net: LookupHost IDN support

[olegrok]

What version of Go are you using (go version)?

go version go1.9 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN="/home/oleg/go/work"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/oleg/go/"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build065227263=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"

What did you do?

I tried to resolve hostname that contains special symbols, e.g. '/', '='. But I got error 'no such host'. Utilities host, dig and nslookup provides this opportunity. Also they allow to resolve hostnames with length is more then 255 symbols unlike net.LookupHost.

Example:

_, err := net.LookupHost("/.yandex.ru")
fmt.Println(err)      // lookup /.yandex.ru: no such host

What did you expect to see?

For example, host:

$ host /.yandex.ru
/.yandex.ru has address 213.180.204.242
/.yandex.ru mail is handled by 10 not-for-mail.yandex.net.

[ianlancetaylor]

CC @mikioh @mdempsky

[mikioh]

FWIW, there's a good documentation: https://tools.ietf.org/html/rfc6055. It covers required areas comprehensively.

I feel like it's not a good idea to make the existing lookup API more generic name resolution stuff because it's still hard to implement correct and safe resolution under any environments. Instead, probably it'd be better for user applications to take the responsibility for selecting label encodings and recursive servers. The net package may help such applications under heterogeneous environments by providing improved type Resolver struct and upcoming DNS signaling/messaging/transportation stuff.

[gopherbot]

Change https://golang.org/cl/99623 mentions this issue: dns/dnsmessage: fix handling of non-LDH domain names