Add UDP prospector type with plain harvester

event looks as following

```
{
  "@timestamp": "2017-05-26T11:57:17.68711727+02:00",
  "beat": {
    "hostname": "ruflin",
    "name": "ruflin",
    "version": "6.0.0-alpha2"
  },
  "message": "Hello, World!99",
  "prospector": {
    "harvester": "plain",
    "type": "udp"
  }
}
```

CHANGELOG.asciidoc

@@ -61,6 +61,7 @@ https://github.com/elastic/beats/compare/v6.0.0-alpha1...master[Check the HEAD d
 - Nginx module: use the first not-private IP address as the remote_ip. {pull}4417[4417]
 - Load Ingest Node pipelines when the Elasticsearch connection is established, instead of only once at startup. {pull}4479[4479]
 - Add support for loading Xpack Machine Learning configurations from the modules, and added sample configurations for the Nginx module. {pull}4506[4506]
+- Add udp prospector type. {pull}4452[4452]
 
 *Heartbeat*
 

filebeat/_meta/common.full.p2.yml

@@ -231,6 +231,10 @@ filebeat.prospectors:
   # Redis AUTH password. Empty by default.
   #password: foobared
 
+#- type: udp
+
+  # Maximum size of the message received over UDP
+  #max_message_size: 10240
 #========================= Filebeat global options ============================
 
 # Event count spool threshold - forces network flush if exceeded

filebeat/filebeat.full.yml

@@ -400,6 +400,10 @@ filebeat.prospectors:
   # Redis AUTH password. Empty by default.
   #password: foobared
 
+#- type: udp
+
+  # Maximum size of the message received over UDP
+  #max_message_size: 10240
 #========================= Filebeat global options ============================
 
 # Event count spool threshold - forces network flush if exceeded

filebeat/harvester/util.go

@@ -7,13 +7,15 @@ const (
 	LogType   = "log"
 	StdinType = "stdin"
 	RedisType = "redis"
+	UdpType   = "udp"
 )
 
 // ValidType of valid input types
 var ValidType = map[string]struct{}{
 	StdinType: {},
 	LogType:   {},
 	RedisType: {},
+	UdpType:   {},
 }
 
 // MatchAny checks if the text matches any of the regular expressions

filebeat/prospector/prospector.go

@@ -13,6 +13,7 @@ import (
 	"github.com/elastic/beats/filebeat/prospector/log"
 	"github.com/elastic/beats/filebeat/prospector/redis"
 	"github.com/elastic/beats/filebeat/prospector/stdin"
+	"github.com/elastic/beats/filebeat/prospector/udp"
 	"github.com/elastic/beats/libbeat/common"
 	"github.com/elastic/beats/libbeat/logp"
 )
@@ -77,6 +78,8 @@ func (p *Prospector) initProspectorer(outlet channel.Outleter, states []file.Sta
 		prospectorer, err = redis.NewProspector(config, outlet)
 	case harvester.LogType:
 		prospectorer, err = log.NewProspector(config, states, outlet, p.done)
+	case harvester.UdpType:
+		prospectorer, err = udp.NewProspector(config, outlet)
 	default:
 		return fmt.Errorf("invalid prospector type: %v. Change type", p.config.Type)
 	}

filebeat/prospector/udp/config.go

@@ -0,0 +1,20 @@
+package udp
+
+import (
+	"github.com/elastic/beats/filebeat/harvester"
+)
+
+var defaultConfig = config{
+	ForwarderConfig: harvester.ForwarderConfig{
+		Type: "udp",
+	},
+	MaxMessageSize: 10240,
+	// TODO: What should be default port?
+	Host: "localhost:8080",
+}
+
+type config struct {
+	harvester.ForwarderConfig `config:",inline"`
+	Host                      string `config:"host"`
+	MaxMessageSize            int    `config:"max_message_size"`
+}

filebeat/prospector/udp/harvester.go

@@ -0,0 +1,72 @@
+package udp
+
+import (
+	"net"
+	"time"
+
+	"github.com/elastic/beats/filebeat/harvester"
+	"github.com/elastic/beats/filebeat/util"
+	"github.com/elastic/beats/libbeat/common"
+	"github.com/elastic/beats/libbeat/logp"
+)
+
+type Harvester struct {
+	forwarder *harvester.Forwarder
+	done      chan struct{}
+	cfg       *common.Config
+	listener  net.PacketConn
+}
+
+func NewHarvester(forwarder *harvester.Forwarder, cfg *common.Config) *Harvester {
+	return &Harvester{
+		done:      make(chan struct{}),
+		cfg:       cfg,
+		forwarder: forwarder,
+	}
+}
+
+func (h *Harvester) Run() error {
+
+	config := defaultConfig
+	err := h.cfg.Unpack(&config)
+	if err != nil {
+		return err
+	}
+
+	h.listener, err = net.ListenPacket("udp", config.Host)
+	if err != nil {
+		return err
+	}
+	defer h.listener.Close()
+
+	logp.Info("Started listening for udp on: %s", config.Host)
+
+	buffer := make([]byte, config.MaxMessageSize)
+
+	for {
+		select {
+		case <-h.done:
+			return nil
+		default:
+		}
+
+		length, _, err := h.listener.ReadFrom(buffer)
+		if err != nil {
+			logp.Err("Error reading from buffer: %v", err.Error())
+			continue
+		}
+		data := util.NewData()
+		event := common.MapStr{
+			"message":    string(buffer[:length]),
+			"@timestamp": time.Now(),
+		}
+		data.Event = event
+		h.forwarder.Send(data)
+	}
+}
+
+func (h *Harvester) Stop() {
+	logp.Info("Stopping udp harvester")
+	close(h.done)
+	h.listener.Close()
+}

filebeat/prospector/udp/prospector.go

@@ -0,0 +1,47 @@
+package udp
+
+import (
+	"github.com/elastic/beats/filebeat/channel"
+	"github.com/elastic/beats/filebeat/harvester"
+	"github.com/elastic/beats/libbeat/common"
+	"github.com/elastic/beats/libbeat/logp"
+)
+
+type Prospector struct {
+	harvester *Harvester
+	started   bool
+}
+
+func NewProspector(cfg *common.Config, outlet channel.Outleter) (*Prospector, error) {
+	forwarder, err := harvester.NewForwarder(cfg, outlet)
+	if err != nil {
+		return nil, err
+	}
+	return &Prospector{
+		harvester: NewHarvester(forwarder, cfg),
+		started:   false,
+	}, nil
+}
+
+func (p *Prospector) Run() {
+	logp.Info("Starting udp prospector")
+
+	if !p.started {
+		p.started = true
+		go func() {
+			err := p.harvester.Run()
+			if err != nil {
+				logp.Err("Error running harvester:: %v", err)
+			}
+		}()
+	}
+}
+
+func (p *Prospector) Stop() {
+	logp.Info("Stopping udp prospector")
+	p.harvester.Stop()
+}
+
+func (p *Prospector) Wait() {
+	p.Stop()
+}

filebeat/tests/system/test_udp.py

@@ -0,0 +1,41 @@
+from filebeat import BaseTest
+import socket
+import time
+
+
+class Test(BaseTest):
+
+    def test_udp(self):
+
+        host = "127.0.0.1"
+        port = 8080
+        prospector_raw = """
+- type: udp
+  host: "{}:{}"
+  enabled: true
+"""
+
+        prospector_raw = prospector_raw.format(host, port)
+
+        self.render_config_template(
+            prospector_raw=prospector_raw,
+            prospectors=False,
+        )
+
+        filebeat = self.start_beat()
+
+        self.wait_until(lambda: self.log_contains("Started listening for udp"))
+
+        sock = socket.socket(socket.AF_INET, socket.SOCK_DGRAM)  # UDP
+
+        for n in range(0, 2):
+            sock.sendto("Hello World: " + str(n), (host, port))
+
+        self.wait_until(lambda: self.output_count(lambda x: x >= 2))
+
+        filebeat.check_kill_and_wait()
+
+        output = self.read_output()
+
+        assert len(output) == 2
+        assert output[0]["prospector.type"] == "udp"