net: AAAA DNS queries are made despite ip4 on LookupIP

[melardev]

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

$ go version
go version go1.15.8 linux/amd64

Does this issue reproduce with the latest release?

Yes

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

go env Output

$ go env
GO111MODULE=""
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/laro/.cache/go-build"
GOENV="/home/laro/.config/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/laro/Desktop/SSDStorage/workspace/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/laro/Desktop/SSDStorage/workspace/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/go"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build927622048=/tmp/go-build -gno-record-gcc-switches"

What did you do?

The following snippets may not run on golang playground, maybe they restrict network connections?

I prepared the following snippet(https://play.golang.org/p/CTmnriuHXcU), run it with a network sniffer turned on, I used Wireshark with the filter "udp port 53" so I only capture DNS traffic.
There are two DNS queries being made, A, and AAA, this should not happen. I thought this may be reproducible on the default resolver too, so I prepared this other snipped(https://play.golang.org/p/6wIdTxjI4dd), it does not make the AAA DNS query, this shows it is a bug, because if it was by design then I would have expected both snippets to behave the same.

What did you expect to see?

If I ask for an A DNS record, why am I making a query for AAA (ip6)?

What did you see instead?

Two DNS queries, one for IP4(A) and other for IP6 (AAA).

[melardev]

Maybe it would help you track this down, this is only reproducible if PreferGo is set to true, I don't see why this flag should determine what DNS requests should be sent. So the issue must be on cgLookupIp I guess? (https://golang.org/src/net/lookup_unix.go)

[dmitshur]

CC @bradfitz, @ianlancetaylor via owners.

[antong]

I believe this is the same as #45024 (filed some days later), so fixed in Go 1.17.

[seankhliao]

Duplicate of #45024