Add support for SNMP over TCP (#5870)

plugins/inputs/snmp/README.md

@@ -98,7 +98,7 @@ Resulting output:
 ### Config parameters
 
 * `agents`: Default: `[]`
-List of SNMP agents to connect to in the form of `IP[:PORT]`. If `:PORT` is unspecified, it defaults to `161`.
+List of SNMP agents to connect to in the form of `[tcp://]IP[:PORT]`. If `:PORT` is unspecified, it defaults to `161`. When using the optional prefix `tcp://`, SNMP over TCP will be used. Otherwise UDP is used as the transport protocol.
 
 * `version`: Default: `2`
 SNMP protocol version to use.

plugins/inputs/snmp/snmp.go

@@ -623,6 +623,10 @@ func (s *Snmp) getConnection(idx int) (snmpConnection, error) {
 	gs := gosnmpWrapper{&gosnmp.GoSNMP{}}
 	s.connectionCache[idx] = gs
 
+	if strings.HasPrefix(agent, "tcp://") {
+		agent = strings.TrimPrefix(agent, "tcp://")
+		gs.Transport = "tcp"
+	}
 	host, portStr, err := net.SplitHostPort(agent)
 	if err != nil {
 		if err, ok := err.(*net.AddrError); !ok || err.Err != "missing port in address" {

plugins/inputs/snmp/snmp_test.go

@@ -272,12 +272,46 @@ func TestGetSNMPConnection_v2(t *testing.T) {
 	assert.EqualValues(t, 567, gs.Port)
 	assert.Equal(t, gosnmp.Version2c, gs.Version)
 	assert.Equal(t, "foo", gs.Community)
+	assert.Equal(t, "udp", gs.Transport)
 
 	gsc, err = s.getConnection(1)
 	require.NoError(t, err)
 	gs = gsc.(gosnmpWrapper)
 	assert.Equal(t, "1.2.3.4", gs.Target)
 	assert.EqualValues(t, 161, gs.Port)
+	assert.Equal(t, "udp", gs.Transport)
+}
+
+func TestGetSNMPConnectionTCP(t *testing.T) {
+	var wg sync.WaitGroup
+	wg.Add(1)
+	go stubTCPServer(&wg)
+	wg.Wait()
+
+	s := &Snmp{
+		Agents: []string{"tcp://127.0.0.1:56789"},
+	}
+	err := s.init()
+	require.NoError(t, err)
+
+	wg.Add(1)
+	gsc, err := s.getConnection(0)
+	require.NoError(t, err)
+	gs := gsc.(gosnmpWrapper)
+	assert.Equal(t, "127.0.0.1", gs.Target)
+	assert.EqualValues(t, 56789, gs.Port)
+	assert.Equal(t, "tcp", gs.Transport)
+	wg.Wait()
+}
+
+func stubTCPServer(wg *sync.WaitGroup) {
+	defer wg.Done()
+	tcpAddr, _ := net.ResolveTCPAddr("tcp", "127.0.0.1:56789")
+	tcpServer, _ := net.ListenTCP("tcp", tcpAddr)
+	defer tcpServer.Close()
+	wg.Done()
+	conn, _ := tcpServer.AcceptTCP()
+	defer conn.Close()
 }
 
 func TestGetSNMPConnection_v3(t *testing.T) {