agent: Support CNAME recursion if we have a recursor

command/agent/dns.go

@@ -175,7 +175,11 @@ func (d *DNSServer) handleQuery(resp dns.ResponseWriter, req *dns.Msg) {
 	m := new(dns.Msg)
 	m.SetReply(req)
 	m.Authoritative = true
-	d.addSOA(d.domain, m)
+
+	// Only add the SOA if requested
+	if req.Question[0].Qtype == dns.TypeSOA {
+		d.addSOA(d.domain, m)
+	}
 
 	// Dispatch the correct handler
 	d.dispatch(network, req, m)
@@ -310,6 +314,14 @@ func (d *DNSServer) nodeLookup(network, datacenter, node string, req, resp *dns.
 	record := formatNodeRecord(&out.NodeServices.Node, req.Question[0].Name, qType)
 	if record != nil {
 		resp.Answer = append(resp.Answer, record)
+
+		// Try to recursively resolve the CNAME
+		if cnRec, ok := record.(*dns.CNAME); ok {
+			aRecs := d.resolveCNAME(cnRec.Target)
+			if len(aRecs) > 0 {
+				resp.Extra = append(resp.Extra, aRecs[0])
+			}
+		}
 	}
 }
 
@@ -399,6 +411,9 @@ func (d *DNSServer) serviceLookup(network, datacenter, service, tag string, req,
 	if qType == dns.TypeANY || qType == dns.TypeSRV {
 		d.serviceSRVRecords(datacenter, out.Nodes, req, resp)
 	}
+
+	// Cleanup duplicate extra entries
+	resp.Extra = removeDuplicates(resp.Extra)
 }
 
 // filterServiceNodes is used to filter out nodes that are failing
@@ -446,6 +461,14 @@ func (d *DNSServer) serviceNodeRecords(nodes structs.CheckServiceNodes, req, res
 		record := formatNodeRecord(&node.Node, qName, qType)
 		if record != nil {
 			resp.Answer = append(resp.Answer, record)
+
+			// Try to recursively resolve the CNAME
+			if cnRec, ok := record.(*dns.CNAME); ok {
+				aRecs := d.resolveCNAME(cnRec.Target)
+				if len(aRecs) > 0 {
+					resp.Extra = append(resp.Extra, aRecs[0])
+				}
+			}
 		}
 	}
 }
@@ -489,6 +512,14 @@ func (d *DNSServer) serviceSRVRecords(dc string, nodes structs.CheckServiceNodes
 		record := formatNodeRecord(&node.Node, srvRec.Target, dns.TypeANY)
 		if record != nil {
 			resp.Extra = append(resp.Extra, record)
+
+			// Try to recursively resolve the CNAME
+			if cnRec, ok := record.(*dns.CNAME); ok {
+				aRecs := d.resolveCNAME(cnRec.Target)
+				if len(aRecs) > 0 {
+					resp.Extra = append(resp.Extra, aRecs[0])
+				}
+			}
 		}
 	}
 }
@@ -526,3 +557,47 @@ func (d *DNSServer) handleRecurse(resp dns.ResponseWriter, req *dns.Msg) {
 		d.logger.Printf("[WARN] dns: failed to respond: %v", err)
 	}
 }
+
+// resolveCNAME is used to recursively resolve CNAME records
+func (d *DNSServer) resolveCNAME(name string) []dns.RR {
+	// Do nothing if we don't have a recursor
+	if d.recursor == "" {
+		return nil
+	}
+
+	// Ask for any A records
+	m := new(dns.Msg)
+	m.SetQuestion(name, dns.TypeA)
+
+	// Make a DNS lookup request
+	c := &dns.Client{Net: "udp"}
+	r, rtt, err := c.Exchange(m, d.recursor)
+	if err != nil {
+		d.logger.Printf("[ERR] dns: cname recurse failed: %v", err)
+		return nil
+	}
+	d.logger.Printf("[DEBUG] dns: cname recurse RTT for %v (%v)", name, rtt)
+
+	// Return all the answers
+	return r.Answer
+}
+
+// removeDuplicates is used to remove the duplicate entries.
+// This only deduplicates on the QName and QType
+func removeDuplicates(rr []dns.RR) []dns.RR {
+	handled := make(map[string]struct{})
+	n := len(rr)
+	for i := 0; i < n; i++ {
+		rec := rr[i]
+		hdr := rec.Header()
+		key := fmt.Sprintf("%s:%d", hdr.Name, hdr.Rrtype)
+		if _, ok := handled[key]; ok {
+			// Remove duplicate
+			rr[i], rr[n-1] = rr[n-1], nil
+			n--
+			i--
+		}
+		handled[key] = struct{}{}
+	}
+	return rr[:n]
+}

command/agent/dns_test.go

@@ -587,14 +587,22 @@ func TestDNS_ServiceLookup_CNAME(t *testing.T) {
 		t.Fatalf("Bad: %#v", srvRec)
 	}
 
-	cnRec, ok = in.Extra[0].(*dns.CNAME)
+	aRec, ok := in.Extra[0].(*dns.A)
 	if !ok {
 		t.Fatalf("Bad: %#v", in.Extra[0])
 	}
-	if cnRec.Hdr.Name != "google.node.dc1.consul." {
+	if aRec.Hdr.Name != "www.google.com." {
 		t.Fatalf("Bad: %#v", in.Extra[0])
 	}
+
+	cnRec, ok = in.Extra[1].(*dns.CNAME)
+	if !ok {
+		t.Fatalf("Bad: %#v", in.Extra[1])
+	}
+	if cnRec.Hdr.Name != "google.node.dc1.consul." {
+		t.Fatalf("Bad: %#v", in.Extra[1])
+	}
 	if cnRec.Target != "www.google.com." {
-		t.Fatalf("Bad: %#v", in.Extra[0])
+		t.Fatalf("Bad: %#v", in.Extra[1])
 	}
 }