+ dnsforward: support IPv6 (AAAA response)

If question type is AAAA:
 Before this patch we responded with NXDOMAIN.
 Now we send an empty response if host rule is IPv4;
 or we send an AAAA answer if host rule is IPv6.

+ block ipv6 if rule is "0.0.0.0 blockdomain"

dnsfilter/dnsfilter.go

@@ -555,9 +555,18 @@ func (d *Dnsfilter) matchHost(host string, qtype uint16) (Result, error) {
 				// either IPv4 or IPv4-mapped IPv6 address
 				res.IP = hostRule.IP.To4()
 				return res, nil
-			} else if qtype == dns.TypeAAAA && hostRule.IP.To4() == nil {
-				res.IP = hostRule.IP
-				return res, nil
+
+			} else if qtype == dns.TypeAAAA {
+				ip4 := hostRule.IP.To4()
+				if ip4 == nil {
+					res.IP = hostRule.IP
+					return res, nil
+				}
+				if bytes.Equal(ip4, []byte{0, 0, 0, 0}) {
+					// send IP="::" response for a rule "0.0.0.0 blockdomain"
+					res.IP = net.IPv6zero
+					return res, nil
+				}
 			}
 			continue