Re: Re: Would love to see reconsideration for domain and search

[Rich Felker <dalias@libc.org>]

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.

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