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

Paul Hoffman <> Sat, 27 June 2020 15:05 UTC

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From: Paul Hoffman <>
To: Eric Rescorla <>
CC: ADD Mailing list <>
Thread-Topic: [Add] [Ext] Draft Posting: CNAME Discovery of Local DoH Resolvers
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Date: Sat, 27 Jun 2020 15:04:55 +0000
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Subject: Re: [Add] [Ext] Draft Posting: CNAME Discovery of Local DoH Resolvers
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On Jun 26, 2020, at 5:36 PM, Eric Rescorla <> wrote:
>> Point 1 feels overstated. The clients we are talking about here are modern web browsers, which can clearly write whatever they want in their DNS queries.
> "clearly" is doing a lot of work here. Sure, it's just software so we could do this. However, Firefox uses the system resolver and in particular calls getaddrinfo(). So while yes, we could in theory write our own resolver or plumb to the OS-specific more flexible interfaces (e.g. res_query()) up to the part of the code that needs this information. Hence, as I said, "somewhat more difficult (though not impossible)".

I assumed that Firefox was using an internal mechanism for creating the DNS queries in the draft. How can Firefox send a CNAME query if it "uses the system resolver and in particular calls getaddrinfo()"? I admit weakness here, but I thought that getaddrinfo() doesn't let you specify the RRtype, just "give me an address" where "address" is defintely not "a domain name".

If Firefox has the means of sending a constructed CNAME query and process the response, shouldn't it also (ahem) clearly have the means of sending a constructed TBD1 query and process the response?

--Paul Hoffman