Please answer these questions before submitting your issue. Thanks!

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

go version go1.9.2 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

greg@lois:~ ❯ go env
GOARCH="amd64"
GOBIN="/home/greg/dev/go/bin"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/greg/dev/go"
GORACE=""
GOROOT="/usr/lib/go"
GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build769697684=/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'm trying to use custom resolvers to resolve the same host using concurrent goroutines. The current pure go resolver uses a global lookupGroup which merges LookupIPAddr calls together for lookups for the same host. As a result only one of the resolvers is used to permform the dns lookup but the result is shared by all the resolvers.

Example code: http://play.golang.org/p/eyR2mhBs1C

What did you expect to see?

I'm expecting each resolver to dial for its own lookups

What did you see instead?

This is the output of the example script:

Creating resolver 0
Creating resolver 1
dial using the resolver 1
dial using the resolver 1
result from 0: [{216.58.198.206 } {2a00:1450:4007:816::200e }]
result from 1: [{216.58.198.206 } {2a00:1450:4007:816::200e }]

Proposal

Maybe the lookupGroup should be local the resolver so that each resolver is really independent.