Date: Thu, 22 Oct 2015 23:04:20 -0700 From: Tim Hockin <thockin@...gle.com> To: musl@...ts.openwall.com Subject: Re: Re: Would love to see reconsideration for domain and search On Thu, Oct 22, 2015 at 11:00 PM, Rich Felker <dalias@...c.org> wrote: > On Thu, Oct 22, 2015 at 10:37:53PM -0700, Tim Hockin wrote: >> I saw from a different thread that musl doesn't or didn't do TCP >> fallbacks - is that still the case? I know we need that for things >> like large multi-SRV sets (which I do not expect libc to support), and >> we have some people who have large A sets (which I do expect libc to >> support). > > Indeed. The only way you can overflow the UDP size limit with the > records the stub resolver uses is with a max-length CNAME pointing to > a max or near-max length record with little or no overlap to allow for > compression. Of course you might run out of space for all the address > results in other cases, but the truncated packet will still have > usable results. While I'm not aware of any official document to this > effect, for practical purposes you just have to avoid making names > that long. There are too many nameservers that don't do TCP at all, as > well as locked-down networks that don't allow TCP except on a few > specific ports, to be able to rely on doing DNS over TCP. Our case is exposing sets of fungible backends as a DNS name with multiple A records. Truncating the set will cause incorrect results for clients who need to discover the whole set. We can cross that bridge when we get there. > Naturally other non-stub-resolver things like zone transfers may need > TCP, but that's outside the domain of the stub resolver. Note that the > libc res_*/dn_*/ns_* APIs should be capable of working with longer > messages over TCP as long as you setup the socket and do the send/recv > yourself. > > Rich
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