Reverse zones made easy: suggested, filled, in the right view, PTRs kept in step

Missing reverse zones are suggested for the private views' A and AAAA records (/24 for private IPv4, /32 for a public address, /48 for IPv6) and created filled with PTR records (jarl zone suggest-reverse, the Zones page). zone create --fill and zone fill-ptr add the PTRs a reverse zone lacks; a reverse zone created without a view goes to the private view using its addresses. Per zone, auto_ptr keeps the PTR records of A and AAAA records in step on every change (database version 12). Zone commands take a network.

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

internal/dnsname/dnsname.go

@@ -469,3 +469,47 @@ func BestZone(name string, zones []string) string {
 	}
 	return best
 }
+
+// PrefixFromReverse returns the network a reverse zone answers for:
+// 10.0.10.in-addr.arpa. is 10.0.10.0/24, a /32 zone a single address, and
+// an ip6.arpa zone of 12 nibbles a /48. False for names that aren't whole
+// octets or nibbles (classless delegations such as 0-63.1.0.10.in-addr.arpa.).
+func PrefixFromReverse(zone string) (netip.Prefix, bool) {
+	zone = strings.ToLower(Fqdn(zone))
+	switch {
+	case strings.HasSuffix(zone, ".in-addr.arpa."):
+		labels := strings.Split(strings.TrimSuffix(zone, ".in-addr.arpa."), ".")
+		if len(labels) > 4 {
+			return netip.Prefix{}, false
+		}
+		var b [4]byte
+		for i, l := range labels {
+			v, err := strconv.Atoi(l)
+			if err != nil || v < 0 || v > 255 || strconv.Itoa(v) != l {
+				return netip.Prefix{}, false
+			}
+			b[len(labels)-1-i] = byte(v)
+		}
+		return netip.PrefixFrom(netip.AddrFrom4(b), 8*len(labels)), true
+	case strings.HasSuffix(zone, ".ip6.arpa."):
+		labels := strings.Split(strings.TrimSuffix(zone, ".ip6.arpa."), ".")
+		if len(labels) > 32 {
+			return netip.Prefix{}, false
+		}
+		var b [16]byte
+		for i, l := range labels {
+			v := strings.IndexByte("0123456789abcdef", l[0])
+			if len(l) != 1 || v < 0 {
+				return netip.Prefix{}, false
+			}
+			n := len(labels) - 1 - i // nibble position from the left
+			if n%2 == 0 {
+				b[n/2] |= byte(v) << 4
+			} else {
+				b[n/2] |= byte(v)
+			}
+		}
+		return netip.PrefixFrom(netip.AddrFrom16(b), 4*len(labels)), true
+	}
+	return netip.Prefix{}, false
+}

internal/dnsname/dnsname_test.go

@@ -217,3 +217,35 @@ func TestReverseZones(t *testing.T) {
 		t.Errorf("ReverseZone /22: %v", err)
 	}
 }
+
+func TestPrefixFromReverse(t *testing.T) {
+	for zone, want := range map[string]string{
+		"10.0.10.in-addr.arpa.":             "10.0.10.0/24",
+		"10.in-addr.arpa":                   "10.0.0.0/8",
+		"154.144.144.139.in-addr.arpa.":     "139.144.144.154/32",
+		"d.3.f.7.3.1.0.8.2.0.a.2.ip6.arpa.": "2a02:8013:7f3d::/48",
+		"b.a.b.a.b.a.b.a.b.a.d.f.ip6.arpa.": "fdab:abab:abab::/48",
+		"0-63.1.0.10.in-addr.arpa.":         "",
+		"example.com.":                      "",
+		"256.10.in-addr.arpa.":              "",
+	} {
+		p, ok := PrefixFromReverse(zone)
+		got := ""
+		if ok {
+			got = p.String()
+		}
+		if got != want {
+			t.Errorf("PrefixFromReverse(%q) = %q, want %q", zone, got, want)
+		}
+	}
+	// Round trip with ReverseZone.
+	for _, s := range []string{"192.168.1.0/24", "2a02:8013:7f3d::/48", "fd00::/8", "10.0.0.5/32"} {
+		z, err := ReverseZone(netip.MustParsePrefix(s))
+		if err != nil {
+			t.Fatal(err)
+		}
+		if p, ok := PrefixFromReverse(z); !ok || p.String() != s {
+			t.Errorf("%s -> %s -> %v", s, z, p)
+		}
+	}
+}