net: net.ListenUDP() only binds to IPv4 when 127.0.0.1 is not present #61310
Description
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.