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Date: Sun, 25 Sep 2022 23:12:38 -0400
From: Rich Felker <>
Subject: Re: [PATCH] [draft] dns tcp fallback

On Thu, Sep 22, 2022 at 04:28:34PM -0400, Rich Felker wrote:
> Here is my first draft of the DNS TCP fallback support that seems to
> be basically working. There may of course be bugs, and there are cases
> where I probably don't want it to fall back like it's doing, but it's
> at a stage where review and [smoke-]testing would be helpful.
> Some known issues:
> - getaddrinfo should use larger buffers (probably 640-768 bytes) or
>   the "huge CNAME" problem iss not solved.
> - If res_send is used to send EDNS0 queries crafted by the caller and
>   the caller provides a buffer smaller than what the OPT header
>   advertises, the answer will be silently truncated to the caller's
>   buffer size.
> I've tested querying TXT for (>7k RRset) and
> basic non-fallback usage. Further large-answer query test cases would
> be appreciated, especially names where one or both of the A or AAAA
> RRset are large so that fallback in parallel lookup can be tested, and
> later so that getaddrinfo buffer size increase can be tested.

OK, I have patches for both of the above now.

View attachment "0004-getaddrinfo-dns-lookup-use-larger-answer-buffer-to-h.patch" of type "text/plain" (2283 bytes)

View attachment "0005-dns-query-core-detect-udp-truncation-at-recv-time.patch" of type "text/plain" (2416 bytes)

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