x/net/idna: Registration.ToASCII() accepts empty TLD label

[jusmarks]

What did you do?

https://go.dev/play/p/oGhflDafvrL

package main

import (
	"fmt"

	"golang.org/x/net/idna"
)

func main() {
	// Returns an error (as expected)
	fmt.Println(idna.Registration.ToASCII("gólang.órg.."))

	// Doesn't return an error
	fmt.Println(idna.Registration.ToASCII("golang.org.."))
}

What did you expect to see?

xn--glang-0ta.xn--rg-4ja.. idna: invalid label ""
golang.org.. idna: invalid label ""

What did you see instead?

xn--glang-0ta.xn--rg-4ja.. idna: invalid label ""
golang.org.. <nil>

It appears if the domain contains an empty TLD label but contains only ASCII characters, then no error is returned. And only for the TLD (empty labels elsewhere return an error as expected).

[cherrymui]

cc @neild @ianlancetaylor

[elliotwutingfeng]

From this section in the Unicode Technical Standard 46, both idna.Registration.ToASCII("gólang.órg..") and idna.Registration.ToASCII("golang.org..") should not be accepted since verifyDnsLength is set to true.

Currently idna.Registration.ToASCII("golang.org..") is being wrongly accepted, this can be fixed by setting *labelIter.next() to skip last label only if it is empty and verifyDNSLength is false.

I have drafted a possible fix here.

[lomkju]

Can you verify if this interim fix works?
Adding an extra trailing dot and removing it after verification seems to work.

package main

import (
	"fmt"

	"golang.org/x/net/idna"
)

var domainList = []string{
	"excloud.in",
	"excloud..in",
	"excloud..in.",
	"excloud.-.in",
	"excloud.in...",
	"excloud.in..",
	"excloud.in.",
	".excloud.in.",
}

func main() {
	p := idna.New(idna.ValidateForRegistration(), idna.RemoveLeadingDots(false))
	for _, v := range domainList {
		// Add Trailing extra dot at end to fix golang issue.
		// https://github.com/golang/go/issues/47182#issuecomment-1592296932
		v = v + "."
		r, err := p.ToASCII(v)
		isValid := "Valid"
		if err != nil {
			isValid = "Invalid"
		}
		// Remove Trailing extra dot at end to fix golang issue.
		// https://github.com/golang/go/issues/47182#issuecomment-1592296932
		r = r[:len(r)-1]
		fmt.Printf("%s : %s\n", r, isValid)
	}
}