Package org.snmp4j
The org.snmp4j
classes are capable of creating, sending, and
receiving SNMPv1/v2c/v3 messages. A SNMP message is composed of its message
header and its PDU payload. This package contains three main groups of classes
and interfaces:
- Classes for SNMP message and target creation
- Classes for SNMP message sending (command generation)
- Classes for SNMP message dispatching (command responding)
The following UML package diagram illustrates the dependencies between the
packages of the core SNMP4J API. Users of the API normally only need to use the
org.snmp4j
and the org.snmp4j.smi
packages directly.
The following UML class diagram shows the most important classes of the org.snmp4j package and their relationships (relationships to other packages are not shown):.
SNMP Messages and Targets
To exchange a SNMP message with a remote system, that system has to be identified,
retransmission, and timeout policy information about the message exchange has
to be specified. A remote system is specified with SNMP4J by creating a
Target
instance appropriate for the SNMP protocol to be used.
- For SNMPv1 and SNMPv2c the
CommunityTarget
has to be used which provides community information in addition to the address, retransmission, and timeout policy information defined by theTarget
interface. -
For SNMPv3 the
UserTarget
has to be used instead. It extends theSecureTarget
abstract class and provides the following User Based Security Model (USM) user information: security name, security level, security model (i.e. USM), and authoritative engine ID.
A SNMP message consists of the message's payload, the SNMP Protocol Data Unit
(PDU) and a message header. Simplified said, in SNMP4J the message header
information is represented by Target
instances and the PDU is
represented by one of the following classes:
-
PDUv1
(SNMPv1) -
PDU
(SNMPv2c) -
ScopedPDU
(SNMPv3)
PDU
instance and a Target
instance have to be created.
PDU Examples
- SNMPv1/v2c GETNEXT PDU
import org.snmp4j.PDU; import org.snmp4j.smi.*; ... PDU pdu = new PDU(); pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.1.1"))); // sysDescr pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.2.1"))); // ifNumber pdu.setType(PDU.GETNEXT); ...
- SNMPv3 GETBULK PDU
import org.snmp4j.ScopedPDU; import org.snmp4j.smi.*; ... ScopedPDU pdu = new ScopedPDU(); pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.2.1"))); // ifNumber pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.2.2.1.10"))); // ifInOctets pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.2.2.1.16"))); // ifOutOctets pdu.setType(PDU.GETBULK); pdu.setMaxRepetitions(50); // Get ifNumber only once pdu.setNonRepeaters(1); // set context non-default context (default context does not need to be set) pdu.setContextName(new OctetString("subSystemContextA")); // set non-default context engine ID (to use targets authoritative engine ID // use an empty (size == 0) octet string) pdu.setContextEngineID(OctetString.fromHexString("80:00:13:70:c0:a8:01:0d")); ...
- SNMPv1 TRAP PDU
import org.snmp4j.PDUv1; ... PDUv1 pdu = new PDUv1(); pdu.setType(PDU.V1TRAP); pdu.setGenericTrap(PDUv1.COLDSTART); ...
- SNMPv2c/SNMPv3 INFORM PDU
import org.snmp4j.ScopedPDU; ... ScopedPDU pdu = new ScopedPDU(); pdu.setType(PDU.INFORM); // sysUpTime long sysUpTime = (System.nanoTime() - startTime) / 10000000; // 10^-7 pdu.add(new VariableBinding(SnmpConstants.sysUpTime, new TimeTicks(sysUpTime))); pdu.add(new VariableBinding(SnmpConstants.snmpTrapOID, SnmpConstants.linkDown)); // payload pdu.add(new VariableBinding(new OID("1.3.6.1.2.1.2.2.1.1"+downIndex), new Integer32(downIndex))); ...
Target Examples
- Community Target
CommunityTarget target = new CommunityTarget(); target.setCommunity(new OctetString("public")); target.setAddress(targetAddress); target.setVersion(SnmpConstants.version1);
- User Target
UserTarget target = new UserTarget(); target.setAddress(targetAddress); target.setRetries(1); // set timeout to 500 milliseconds: 2*500ms = 1s total timeout target.setTimeout(500); target.setVersion(SnmpConstants.version3); target.setSecurityLevel(SecurityLevel.AUTH_PRIV); target.setSecurityName(new OctetString("MD5DES"));
Sending SNMP messages
SNMP message are sent with SNMP4J by using a instance of the SNMP
Session
interface. The default implementation of this interface
is the Snmp
class.
To setup a Snmp
instance it is sufficient to call its
constructor with a TransportMapping
instance. The transport
mapping is used by the SNMP session to send
(and receive) SNMP message to a remote systems by using a transport protocol,
for example the User Datagram Protocol (UDP).
A SNMP4J Snmp
instance supports SNMP v1, v2c, and v3 by default.
By sub-classing Snmp
other combinations of those SNMP protocol
versions can be supported.
With SNMP4J, SNMP messages can be sent synchronously (blocking) and
asynchronously (non-blocking). The Snmp
class does not use
an internal thread to process responses on asynchronous and synchronous
requests. Nevertheless it uses the receiver threads of the transport mappings
to process responses.
Asynchronous responses are returned by calling a callback method
on an object instance that implements the ResponseListener
interface. The callback is carried out on behalf of the transport mapping
thread that received the response packet from the wire.
Thus, if the called method blocks, the delivery of synchronous and
asynchronous messages received on the listen port of that transport mapping
will be also blocked. Other transport mapping will not be affected.
Blocking can be avoided by either using synchronous messages only or by
decoupling the processing within the callback method.
Example for Sending a Synchronous Message
import org.snmp4j.*; ... Snmp snmp = new Snmp(new DefaultUdpTransportMapping()); snmp.listen(); ... ResponseEvent response = snmp.send(requestPDU, target); if (response.getResponse() == null) { // request timed out ... } else { System.out.println("Received response from: "+ response.getPeerAddress()); // dump response PDU System.out.println(response.getResponse().toString()); }
Example for Sending an Asynchronous Message
import org.snmp4j.*; import org.snmp4j.event.*; ... Snmp snmp = new Snmp(new DefaultUdpTransportMapping()); snmp.listen(); ... ResponseListener listener = new ResponseListener() { public void onResponse(ResponseEvent event) { // Always cancel async request when response has been received // otherwise a memory leak is created! Not canceling a request // immediately can be useful when sending a request to a broadcast // address. ((Snmp)event.getSource()).cancel(event.getRequest(), this); PDU response = event.getResponse(); PDU request = event.getRequest(); if (response == null) { System.out.println("Request "+request+" timed out"); } else { System.out.println("Received response "+response+" on request "+ request); } } }; snmp.sendPDU(request, target, null, listener); ...
Receiving SNMP messages
SNMP4J receives SNMP messages through the listen port of transport mappings.
In order to be able to receive responses or requests, that port needs to be
set into listen mode. This has to be done by calling the listen()
method of the TransportMapping
instance to start the transport
mappings internal listen thread. The internal thread is stopped and the listen
port is closed by calling the close()
method on the
TransportMapping
instance or the associated Snmp
instance.
The transport mapping just receives the SNMP mesage as a stream of bytes and
forwards the message to associated MessageDispatcher
instances.
By default, SNMP4J uses one instance of the MessageDispatcherImpl
class for decoding and dispatching incoming messages. That instance is created
and used internally by the Snmp
class.
The Snmp
class processes responses to outstanding requests and
forwards PDUs of other SNMP messages to registered CommandResponder
listener instances. To receive SNMP messages it is thus sufficient to
-
Create a
TransportMapping
and initialize its listen port by callingTransportMapping.listen()
. -
Create a
Snmp
instance with the aboveTransportMapping
. -
Instantiate a class that implements the
CommandResponder
interface and register it with theSnmp
instance by callingSnmp.addCommandResponder(CommandResponder)
.
When a unhandled SNMP message (thus a SNMP message where no corresponding
outstanding request exists) is received, then the
processPdu(CommandResponderEvent)
method of the
CommandResponder
will be called with the decoded PDU and
additional information about the received SNMP message provided by the
message processing model that has decoded the SNMP message.
Example for Receiving SNMP Messages
import org.snmp4j.*; import org.snmp4j.smi.*; import org.snmp4j.mp.SnmpConstants; ... TransportMapping transport = new DefaultUdpTransportMapping(new UdpAddress("0.0.0.0/161")); Snmp snmp = new Snmp(transport); if (version == SnmpConstants.version3) { byte[] localEngineID = ((MPv3)snmp.getMessageProcessingModel(MessageProcessingModel.MPv3)).createLocalEngineID(); USM usm = new USM(SecurityProtocols.getInstance(), new OctetString(localEngineID), 0); SecurityModels.getInstance().addSecurityModel(usm); snmp.setLocalEngine(localEngineID, 0, 0); // Add the configured user to the USM ... } snmp.addCommandResponder(this); snmp.listen(); ... public synchronized void processPdu(CommandResponderEvent e) { PDU command = e.getPdu(); if (command != null) { ... } }
-
ClassDescriptionAbstractTarget<A extends Address>A
AbstractTarget
class is an abstract representation of a remote SNMP entity.TheCertifiedIdentity
interface describes an identity that is associated with a client certificate fingerprint and a server certificate fingerprint.CertifiedTarget<A extends Address>TheCertifiedTarget
class implements aSecureTarget
for usage withSecurityModel
s that support secured connections using client and server certificates.CommandResponder
process incoming request, report and notification PDUs.CommandResponderEvent<A extends Address>TheCommandResponderEvent
is fired by theMessageDispatcher
to listeners that potentially can process the included request, report, or trap/notification.CommunityTarget<A extends Address>ACommunityTarget
represents SNMP target properties for community based message processing models (SNMPv1 and SNMPv2c).TheDefaultTimeoutModel
implements a timeout model that uses constant timeouts between retries.DirectUserTarget<A extends Address>User based target for SNMPv3 User Based Security ModelUSM
or later that includes any necessary authentication and privacy information, i.e.TheMessageDispatcher
interface defines common services of instances that process incoming SNMP messages and dispatch them to interestedCommandResponder
instances.TheMessageDispatcherImpl
decodes and dispatches incoming messages usingMessageProcessingModel
instances and encodes and sends outgoing messages using an appropriateTransportMapping
instances.TheMessageException
represents information about an exception occurred during message processing.TheMutablePDU
is a container for aPDU
instance.ThePDU
class represents a SNMP protocol data unit.ThePDUv1
represents SNMPv1 PDUs.TheScopedPDU
class represents a SNMPv3 scoped PDU.SecureTarget<A extends Address>TheSecureTarget
is an security model independent abstract class for all targets supporting secure SNMP communication.Session
defines a common interface for all classes that implement SNMP protocol operations based on SNMP4J.TheSnmp
class is the core of SNMP4J.Interface for handling reports.TheSNMP4JSettings
class implements a central configuration class of the SNMP4J framework.Specifies the how the security level of retry requests after a REPORT PDU is set.ATarget
interface defines an abstract representation of a remote SNMP entity.TheTimeoutModel
is the common interface for all models of timing out a SNMP request.TransportMapping<A extends Address>TheTransportMapping
defines the common interface for SNMP transport mappings.TheTransportStateReference
class holds information defined by RFC 5343 for the tmStateReference ASI elements.Common interface that has to be implemented by all user based security models user classes.UserTarget<A extends Address>User based target for SNMPv3 or later.