supply default dns server

conf/input.dns_query/dns_query.toml

@@ -8,6 +8,9 @@
 # # interval = global.interval * interval_times
 # interval_times = 1
 
+# # 
+enable = false
+
 ## servers to query
 # servers = ["8.8.8.8"]
 servers = []

inputs/dns_query/dns_query.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 	"log"
 	"net"
+	"os"
 	"strconv"
 	"sync"
 	"time"
@@ -48,6 +49,7 @@ func (dq *DnsQuery) GetInstances() []inputs.Instance {
 type Instance struct {
 	config.InstanceConfig
 
+	Enable bool `toml:"enable"`
 	// Domains or subdomains to query
 	Domains []string `toml:"domains"`
 
@@ -68,8 +70,23 @@ type Instance struct {
 }
 
 func (ins *Instance) Init() error {
+	if ins.Enable == false {
+		return nil
+	}
 	if len(ins.Servers) == 0 {
-		return types.ErrInstancesEmpty
+		resolvPath := "/etc/resolv.conf"
+		if _, err := os.Stat(resolvPath); os.IsNotExist(err) {
+			return nil
+		}
+		config, _ := dns.ClientConfigFromFile(resolvPath)
+		Servers := []string{}
+		for _, ipAddress := range config.Servers {
+			Servers = append(Servers, ipAddress)
+		}
+		ins.Servers = Servers
+		if len(ins.Servers) == 0 {
+			return nil
+		}
 	}
 
 	if ins.Network == "" {