Registrar: only for zones that are a registered domain

With porkbun.domains empty the Registrar tab was offered for every zone
and always failed for subdomain zones, reverse zones and private names.
dnsname.RegisteredDomain (public suffix list) now decides; the others
get a message naming what delegates them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/dnsname/dnsname.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/miekg/dns"
 	"golang.org/x/net/idna"
+	"golang.org/x/net/publicsuffix"
 )
 
 // Root is the root domain name.
@@ -305,6 +306,28 @@ func EmailFromMailbox(rname string) string {
 	return local + "@" + strings.Join(labels[1:], ".")
 }
 
+// RegisteredDomain returns the domain registered at a registrar that name
+// is in, without a trailing dot: adbraun.lol for lab.adbraun.lol, from
+// the public suffix list. ok is false when no registrar is involved:
+// reverse zones and the rest of .arpa (delegated by whoever holds the
+// addresses), names under a suffix the list doesn't know (home.lan,
+// internal) and public suffixes themselves (co.uk).
+func RegisteredDomain(name string) (string, bool) {
+	n := strings.ToLower(strings.TrimSuffix(name, "."))
+	if n == "" || n == "arpa" || strings.HasSuffix(n, ".arpa") {
+		return "", false
+	}
+	suffix, icann := publicsuffix.PublicSuffix(n)
+	if !icann || suffix == n {
+		return "", false
+	}
+	d, err := publicsuffix.EffectiveTLDPlusOne(n)
+	if err != nil {
+		return "", false
+	}
+	return d, true
+}
+
 // IsReverse reports whether name is in in-addr.arpa or ip6.arpa.
 func IsReverse(name string) bool {
 	return IsSubdomain(name, "in-addr.arpa.") || IsSubdomain(name, "ip6.arpa.") || IsSubdomain(name, "ip6.int.")

internal/dnsname/dnsname_test.go

@@ -249,3 +249,23 @@ func TestPrefixFromReverse(t *testing.T) {
 		}
 	}
 }
+
+func TestRegisteredDomain(t *testing.T) {
+	for name, want := range map[string]string{
+		"adbraun.lol.":          "adbraun.lol",
+		"lab.adbraun.lol.":      "adbraun.lol",
+		"Example.CO.UK":         "example.co.uk",
+		"co.uk.":                "",
+		"lol.":                  "",
+		"10.0.10.in-addr.arpa.": "",
+		"d.f.ip6.arpa.":         "",
+		"home.arpa.":            "",
+		"nas.home.lan.":         "",
+		"":                      "",
+	} {
+		got, ok := RegisteredDomain(name)
+		if got != want || ok != (want != "") {
+			t.Errorf("RegisteredDomain(%q) = %q, %v; want %q", name, got, ok, want)
+		}
+	}
+}