[packetbeat] Fix interface device parsing for packetbeat protocols (#…

…37946)

* Fix interface device parsing for packetbeat protocols

* Add unit test

* Move device check out of validation function

* Add err check on condition for clarity

CHANGELOG.next.asciidoc

@@ -107,6 +107,7 @@ fields added to events containing the Beats version. {pull}37553[37553]
 
 *Packetbeat*
 
+- Fix interface device parsing for packetbeat protocols. {pull}37946[37946]
 
 *Winlogbeat*
 

packetbeat/protos/protos.go

@@ -173,16 +173,9 @@ func (s ProtocolsStruct) configureProtocol(test bool, device string, pub reporte
 
 	if device != "" {
 		// This could happen earlier, but let any errors be found first.
-		var protocol struct {
-			Device string `config:"interface"`
-		}
-		err := config.Unpack(&protocol)
-		if err != nil {
+		if isValid, err := validateProtocolDevice(device, config); !isValid || err != nil {
 			return err
 		}
-		if protocol.Device != "" && protocol.Device != device {
-			return nil
-		}
 	}
 
 	var client beat.Client
@@ -205,6 +198,24 @@ func (s ProtocolsStruct) configureProtocol(test bool, device string, pub reporte
 	return nil
 }
 
+func validateProtocolDevice(device string, config *conf.C) (bool, error) {
+	var protocol struct {
+		Interface struct {
+			Device string `config:"device"`
+		} `config:"interface"`
+	}
+
+	if err := config.Unpack(&protocol); err != nil {
+		return false, err
+	}
+
+	if protocol.Interface.Device != "" && protocol.Interface.Device != device {
+		return false, nil
+	}
+
+	return true, nil
+}
+
 func (s ProtocolsStruct) register(proto Protocol, client beat.Client, plugin Plugin) {
 	if _, exists := s.all[proto]; exists {
 		logp.Warn("Protocol (%s) plugin will overwritten by another plugin", proto.String())

packetbeat/protos/protos_test.go

@@ -24,6 +24,8 @@ import (
 	"time"
 
 	"github.com/elastic/beats/v7/libbeat/common"
+	conf "github.com/elastic/elastic-agent-libs/config"
+	"github.com/elastic/go-ucfg"
 
 	"github.com/stretchr/testify/assert"
 )
@@ -200,3 +202,58 @@ func TestGetUDP(t *testing.T) {
 	assert.NotNil(t, udp)
 	assert.Contains(t, udp.GetPorts(), 53)
 }
+
+func TestValidateProtocolDevice(t *testing.T) {
+	tcs := []struct {
+		testCase, device string
+		config           map[string]interface{}
+		expectedValid    bool
+		expectedErr      string
+	}{
+		{
+			"DeviceIsIncorrect",
+			"eth0",
+			map[string]interface{}{
+				"interface": map[string]interface{}{
+					"device": "eth1",
+				},
+			},
+			false,
+			"",
+		},
+		{
+			"DeviceIsCorrect",
+			"eth1",
+			map[string]interface{}{
+				"interface": map[string]interface{}{
+					"device": "eth1",
+				},
+			},
+			true,
+			"",
+		},
+		{
+			"ConfigIsInvalid",
+			"eth0",
+			map[string]interface{}{
+				"interface": "eth1",
+			},
+			false,
+			"required 'object', but found 'string' in field 'interface'",
+		},
+	}
+
+	for _, tc := range tcs {
+		tc := tc
+		t.Run(tc.testCase, func(t *testing.T) {
+			cfg := (*conf.C)(ucfg.MustNewFrom(tc.config))
+			isValid, err := validateProtocolDevice(tc.device, cfg)
+			assert.Equal(t, tc.expectedValid, isValid)
+			if tc.expectedErr == "" {
+				assert.Nil(t, err)
+			} else {
+				assert.EqualError(t, err, tc.expectedErr)
+			}
+		})
+	}
+}