Only allow 1 CNAME when querying for a service.

This just makes sure that if multiple services are registered with unique service addresses that we don’t blast back multiple CNAMEs for the same service DNS name and keeps us within the DNS specs.

agent/dns.go

@@ -1143,6 +1143,7 @@ func (d *DNSServer) serviceNodeRecords(dc string, nodes structs.CheckServiceNode
 	qType := req.Question[0].Qtype
 	handled := make(map[string]struct{})
 	edns := req.IsEdns0() != nil
+	haveCNAME := false
 
 	count := 0
 	for _, node := range nodes {
@@ -1169,6 +1170,16 @@ func (d *DNSServer) serviceNodeRecords(dc string, nodes structs.CheckServiceNode
 		// Add the node record
 		records := d.formatNodeRecord(node.Node, addr, qName, qType, ttl, edns, true)
 		if records != nil {
+			// only allow at most 1 CNAME record
+			switch records[0].(type) {
+			case *dns.CNAME:
+				if haveCNAME {
+					continue
+				} else {
+					haveCNAME = true
+				}
+			}
+
 			resp.Answer = append(resp.Answer, records...)
 			count++
 			if count == d.config.ARecordLimit {