Re: [TLS] ESNI TXT RData, add leading distinguisher?

Eric Rescorla <ekr@rtfm.com> Mon, 03 December 2018 22:03 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Mon, 3 Dec 2018 14:02:49 -0800
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Subject: Re: [TLS] ESNI TXT RData, add leading distinguisher?
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Currently, the record has a checksum at the front which would allow
rejection of malformed records.

However, I think it is likely we will stop using TXT.

-Ekr


On Mon, Dec 3, 2018 at 12:24 PM Viktor Dukhovni <ietf-dane@dukhovni.org>;
wrote:

> On Mon, Oct 22, 2018 at 10:19:54AM -0700, internet-drafts@ietf.org wrote:
>
> > https://datatracker.ietf.org/doc/html/draft-ietf-tls-esni-02
>
> I see that the TXT record does not carry any sort of distinguishing
> identifier in front of the payload:
>
>     $ dig +short txt _esni.www.cloudflare.com
>
> "/wHdTAKgACQAHQAgnkJCWxSqQ75Vaxti1Q/S2XEbZa49aRA5/wtNLK2yA38AAhMBAQQAAAAAXAGIsAAAAABcCXGwAAA="
>
> Given widely deployed wildcard (mostly SPF) TXT records implementations
> need to be prepared to ignore responses that are not well-formed
> base64 encodings of the expected data structure.  Perhaps a short
> leading identifier such as "ESNI;" or similar would make it easier
> to quickly reject non-ESNI RData.
>
> For example, Rapid7's "mta-sts" survey dataset contains over a
> million TXT records with owner name "_mta-sts.<domain-suffix>", but
> only O(100) are actual "v=STSv1" MTA-STS TXT records, the rest are
> largely SPF.  So one can't rely on the "_esni" prefix to be an
> effective indication of intent to provide an actual ESNI response.
>
> --
>         Viktor.
>
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