+ 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

@@ -555,10 +555,19 @@ 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 {
+
+			} 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
 
 		} else {

dnsforward/dnsforward.go

@@ -378,7 +378,7 @@ func (s *Server) filterDNSRequest(d *proxy.DNSContext) (*dnsfilter.Result, error
 	var res dnsfilter.Result
 	var err error
 
-	res, err = dnsFilter.CheckHost(host)
+	res, err = dnsFilter.CheckHost(host, d.Req.Question[0].Qtype)
 	if err != nil {
 		// Return immediately if there's an error
 		return nil, errorx.Decorate(err, "dnsfilter failed to check host '%s'", host)
@@ -394,7 +394,7 @@ func (s *Server) filterDNSRequest(d *proxy.DNSContext) (*dnsfilter.Result, error
 func (s *Server) genDNSFilterMessage(d *proxy.DNSContext, result *dnsfilter.Result) *dns.Msg {
 	m := d.Req
 
-	if m.Question[0].Qtype != dns.TypeA {
+	if m.Question[0].Qtype != dns.TypeA && m.Question[0].Qtype != dns.TypeAAAA {
 		return s.genNXDomain(m)
 	}
 
@@ -405,11 +405,25 @@ func (s *Server) genDNSFilterMessage(d *proxy.DNSContext, result *dnsfilter.Resu
 		return s.genBlockedHost(m, parentalBlockHost, d)
 	default:
 		if result.IP != nil {
+			if m.Question[0].Qtype == dns.TypeA {
 				return s.genARecord(m, result.IP)
+			} else if m.Question[0].Qtype == dns.TypeAAAA {
+				return s.genAAAARecord(m, result.IP)
+			}
+
+			// empty response
+			resp := dns.Msg{}
+			resp.SetReply(m)
+			return &resp
 		}
 
 		if s.conf.BlockingMode == "null_ip" {
-			return s.genARecord(m, net.IPv4zero)
+			switch m.Question[0].Qtype {
+			case dns.TypeA:
+				return s.genARecord(m, []byte{0, 0, 0, 0})
+			case dns.TypeAAAA:
+				return s.genAAAARecord(m, net.IPv6zero)
+			}
 		}
 
 		return s.genNXDomain(m)
@@ -426,15 +440,41 @@ func (s *Server) genServerFailure(request *dns.Msg) *dns.Msg {
 func (s *Server) genARecord(request *dns.Msg, ip net.IP) *dns.Msg {
 	resp := dns.Msg{}
 	resp.SetReply(request)
-	answer, err := dns.NewRR(fmt.Sprintf("%s %d A %s", request.Question[0].Name, s.conf.BlockedResponseTTL, ip.String()))
+	resp.Answer = append(resp.Answer, s.genAAnswer(request, ip))
-	if err != nil {
-		log.Printf("Couldn't generate A record for replacement host '%s': %s", ip.String(), err)
-		return s.genServerFailure(request)
-	}
-	resp.Answer = append(resp.Answer, answer)
 	return &resp
 }
 
+func (s *Server) genAAAARecord(request *dns.Msg, ip net.IP) *dns.Msg {
+	resp := dns.Msg{}
+	resp.SetReply(request)
+	resp.Answer = append(resp.Answer, s.genAAAAAnswer(request, ip))
+	return &resp
+}
+
+func (s *Server) genAAnswer(req *dns.Msg, ip net.IP) *dns.A {
+	answer := new(dns.A)
+	answer.Hdr = dns.RR_Header{
+		Name:   req.Question[0].Name,
+		Rrtype: dns.TypeA,
+		Ttl:    s.conf.BlockedResponseTTL,
+		Class:  dns.ClassINET,
+	}
+	answer.A = ip
+	return answer
+}
+
+func (s *Server) genAAAAAnswer(req *dns.Msg, ip net.IP) *dns.AAAA {
+	answer := new(dns.AAAA)
+	answer.Hdr = dns.RR_Header{
+		Name:   req.Question[0].Name,
+		Rrtype: dns.TypeAAAA,
+		Ttl:    s.conf.BlockedResponseTTL,
+		Class:  dns.ClassINET,
+	}
+	answer.AAAA = ip
+	return answer
+}
+
 func (s *Server) genBlockedHost(request *dns.Msg, newAddr string, d *proxy.DNSContext) *dns.Msg {
 	// look up the hostname, TODO: cache
 	replReq := dns.Msg{}