net: net.ListenUDP() only binds to IPv4 when 127.0.0.1 is not present

[asdfjkluiop]

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

$ go version
go version go1.20.5 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/user/.cache/go-build"
GOENV="/home/user/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/user/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/user/go"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/lib/go-1.20"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/lib/go-1.20/pkg/tool/linux_amd64"
GOVCS=""
GOVERSION="go1.20.5"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/home/user/sslip.io/src/sslip.io-dns-server/go.mod"
GOWORK=""
CGO_CFLAGS="-O2 -g"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-O2 -g"
CGO_FFLAGS="-O2 -g"
CGO_LDFLAGS="-O2 -g"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build3799498623=/tmp/go-build -gno-record-gcc-switches"

What did you do?

When running on a system with no IPv4 loopback address(127.0.0.1) present net.ListenUDP("udp", &net.UDPAddr{Port: 1234}) binds to 0.0.0.0:1234 instead of [::]:1234. On IPv6 only systems with no IPv4 loopback this is undesirable.

What did you expect to see?

net.ListenUDP("udp", &net.UDPAddr{Port: 1234}) should bind to [::]:1234 when no IPv4 loopback is present on IPv6 only systems. Manually specifying the IPv6 wildcard address :: in the net.UDPAddr struct works as expected. This is only present when no bind address is given.

What did you see instead?

net.ListenUDP("udp", &net.UDPAddr{Port: 1234}) binds to 0.0.0.0:1234 even though only IPv6 is present.

[cherrymui]

cc @ianlancetaylor @neild