net/netip: Document of AddrFromSlice doesn't mention that passing IPv4 net.IP made with net.ParseIP to AddrFromSlice creates IPv4 mapped IPv6 netip.Addr

[YutaroHayakawa]

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

$ go version
go version go1.19 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/yutaro/.cache/go-build"
GOENV="/home/yutaro/.config/go/env"
GOEXE=""
GOEXPERIMENT=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOINSECURE=""
GOMODCACHE="/home/yutaro/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="linux"
GOPATH="/home/yutaro/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"
GOVCS=""
GOVERSION="go1.19"
GCCGO="gccgo"
GOAMD64="v1"
AR="ar"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD="/dev/null"
GOWORK=""
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 -Wl,--no-gc-sections -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build2759925210=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://go.dev/play/p/Dnz95xrGio-

What did you expect to see?

isIPv4: true, string: 10.0.0.1
isIPv4: true, string: 10.0.0.1

What did you see instead?

isIPv4: true, string: 10.0.0.1
isIPv4: false, string: ::ffff:10.0.0.1

The issue happens because net.IP holds an IPv4 address in the form of an IPv4 mapped IPv6 address if I understand correctly. Since AddrFromSlice is not aware of the net.IP and it only sees the length of the slice, so it is working as intended in that sense. However, the document of AddrFromSlice is a bit misleading.

Note that a net.IP can be passed directly as the []byte argument.

It can be read like we can use AddrFromSlice to convert net.IP to netip.Addr, but as mentioned in the #37921, net.IP cannot distinguish IPv4 address and IPv4 mapped IPv6 address. Thus, users should always be aware of the address family of net.IP before calling AddrFromSlice and need to do net.IP.To4() when it is an IPv4 address. I think users should know this.

[heschi]

cc @ianlancetaylor @neild

[ianlancetaylor]

CC @bradfitz @josharian

[SrijanSriv]

I believe this has been cleared?

[ianwoolf]

i send a cl to add ipv4 check to netip.AddrFromSlice.

https://go-review.googlesource.com/c/go/+/443196

please feel free to review it

[gopherbot]

Change https://go.dev/cl/443196 mentions this issue: net/netip: update ipv4 check of AddrFromSlice

[ianwoolf]

As the comment of net.IP said, 16-byte slices can still be IPv4 addresses. And IP.String() checks the version of ip and returns a different address string.

But AddrFromSlice only checks the length of the slice. So I think AddrFromSlice should check the ip version when the slice length is 16-byte.

[ainar-g]

See also:

• proposal: net: UDPAddr/TCPAddr AddrPort should map 4in6 addresses to IPv4 netip.Addrs #53607
• net: LookupNetIP returns IPv4 in IPv6 instead of IPv4 #53554

[gtataranni]

But AddrFromSlice only checks the length of the slice. So I think AddrFromSlice should check the ip version when the slice length is 16-byte.

In fact, AddrFromSlice over ParseIP or directly produces slightly different results.
The two objects compare successfully on a reflect.DeepEqual, but the .Is4 returns two different values.

This is confusing. If you really don't want to change it, at least please document it clearly.

func TestAddrFromSlice(t *testing.T) {

	addr1, _ := netip.AddrFromSlice(net.ParseIP("192.168.1.1"))

	if !addr1.Is4() { // this fails
		t.Logf("addr1 is not ipv4: %#v", addr1) // addr1 is not ipv4: netip.Addr{addr:netip.uint128{hi:0x0, lo:0xffffc0a80101}, z:(*intern.Value)(0x140000be018)}
		t.Fail()
	}

	addr2, _ := netip.AddrFromSlice([]byte{192, 168, 1, 1})
	if !addr2.Is4() {
		t.Logf("addr2 is not ipv4: %#v", addr2)
		t.Fail()
	}

	if !reflect.DeepEqual(addr1, addr2) {
		t.Logf("DeepEqual failed, addr1 %#v, addr2 %#v", addr1, addr2)
		t.Fail()
	}
}