feat(go.d/dnsquery): support system DNS servers from /etc/resolv.conf

src/go/go.mod

@@ -71,6 +71,7 @@ require (
 	github.com/Masterminds/semver/v3 v3.3.0 // indirect
 	github.com/Microsoft/go-winio v0.6.1 // indirect
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
+	github.com/containerd/log v0.1.0 // indirect
 	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/dgryski/go-metro v0.0.0-20180109044635-280f6062b5bc // indirect
 	github.com/dgryski/go-rendezvous v0.0.0-20200823014737-9f7001d12a5f // indirect

src/go/plugin/go.d/collector/dnsquery/collector.go

@@ -83,6 +83,10 @@ func (c *Collector) Init(context.Context) error {
 		return fmt.Errorf("config validation: %v", err)
 	}
 
+	if err := c.initServers(); err != nil {
+		return fmt.Errorf("failed to initialize servers: %v", err)
+	}
+
 	rt, err := c.initRecordTypes()
 	if err != nil {
 		return fmt.Errorf("init record type: %v", err)

src/go/plugin/go.d/collector/dnsquery/collector_test.go

@@ -77,17 +77,6 @@ func TestCollector_Init(t *testing.T) {
 				Timeout:     confopt.Duration(time.Second),
 			},
 		},
-		"fail when servers not set": {
-			wantFail: true,
-			config: Config{
-				Domains:     []string{"example.com"},
-				Servers:     nil,
-				Network:     "udp",
-				RecordTypes: []string{"A"},
-				Port:        53,
-				Timeout:     confopt.Duration(time.Second),
-			},
-		},
 		"fail when network is invalid": {
 			wantFail: true,
 			config: Config{

src/go/plugin/go.d/collector/dnsquery/config_schema.json

@@ -65,7 +65,7 @@
       },
       "servers": {
         "title": "Servers",
-        "description": "List of DNS servers to query.",
+        "description": "List of DNS servers to query. If empty, the collector will automatically use DNS servers from `/etc/resolv.conf`.",
         "type": [
           "array",
           "null"
@@ -78,8 +78,7 @@
         "default": [
           "8.8.8.8"
         ],
-        "uniqueItems": true,
-        "minItems": 1
+        "uniqueItems": true
       },
       "domains": {
         "title": "Domains",
@@ -107,7 +106,6 @@
     },
     "required": [
       "domains",
-      "servers",
       "network"
     ],
     "patternProperties": {

src/go/plugin/go.d/collector/dnsquery/init.go

@@ -16,10 +16,6 @@ func (c *Collector) verifyConfig() error {
 		return errors.New("no domains specified")
 	}
 
-	if len(c.Servers) == 0 {
-		return errors.New("no servers specified")
-	}
-
 	if !(c.Network == "" || c.Network == "udp" || c.Network == "tcp" || c.Network == "tcp-tls") {
 		return fmt.Errorf("wrong network transport : %s", c.Network)
 	}
@@ -36,6 +32,24 @@ func (c *Collector) verifyConfig() error {
 	return nil
 }
 
+func (c *Collector) initServers() error {
+	if len(c.Servers) != 0 {
+		return nil
+	}
+	servers, err := getResolvConfNameservers()
+	if err != nil {
+		return err
+	}
+	if len(servers) == 0 {
+		return errors.New("no resolv conf nameservers")
+	}
+
+	c.Debugf("resolv conf nameservers: %v", servers)
+	c.Servers = servers
+
+	return nil
+}
+
 func (c *Collector) initRecordTypes() (map[string]uint16, error) {
 	types := make(map[string]uint16)
 	for _, v := range c.RecordTypes {

src/go/plugin/go.d/collector/dnsquery/metadata.yaml

@@ -62,9 +62,9 @@ modules:
               default_value: ""
               required: true
             - name: servers
-              description: Servers to query.
+              description: Servers to query. If empty, the collector will automatically use DNS servers from `/etc/resolv.conf`.
               default_value: ""
-              required: true
+              required: false
             - name: port
               description: DNS server port.
               default_value: 53
@@ -86,8 +86,8 @@ modules:
             title: Config
             enabled: true
           list:
-            - name: Basic
-              description: An example configuration.
+            - name: Specific DNS servers
+              description: An example configuration using Google's public DNS servers.
               config: |
                 jobs:
                   - name: job1
@@ -101,6 +101,18 @@ modules:
                     servers:
                       - 8.8.8.8
                       - 8.8.4.4
+            - name: System DNS
+              description: An example configuration using DNS servers from `/etc/resolv.conf`.
+              config: |
+                jobs:
+                  - name: job1
+                    record_types:
+                      - A
+                      - AAAA
+                    domains:
+                      - google.com
+                      - github.com
+                      - reddit.com
     troubleshooting:
       problems:
         list: []

src/go/plugin/go.d/collector/dnsquery/resolvconf.go

@@ -0,0 +1,16 @@
+package dnsquery
+
+import (
+	"os"
+
+	"github.com/docker/docker/libnetwork/resolvconf"
+)
+
+func getResolvConfNameservers() ([]string, error) {
+	path := resolvconf.Path()
+	bs, err := os.ReadFile(path)
+	if err != nil {
+		return nil, err
+	}
+	return resolvconf.GetNameservers(bs, resolvconf.IP), nil
+}