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Date: Wed, 5 Jul 2023 12:18:11 +1200
From: Hamish Forbes <hamish.forbes@...il.com>
To: Rich Felker <dalias@...c.org>
Cc: musl@...ts.openwall.com
Subject: Re: DNS answer buffer is too small
On Wed, 5 Jul 2023 at 04:06, Rich Felker <dalias@...c.org> wrote:
>
> On Tue, Jul 04, 2023 at 04:19:16PM +1200, Hamish Forbes wrote:
> > On Tue, 4 Jul 2023 at 15:29, Rich Felker <dalias@...c.org> wrote:
> It's as safe as it was before, and it's always been the intended
> behavior. Aside from the CNAME chain issue which wasn't even realized
> at the time, use of TCP for getaddrinfo is not about getting more
> answers than fit in the UDP response size. It's about handling the
> case where the recursive server returns a truncated response with zero
> answer records instead of the max number that fit. It turned out this
> could also occur with a single CNAME where both the queried name and
> the CNAME target take up nearly the full 255 length.
>
> As for why not to care about more results, getaddrinfo does not
> provide a precise view of DNS space. It takes a hostname and gives you
> a set of addresses you can use to attempt to connect to that host (or
> bind if that name is your own, etc.). There's very little utility in
> timing out more than 47 times then continuing to try more addresses
> rather than just failing. "Our name resolves to 100 addresses and you
> have to try all of them to find the one that works" is not a viable
> configuration. (A lot of software does not even iterate and try
> fallbacks at all, but only attempts to use the first one, typically
> round-robin rotated by the nameserver.)
>
> Anyway, if there are objections to this behavior, it's a completely
> separate issue from handling long CNAME chains.
Ah yeah, ok that makes sense.
I wasn't thinking about it as "we just need any address".
No objections to that from me!
> From my reading of your links, and
>
> https://groups.google.com/g/comp.protocols.dns.bind/c/rXici9NvIqI
>
> I don't think max-recursion-depth is related to CNAMEs. It's the depth
> of delegation recursion. The max CNAME chain length is separate, and
> in unbound terminology is the number of "restarts". Unbound's limit as
> you've found is 11. BIND's is supposedly hard-coded at 16.
>
> Assuming the recursive server uses pointers properly, max size of a
> length-N CNAME chain is (N+1)*(255+epsilon). This comes out to a
> little over 4k for the BIND limit, and that's assuming max-length
> names with no further redundancy. I would expect the real-world need
> is considerably lower than this, and that the Unbound default limit on
> chain length also suffices in practice (or it wouldn't be the default
> for a widely used recursive server). So, for example, using a 4k
> buffer (adding a little over 3k to what we have now, which already had
> enough for one CNAME) should solve the problem entirely.
>
> Does this sound like an okay fix to you?
Sounds good to me!
>
> Rich
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