Re: [Add] [Ext] Draft Posting: CNAME Discovery of Local DoH Resolvers

[tirumal reddy <[e-mail]>]

On Tue, 30 Jun 2020 at 21:30, Eric Rescorla <[e-mail]> wrote:

>
>
> On Tue, Jun 30, 2020 at 6:44 AM tirumal reddy <[e-mail]> wrote:
>
>>
>>
>> On Tue, 30 Jun 2020 at 18:28, Eric Rescorla <[e-mail]> wrote:
>>
>>>
>>>
>>> On Tue, Jun 30, 2020 at 2:19 AM Paul Vixie <[e-mail]> wrote:
>>>
>>>> On Tuesday, 30 June 2020 09:07:43 UTC Martin Thomson wrote:
>>>> > Hi Paul,
>>>> >
>>>> > On Tue, Jun 30, 2020, at 17:51, Paul Vixie wrote:
>>>> > > Eric Rescorla wrote on 2020-06-29 20:08:
>>>> > > > On Mon, Jun 29, 2020 at 8:05 PM Daniel Migault <
>>>> [e-mail]
>>>> > > >
>>>> > > > <mailto:[e-mail]>> wrote:
>>>> > > >     If the lookup takes as input the IP addresses or something
>>>> provided
>>>> > > >     by the ISP (like the local resolver IP address), the
>>>> resulting chain
>>>> > > >     is likely to be from the ISP. DNSSEC is needed to assert it.
>>>> > > >
>>>> > > > Why do you assume that the IP is delivered securely?
>>>> > >
>>>> > > because dnssec allows me to verify end-to-end authenticity of
>>>> > > name/address bindings (and other dns content.)
>>>> >
>>>> > DNSSEC allows you to be sure of the veracity of what comes from
>>>> DNSSEC, but
>>>> > in this case the IP address didn't come from DNSSEC.  It's not DNS
>>>> content.
>>>>
>>>> i have badly misunderstood.
>>>>
>>>> the way i know that the ip address provided by the isp was delivered
>>>> securely
>>>> today is because off-net DHCP forgery is hard,
>>>
>>>
>>> Let's start here:
>>> I agree that off-net DHCP forgery is hard. However, once you assume that
>>> you are off-path, then Do53 interception is *also* hard. So for this to be
>>> useful you need an environment in which the attacker is able to attack Do53
>>> but *not* to attack DHCP. What I'm asking for is for someone to define that
>>> threat model precisely so that we can design protocols that match it.
>>>
>>
>> The various possible attacks in a home network are discussed in
>> https://tools.ietf.org/html/draft-btw-add-home-06#section-9
>>
>
> As far as I can tell, what this text says is that authenticating the
> server based on information received in DHCP is not useful.
>

No, the draft discusses new DHCP/RA options to convey the DoH/DoT server
domain name. Section 9 discusses several attacks and mitigations the
endpoint can use to defend from an on-path attacker modifying the DHCP
response.

(1) Attacker modifying the DHCP response to point to the DoH/DoT server
hosted by the attacker.
(2) Attacker modifying the DHCP response to point to a public DoH server
not capable of filtering malware when the local resolver provides malware
filtering capability.

-Tiru


> -Ekr
>