net: net.ParseIP always return ipv6

[lysShub]

Go version

go version go1.21.3 windows/amd64

Output of go env in your module/workspace:

set GO111MODULE=
set GOARCH=amd64
set GOBIN=
set GOCACHE=C:\Users\lys\AppData\Local\go-build
set GOENV=C:\Users\lys\AppData\Roaming\go\env
set GOEXE=.exe
set GOEXPERIMENT=
set GOFLAGS=
set GOHOSTARCH=amd64
set GOHOSTOS=windows
set GOINSECURE=
set GOMODCACHE=D:\gopath\pkg\mod
set GONOPROXY=
set GONOSUMDB=
set GOOS=windows
set GOPATH=D:\gopath
set GOPRIVATE=
set GOPROXY=https://goproxy.io,direct
set GOROOT=D:\Go
set GOSUMDB=sum.golang.org
set GOTMPDIR=
set GOTOOLCHAIN=auto
set GOTOOLDIR=D:\Go\pkg\tool\windows_amd64
set GOVCS=
set GOVERSION=go1.21.3
set GCCGO=gccgo
set GOAMD64=v1
set AR=ar
set CC=gcc
set CXX=g++
set CGO_ENABLED=0
set GOMOD=NUL
set GOWORK=
set CGO_CFLAGS=-O2 -g
set CGO_CPPFLAGS=
set CGO_CXXFLAGS=-O2 -g
set CGO_FFLAGS=-O2 -g
set CGO_LDFLAGS=-O2 -g
set PKG_CONFIG=pkg-config

What did you do?

import (
	"fmt"
	"net"
	"net/netip"
)

func main() {
	ip1 := net.ParseIP("127.0.0.1")
	fmt.Println([]byte(ip1))

	nip1, _ := netip.AddrFromSlice(ip1)
	fmt.Println(nip1.Is4())

	conn, _ := net.DialUDP("udp", nil, &net.UDPAddr{IP: net.ParseIP("8.8.8.8"), Port: 53})
	ip2 := (conn.LocalAddr().(*net.UDPAddr)).IP
	fmt.Println([]byte(ip2))
}

What did you see happen?

[0 0 0 0 0 0 0 0 0 0 255 255 127 0 0 1]
false
[192 168 0 111]

What did you expect to see?

[127 0 0 1]
false
[192 168 0 111]

[mateusz834]

This is intentional, [0 0 0 0 0 0 0 0 0 0 255 255 127 0 0 1] is a v4InV6 address.

go/src/net/ip.go

Lines 27 to 36 in 8e658ee

	// An IP is a single IP address, a slice of bytes.
	// Functions in this package accept either 4-byte (IPv4)
	// or 16-byte (IPv6) slices as input.
	//
	// Note that in this documentation, referring to an
	// IP address as an IPv4 address or an IPv6 address
	// is a semantic property of the address, not just the
	// length of the byte slice: a 16-byte slice can still
	// be an IPv4 address.
	type IP []byte

EDIT: but maybe it should be documented better?

[thediveo]

Oh yes, this should be documented better! 🩹 🤕

I've been bitten in the past on this, more so, when combining with 3rd party modules. I'm basically now having always code in place that converts back to the IPv4 4-octets representation upon detecting this situation, using this as my canonical IPv4 representation anywhere, especially before comparing type IPs, coming from different sources, such as stdlib and, for instance, RTNETLINK...

[gopherbot]

Change https://go.dev/cl/598076 mentions this issue: net: document ParseIP always returns IPv6 addresses