The commit d96a3a2 makes hostname entries in local database lowercase forcibly during go1.6 development. I don't think it's a good idea because /etc/hosts-like local database is a last resort for name resolution and https://tools.ietf.org/html/rfc4343 never says that "don't preserve case on input and output" for some reasons; perhaps it was for internetworking with old-plain systems and it is just for future use.

I don't see any rationale for the change in d96a3a2, so can you please restore the behavior that preserves case on input and output?

Note that we know a few name resolver libraries don't preserve case of hostnames and DNS labels, for example libc for OS X/iOS that supports both unicast and multicast DNS, but I still don't think we need to do case conversion for now especially with inconsistency that providing case-converted hostnames and case-preserved DNS labels.