Snmp4j编程简介之三：Snmp

Snmp4j编程简介之三：Snmp
Class Snmp

java.lang.Object
org.snmp4j.Snmp
All Implemented Interfaces:
java.util.EventListener, CommandResponder, Session
snmp类是SNMP4J的核心，它提供了发送和接收SNMP PDUs的方法，所有的SNMP PDU 类型都可以采用同步或者异步的方式被发送。

Snmp采用独立的传输协议，通过TransportMapping 接口调用addTransportMapping(TransportMapping transportMapping) 方法或者采用默认的构造函数来实现传输映射，以此来实现信息的传输。

下面的代码片段是采用UDP传输方式来实现一个SNMPv3的SNMP session ：

Java代码
Address targetAddress = GenericAddress.parse("udp:127.0.0.1/161");
TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport);
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(MPv3.createLocalEngineID()), 0);
SecurityModels.getInstance().addSecurityModel(usm);
transport.listen();

Address targetAddress = GenericAddress.parse("udp:127.0.0.1/161");
TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport);
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(MPv3.createLocalEngineID()), 0);
SecurityModels.getInstance().addSecurityModel(usm);
transport.listen();

如何实现SNMPv3信息的同步发送，下面举例说明：

Java代码
// add user to the USM
snmp.getUSM().addUser(new OctetString("MD5DES"),
new UsmUser(new OctetString("MD5DES"),
AuthMD5.ID,
new OctetString("MD5DESUserAuthPassword"),
PrivDES.ID,
new OctetString("MD5DESUserPrivPassword")));
// create the target
UserTarget target = new UserTarget();
target.setAddress(targetAddress);
target.setRetries(1);
target.setTimeout(5000);
target.setVersion(SnmpConstants.version3);
target.setSecurityLevel(SecurityLevel.AUTH_PRIV);
target.setSecurityName(new OctetString("MD5DES"));

// create the PDU
PDU pdu = new ScopedPDU();
pdu.add(new VariableBinding(new OID("1.3.6")));
pdu.setType(PDU.GETNEXT);

// send the PDU
ResponseEvent response = snmp.send(pdu, target);
// extract the response PDU (could be null if timed out)
PDU responsePDU = response.getResponse();
// extract the address used by the agent to send the response:
Address peerAddress = response.getPeerAddress();
An asynchronous SNMPv1 request is sent by the following code:

// setting up target
CommunityTarget target = new CommunityTarget();
target.setCommunity(new OctetString("public"));
target.setAddress(targetAddress);
target.setRetries(2);
target.setTimeout(1500);
target.setVersion(SnmpConstants.version1);
// creating PDU
PDU pdu = new PDU();
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,1})));
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,2})));
pdu.setType(PDU.GETNEXT);
// sending request
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);
System.out.println("Received response PDU is: "+event.getResponse());
}
};
snmp.sendPDU(pdu, target, null, listener);

//Traps (notifications) and other SNMP PDUs can be received by adding the folling code to the first code snippet above:
CommandResponder trapPrinter = new CommandResponder() {
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPDU();
if (command != null) {
System.out.println(command.toString());
}
}
};
snmp.addCommandResponder(trapPrinter);



Version:
1.8
Author:
Frank Fock
translate：avery_leo

// add user to the USM
snmp.getUSM().addUser(new OctetString("MD5DES"),
new UsmUser(new OctetString("MD5DES"),
AuthMD5.ID,
new OctetString("MD5DESUserAuthPassword"),
PrivDES.ID,
new OctetString("MD5DESUserPrivPassword")));
// create the target
UserTarget target = new UserTarget();
target.setAddress(targetAddress);
target.setRetries(1);
target.setTimeout(5000);
target.setVersion(SnmpConstants.version3);
target.setSecurityLevel(SecurityLevel.AUTH_PRIV);
target.setSecurityName(new OctetString("MD5DES"));

// create the PDU
PDU pdu = new ScopedPDU();
pdu.add(new VariableBinding(new OID("1.3.6")));
pdu.setType(PDU.GETNEXT);

// send the PDU
ResponseEvent response = snmp.send(pdu, target);
// extract the response PDU (could be null if timed out)
PDU responsePDU = response.getResponse();
// extract the address used by the agent to send the response:
Address peerAddress = response.getPeerAddress();
An asynchronous SNMPv1 request is sent by the following code:

// setting up target
CommunityTarget target = new CommunityTarget();
target.setCommunity(new OctetString("public"));
target.setAddress(targetAddress);
target.setRetries(2);
target.setTimeout(1500);
target.setVersion(SnmpConstants.version1);
// creating PDU
PDU pdu = new PDU();
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,1})));
pdu.add(new VariableBinding(new OID(new int[] {1,3,6,1,2,1,1,2})));
pdu.setType(PDU.GETNEXT);
// sending request
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);
System.out.println("Received response PDU is: "+event.getResponse());
}
};
snmp.sendPDU(pdu, target, null, listener);

//Traps (notifications) and other SNMP PDUs can be received by adding the folling code to the first code snippet above:
CommandResponder trapPrinter = new CommandResponder() {
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPDU();
if (command != null) {
System.out.println(command.toString());
}
}
};
snmp.addCommandResponder(trapPrinter);

Version:
1.8
Author:
Frank Fock
translate：avery_leo

Snmp类提供了一套有关Snmp的功能接口。具体来讲，就是发送、接受、创建Snmp消息。
一个Snmp对象是一个Session，而在Snmp4j中，一个Session可以同多个远程设备通信。

（1） Snmp、Target、PDU三者的关系
Target代表远程设备或者远程实体、PDU代表管理端同Target通信的数据，Snmp就代表管理者管理功能（其实就是数据的收发）的具体执行者。
打个比方：Target就是你远方的恋人，PDU就是你们之间传递的情书、而Snmp就是负责帮你寄信收信的邮差。

（2）Snmp收发数据的两种方式
Snmp可以同步、也可异步收发数据。详细见代码示例说明。

（3）Snmp与传输层协议
Snmp可以定制传输层协议，一般选择udp，也可以选择tcp。详细见代码示例说明。

（4）Snmp与Usm
创建Snmp用来发送Snmpv3版本的消息时候，一般还要创建USM，将它添加至安全模型管理器（SecriryModels）中，同时还需要向Usm中添加相应的USM用户（UsmUser）。详细见代码示例说明。

代码示例：（摘自Snmp4j的API文档）

（1）创建Snmp
1）使用UDP传输协议

TransportMapping transport = new DefaultUdpTransportMapping();
snmp = new Snmp(transport); 2）使用TCP传输协议

TransportMapping transport = new DefaultTcpTransportMapping();
snmp = new Snmp(transport); 3）创建用于Snmpv3的Snmp

// 创建Snmp
TransportMapping transport =
new DefaultUdpTransportMapping();
Snmp snmp = new Snmp(transport);
if (version == SnmpConstants.version3) {
byte[] localEngineID =
((MPv3)snmp.getMessageProcessingModel(MessageProcessingModel.MPv3)).createLocalEngineID();
// 创建USM
USM usm = new USM(SecurityProtocols.getInstance(),
new OctetString(localEngineID), 0);
// 将USM添加至安全模式管理器中
// 安全模型管理器采用了单例模式，它内部可以维护为3个安全模型，分别对应Snmp三个版本
SecurityModels.getInstance().addSecurityModel(usm);
snmp.setLocalEngine(localEngineID, 0, 0);
// 添加用户
snmp.getUSM().addUser(securityName，new UsmUser(securityName,authProtocol,
authPassphrase,privProtocol,privPassphrase));
}（2）同步收发消息

import org.snmp4j.*;
...
Snmp snmp = new Snmp(new DefaultUdpTransportMapping());
...
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());
}（3）异步收发消息

import org.snmp4j.*;
import org.snmp4j.event.*;
...
Snmp snmp = new Snmp(new DefaultUdpTransportMapping());
...
// 增加监听器
ResponseListener listener = new ResponseListener() {
public void onResponse(ResponseEvent event) {
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);
...（4）实现trap
实现trap需要三步：
1）创建Snmp；
2）对于listen（）使处于网络监听（实际上是同于网络编程中的Socket监听）；
3）实现CommandResponder接口的监听器，并且调用Snmp.addCommandResponder(CommandResponder)注册监听器。

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
...
}
// 注册命令响应监听器
snmp.addCommandResponder(this);
transport.listen();
...
// 实现CommandResponder接口
public synchronized void processPdu(CommandResponderEvent e) {
PDU command = e.getPdu();
if (command != null) {
...
}
}
总结
Snmp内含了一个消息分发器，消息分发器中内含了处理网络的线程，在使用完后最好调用close（），将其资源回归处理。
掌握了上面所说的三个概念，基本上可以使用Snmp4j编写Snmp的程序了。
有关Snmp4j编程最好也最详细的资料：API文档和源代码。关于使用Snmp4j编写Snmp程序的例子，多线程的例子可以参看源代码中：org.snmp4j.test包下的MultiThreadedTrapReceiver.java，完整的例子可以参看 org.snmp4j.tools.console包下的SnmpRequest（一个命令行的Snmp管理器）。
不过，要想快速和深入掌握Snmp编程，最好的办法一定是先弄懂Snmp协议，这方面的资料最权威的就是RFC协议了。