI indicating things.)
>

We may need to change this up a bit and then mention that the user
accepts the prompt. E.g.



   3.  The User Equipment's detects that the length of time left
       for its access has fallen below a threshold

   4.  The User Equipment visits the API again to validate the expiry
       time

   5.  If expiry is still imminent, the User Equipment prompts the user
       to access the web-portal URI again

   6. The user equipment accepts the prompt

   7.  The User extends their access through the web-portal via the UI


> Section 4.3
>
>    Whenever a new Portal URI is received by end User Equipment, it
>    SHOULD discard the old URI and use the new one for future requests to
>    the API.
>
> What kind of validation/authorization checks need to be applied to the
> new Portal URI?

This should be the same as the initial one we received when first
accessing the network
(e.g. via DHCP, RAs, etc). Perhaps we don't properly indicate
elsewhere that this should be supported. I thought we did, but I can't
find it...

I think we should add a point to Section 2.1 that the UE SHOULD
support updates to the captive portal URI from the network
provisioning service, and then mention here that that service is what
did it. The same trust as mentioned in section 7.1 still applies.


> (nit: we probably should check the terminology in this section; the
> Section 1.2 lexicon would call this information the "Captive Portal API
> Server URI" and not a "Portal URI".)

Will update
>
> Section 7
>
> This mechanism rather inherently requires having multiple entities track
> the UE's identity (and, thus, likely be tracking a proxy for the user's
> identity).  It seems appropriate to include some discussion of the
> privacy considerations of this tracking, and whether/what kind of
> anonymity support is appropriate!
>

That seems reasonable. I'll need to do some thinking on the text for
this, but a rough
first pass. I'm open to suggestions here, since I'm not a
security/privacy expert. I'll likely
get something wrong/miss something obvious.

Section 7.X  Privacy

  Section 3 describes a mechanism by which all components within the
Captive Network
  agree on a unique identifier for the User Equipment. This identifier
could be abused to
  track the user. Implementers and designers of Captive Networks
should take care to ensure
  that identifiers, if stored, are stored securely. Further,


> Section 7.1
>
>    Given that a user chooses to visit a Captive Portal URI, the URI
>    location SHOULD be securely provided to the user's device.  E.g., the
>    DHCPv6 AUTH option can sign this information.
>
> I'm not sure that I understand the intent behind the "Given that"
> construction here.  Is it trying to emphasize user choice, and thus the
> need for informed choice?

Yes. We should probably rephrase this

E.g.

"The user makes an informed choice in deciding to visit and trust the
Captive Portal URI. Since the network provides Captive Portal URI to
the user equipment, the
network SHOULD do so securely so that the user's trust in the network
can extend to their trust of the Captive Portal URI. E.g., the DHCPv6
AUTH option can sign this information."

>
> Section 7.2
>
> [In the vein of my previous remarks, there are many ways to use TLS, and
> usually we provide more details on how we expect TLS to be used.]
>

We can expand on that here.


> Section 7.3
>
>    The API MUST ensure the integrity of this information, as well as its
>    confidentiality.
>
> Who/what is the attacker(s) that we need to preserve confidentiality from?
>

Without knowing the details of the particular solution, it's a bit
hard to say for sure, but roughly
I'd say it's someone who wants to interact with the API using the
identity of the user. E.g. if we're using an 'unguessable' URI, an
attacker snooping on the communication with the API could determine
the URI, and use it.

Does that sound reasonable?

> Section 7.4
>
>    *  Accesses to the API Server are rate limited, limiting the impact
>       of a repeated attack.
>
> One might consider a flooding attack that tries to get the UE to use all
> its (rate-limited) connections to get some information that is not the
> information that it's most important for the UE to have.  If there's
> only a single operation that can be performed at the API Server (which I
> believe is the intent?) there is no such attack, but it may be worth
> mentioning that there is no such attack.
>

In this case that's the intent (i.e. only visit to get whether we're
still captive). That's a good point, though: if we ever expand on
that, it could open the door to such an attack, so it's worth
mentioning why it's not a problem.

That said, I actually don't quite see how the uni-directional signal
could be used to get information back to the attacker. Could you maybe
describe that in a bit more detail?


> Section 8.1
>
> Interestingly, none of the places where we reference 7710bis have
> surrounding text that clearly incur a normative dependency.
>
> Appendix A
>
> We explain the use of the "canary" term here, but have already used it
> twice (with no forward-reference) in the body of the document.
>

Let's add a forward reference.

>    Another test that can be performed is a DNS lookup to a known address
>    with an expected answer.  If the answer differs from the expected
>    answer, the equipment detects that a captive portal is present.  DNS
>    queries over TCP or HTTPS are less likely to be modified than DNS
>    queries over UDP due to the complexity of implementation.
>
> Is the reader supposed to draw the conclusion that DoTCP/DoH provide
> less-reliable captive-portal detection than Do53?  (I assume "TCP" is
> not a typo for "TLS", here, though am unsure enough to want to check.)

Correct on both accounts. Do you think we should clarify the intent?

>
>    Malicious or misconfigured networks with a captive portal present may
>    not intercept these requests and choose to pass them through or
>    decide to impersonate, leading to the device having a false negative.
>
> nit: I suggest "these 'canary' requests" to clarify which requests we're
> talking about.
>

WIll change.
>
>


Thanks again!

Kyle