improving java.net.UnknownHostException

[Florian Weimer]

* Daniel Fuchs:

> The platform built-in resolver that performs name-to-address
> resolution delegates to `getaddrinfo`. AFAICS `getaddrinfo` doesn't
> seem to provide this level of granularity in the error codes
> that it might return - though maybe distinguishing between
> some of the errors it returns could be envisaged.

I know that glibc has some issues with error reporting from the
low-level libresolv functions.  We can probably improve the situation
somewhat for cases that people actually encounter.  The DNS interface
has limits, though—if the recursive resolver cannot get data, queries
fail with SERVFAIL.  With glibc, timeouts usually mean that the network
between the host and its recursive resolver is down.  The glibc timeouts
are far longer than those of a typical recursive resolver, so the latter
gives up and produces the SERVFAIL error.

Thanks,
Florian