Re: [Doh] [Ext] Fallback to untrusted DOH servers

Mateusz Jończyk <mat.jonczyk@o2.pl> Mon, 23 April 2018 12:57 UTC

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To: Patrick McManus <pmcmanus@mozilla.com>
Cc: Paul Hoffman <paul.hoffman@icann.org>, DoH WG <doh@ietf.org>
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From: Mateusz Jończyk <mat.jonczyk@o2.pl>
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Date: Mon, 23 Apr 2018 14:56:46 +0200
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Subject: Re: [Doh] [Ext] Fallback to untrusted DOH servers
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What I am discussing here are modifications to DOH so that adding support for
discovery would be possible without modifying the future RFC.

Thanks,
Mateusz Jończyk


W dniu 23.04.2018 o 14:40, Patrick McManus pisze:
> I really think you are discussing discovery. The chairs have said that is out of
> scope for this draft but they are soliciting proposals for new work that could
> include it.
> 
> On Mon, Apr 23, 2018, 06:04 Mateusz Jończyk <mat.jonczyk@o2.pl
> <mailto:mat.jonczyk@o2.pl>> wrote:
> 
>     W dniu 22.04.2018 o 17:15, Paul Hoffman pisze:
>     > On Apr 22, 2018, at 6:21 AM, Mateusz Jończyk <mat.jonczyk@o2.pl
>     <mailto:mat.jonczyk@o2.pl>> wrote:
>     >> I think if Your interpretation of DOH is correct, that the text is
>     unclear there
>     >> and should be clarified.
>     >> I would suggest to add the following clarification:
>     >>
>     >>      A client MAY use an untrustworthy DNS API server as a fallback.
>     >
>     > This seems horribly dangerous without a clear definition of "fallback".
> 
>     There was a definition of "fallback" in a modification I proposed several mails
>     ago, but I removed it for sake of simplicity.
> 
>     So, I would propose a following addition:
>        A client MAY use an untrustworthy
>        DNS API server as a fallback, for example: when no trustworthy DNS API
>        server is configured, no configured DNS server works or
>        when the trustworthy DNS API server returned NXDOMAIN (and the client
>        checks whether an untrustworthy DNS API server would resolve the address in
>        question).
> 
>        The client MUST use separate DNS API caches for trustworthy and untrustworthy
>        DNS API servers or drop DNS caches when switching from an untrustworthy
>        DNS API server to a trustworthy one.
> 
>     Ted Lemon suggested in a private e-mail that contacting an untrusworthy DNS API
>     server after the trustworthy DNS API server returned NXDOMAIN exposes all
>     mistyped domains to the untrustworthy DNS API server. This is a valid concern,
>     and applies equally well to using old-school DNS in such a situation.
> 
>     I would therefore propose to add the following warning after the phrase:
>             "If a client of this protocol encounters an HTTP error after sending a
>             DNS query, and then falls back to a different
>             DNS retrieval mechanism, doing so can weaken the privacy and
>             authenticity expected by the user of the client."
> 
>             When a DNS API server returns NXDOMAIN, a client may wish to check
>             whether another server will resolve the domain name (as this may be
>             a local name to be resolved by a local DNS server). Doing so will,
>             however, expose all mistyped domain to that server.
> 
> 
>     >
>     >> I am going to submit a draft that specifies how a fallback DNS API server
>     could
>     >> be retrieved from DHCP.
>     >
>     > That would be quite useful. If you do that, wouldn't it define the DNS API
>     server as trusted?
> 
>     No, the server retrieved via DHCP is going to an untrustworthy DNS API server
>     (as defined above) as DHCP is usually unauthenticated and prone to various
>     manipulations.
> 
>     >
>     > --Paul Hoffman
>     >
> 
>     Greetings,
>     Mateusz Jończyk
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