Re: [OPSAWG] [Mud] Declaring something to be a controller in MUD

Eliot Lear <> Wed, 26 June 2019 11:15 UTC

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From: Eliot Lear <>
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Date: Wed, 26 Jun 2019 13:15:41 +0200
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To: Michael Richardson <>
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Subject: Re: [OPSAWG] [Mud] Declaring something to be a controller in MUD
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> On 25 Jun 2019, at 21:52, Michael Richardson <> wrote:
> Signed PGP part
> Eliot Lear <> wrote:
>> A few of us are just trying to put out an initial draft that addresses
>> one gap in MUD (there are several).
>> In a MUD file one can say that one
>> wants to access a controller in two ways: either "my-controller”
>> meaning a controller that services devices of a particular MUD URL or a
>> “controller” class that services devices based on a particular class
>> name of controller.
> I think that we have two potential avenues for security attacks here:
>  1) a device that claims to be a controller in order to gain access to
>     devices.
>  2) devices that claim to be belong to a controller in order to attack
>     controllers.
> To my mind there are some different things we could do:
> 1) insist that to user the my-controller connections that the two devices
>   have to be signed by the "same" entity.  ["same" could mean literal the
>   same key, the same certificate Issuer/DN,  or something more complex]
> 2) we could have devices declare in an MUD extension the
>   DN/certificate/entity which must sign their controller device.
> 3) (2) above, but with some level of indirection through some URL.

Another thought here:

We could provide a level of dereference for authorized controllers in the device’s MUD file, in the form of a URL list that would look something like:

{  [
   “controller” : “controller-name"
   “mud-urls” : [ some mud urls of valid controllers here ]
   “include” : “” that points to a file that contains a JSON serialization of this grouping

Whether you trust that mud-url without a cert is up to you.


>> In either case, right now the administrator has to manually know and
>> populate information, to say - some device is a controller,
>> either for MUD URL <> or
>> a class <>.
>> That can be laborious.  To assist, we are examining ways to have a
>> controller declare itself as a candidate controller.  That at least
>> provides a hint to the administrator that this particular device is
>> capable of serving in a particular role.
> I think that anything that requires administrator activity to be a fail for
> residential use.  It's too complex.
>> To make that declaration, the device must- Form the declaration; Find
>> the MUD manager; and Send it.
>> Finding the MUD manager depends on one question: Was the device built
>> to be a controller or is it a general purpose device that has an app
>> that is intended to be a controller?
>> If the device was built to be a controller, we can simply cram the
>> declaration into that devices own MUD file as an extension.  If the
>> device is a general purpose computer, things get a bit more
>> interesting.
> Yes... but I think that we have to solve the multi-purpose computer MUD
> anyway.  The intelligent speakers (Echo,Home,Mycroft,etc.) need to gain new
> MUD definitions as they gain "skills", and I think that we can treat a
> smartphone in a similar way.
> This might be a place where IPv6 wins, if we can split off each skill into a
> new provisioning domain, giving it a new IPv6 IID.  I was thinking that maybe
> we can associate the private key that signed the MUD file to the IID via
> something like SEND/CGA, but I'm not sure how many private keys we have
> (one for the app developer, or one per app installed on each device).
>> In this case we have two choices:
>> Either create a MUD file that points somewhere internally - this
>> doesn’t seem very plug and play.  Make the declaration directly to the
>> MUD manager.
>> I’m going to focus on the latter for the moment.  It is easy enough to
>> create a RESTful interface for this purpose, but it requires a
>> mechanism to discovered the MUD manager, which up until now has been an
>> internal part of the network infrastructure.
>> Let me call this out plainly: letting the app itself directly call the
>> MUD manager requires that the MUD manager itself become exposed to the
>> user infrastructure, which is a change.
> Agreed.
> And that the MUD manager have some reason to trust the device is not p0wned,
> and sending a bogus MUD file up.   A certificate chain back to the
> manufacturer is not enough, it has to be signed by a key that an attacker
> can't get access to.  So that requires attested keys if they are "local", or
> for the signature to be done elsewhere.
>> One possibility to address this is to incorporate the new RESTful
>> endpoint into an ANIMA BRSKI join registrar, which may already be
>> exposed.  But that requires that ANIMA BRSKI be in play, which it may
>> not.
> It is, however, a really good idea for the case where it is in play.
>> My thinking is that we do this work in two stages.  First handle the
>> easy case, which is the MUD file extension, and then figure out how to
>> do the app version of this.
>> Thoughts?
> yes.
> --
> Michael Richardson <>ca>, Sandelman Software Works
> -= IPv6 IoT consulting =-