net-snmp agent开发（用net-snmp扩展MIB库）
注：本文经过http://blog.csdn.net/hepeng597/article/details/8782868原文以及网上资料进行归纳整理
用net-snmp扩展MIB库，实现方法可归结为四种：
1）一是静态库方式，通过修改配置头文件，在相应地方包含新引入的mib模块的.c和.h文件，然后重新编译库文件和扩展代码；这种方式不够灵活，每次修改扩展的MIB后，都需要重新编译snmpd和扩展的代码，再重新安装snmpd到系统中。
2）二是编译动态共享库，只需把新引入的MIB模块的.c和.h文件编译成动态库，通过设置能够让代理程序载入。
对于第二种方式，一需要编译成.so动态共享库，二需要原代理程序是否包含dlmod或load命令，三还要看系统是否支持。一般情况下仅支持Unix平台。
3）三是扩展一个子代理，让SNMPD以主代理的模式运行，对于SNMPD我们只要让它启动就可以，不需要任何的更改和配置，把子代理编译生成的程序运行起来就可以扩展自定义的MIB库。
4）用shell脚本来扩展
本文我们以第一种、第二种和第三种方法在linux上开发和测试：
1.环境搭建
对于安装snmpd过程本文暂不做详解网上资料很多，本文所述开发与测试基于net-snmp-5.7.3版本
2.建立自己的MIB库
首先MIB库有什么用？其实作为子代理来说，在server机器上，可以不用MIB库，MIB库只为了让用户访问时方便，有了MIB库，用户就不用记那么多和长的OID，比如把MIB放在windows机器上，在windows机器装一个支持MIB的软件，用该软件打开MIB库，只要点击相应的对象就可以自动发送snmp请求到server端，所以server端是可以不要MIB库的。如果把MIB库放在linux客户端机器上，以下面自定义的MIB库为例，那么就可以直接执行snmpget -v2c -c public Test-MIB::GetTime.0，当然需要linux客户端装有snmp，而且自定义的MIB库必须能让snmpd程序找到。
这里用就一个OID建一个MIB库来简化，命名Test-MIB.my,放在/usr/local/share/snmp/mibs目录下，因为这个目录是snmpd的默认目录，只要把MIB库放入该目录就可以自动加载MIB库，否则需要修改/etc/snmp/snmp.conf文件，添加mibs +/path/to/Test-MIB.my 并重启snmpd。
自定义MIB库，如下：
-- Test-MIB.my
Test-MIB DEFINITIONS ::= BEGIN
IMPORTS
OBJECT-GROUP, MODULE-COMPLIANCE, NOTIFICATION-GROUP
FROM SNMPv2-CONF
enterprises, Integer32, Unsigned32, OBJECT-TYPE, MODULE-IDENTITY,
NOTIFICATION-TYPE
FROM SNMPv2-SMI
DisplayString
FROM SNMPv2-TC;
-- October 09, 2002 at 14:50 GMT
-- 1.3.6.1.4.1.16535
Test MODULE-IDENTITY
LAST-UPDATED "200210091450Z" -- October 09, 2002 at 14:50 GMT
ORGANIZATION
""
CONTACT-INFO
""
DESCRIPTION
"Video's Server MIB."
::= { enterprises 16535 }
-- Node definitions
-- This part will include all details about the Test.
-- 1.3.6.1.4.1.16535.1
Time OBJECT IDENTIFIER ::= { Test 1 }
-- 1.3.6.1.4.1.16535.1.1
GetTime OBJECT-TYPE
SYNTAX DisplayString (SIZE (0..100))
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"Example : 2013/4/11"
::= { Time 1 }
END
-- Test-MIB.my该MIB库只有一个OID，即：1.3.6.1.4.1.16535.1.1，写完后我们测一个MIB库有没有问题，在linux机器上用snmptranslate -Tp -IR Test-MIB::Test显示结果如下：
+--Test(16535)
|
+--Time(1)
|
+-- -R-- String GetTime(1)
Textual Convention: DisplayString
Size: 0..100
mib2c可以根据mib库生成对应的源代码，有多种模板，这里我们要生成子代理的代码，所以选择是固定的，执行env MIBS="+/usr/local/share/snmp/mibs/Test-MIB.my" mib2c Test，会引导你逐渐生成Test.h和Test.c, 先选2再选1，过程如下：
[root@localhost mibs]# env MIBS="+/etc/snmp/mibs/Test-MIB.my" mib2c Test
writing to -
mib2c has multiple configuration files depending on the type of
code you need to write. You must pick one depending on your need.
You requested mib2c to be run on the following part of the MIB tree:
OID: Test
numeric translation: .1.3.6.1.4.1.16535
number of scalars within: 1
number of tables within: 0
number of notifications within: 0
First, do you want to generate code that is compatible with the
ucd-snmp 4.X line of code, or code for the newer Net-SNMP 5.X code
base (which provides a much greater choice of APIs to pick from):
1) ucd-snmp style code
2) Net-SNMP style code
Select your choice : 2
**********************************************************************
GENERATING CODE FOR SCALAR OBJECTS:
**********************************************************************
It looks like you have some scalars in the mib you requested, so I
will now generate code for them if you wish. You have two choices
for scalar API styles currently. Pick between them, or choose not
to generate any code for the scalars:
1) If you're writing code for some generic scalars
(by hand use: "mib2c -c mib2c.scalar.conf Test")
2) If you want to magically "tie" integer variables to integer
scalars
(by hand use: "mib2c -c mib2c.int_watch.conf Test")
3) Don't generate any code for the scalars
Select your choice: 1
using the mib2c.scalar.conf configuration file to generate your code.
writing to Test.h
writing to Test.c
**********************************************************************
* NOTE WELL: The code generated by mib2c is only a template. *YOU* *
* must fill in the code before it'll work most of the time. In many *
* cases, spots that MUST be edited within the files are marked with *
* /* XXX */ or /* TODO */ comments. *
**********************************************************************
running indent on Test.h
running indent on Test.c以上的代码都是自动生成的，我们没有写一行代码，到了这一步，我们需要把Test.c里面的 XXX改成自己的值，也就两行，修改后Test.c文件代码如下：
/*
* Note: this file originally auto-generated by mib2c using
* $
*/
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/agent/net-snmp-agent-includes.h>
#include "Test.h"
#include <time.h>
/** Initializes the Test module */
void
init_Test(void)
{
const oid GetTime_oid[] = { 1, 3, 6, 1, 4, 1, 16535, 1, 1 };
DEBUGMSGTL(("Test", "Initializing\n"));
netsnmp_register_scalar(netsnmp_create_handler_registration
("GetTime", handle_GetTime, GetTime_oid,
OID_LENGTH(GetTime_oid), HANDLER_CAN_RONLY));
}
int
handle_GetTime(netsnmp_mib_handler *handler,
netsnmp_handler_registration *reginfo,
netsnmp_agent_request_info *reqinfo,
netsnmp_request_info *requests)
{
/*
* We are never called for a GETNEXT if it's registered as a
* "instance", as it's "magically" handled for us.
*/
/*
* a instance handler also only hands us one request at a time, so
* we don't need to loop over a list of requests; we'll only get one.
*/
time_t t;
switch (reqinfo->mode) {
case MODE_GET:
time(&t);
char szTime[100];
snprintf(szTime,100,"%s",ctime(&t));
snmp_set_var_typed_value(requests->requestvb, ASN_OCTET_STR,
/*
* XXX: a pointer to the scalar's data
*/ szTime,
/*
* XXX: the length of the data in bytes
*/ strlen(szTime));
break;
default:
/*
* we should never get here, so this is a really bad error
*/
snmp_log(LOG_ERR, "unknown mode (%d) in handle_GetTime\n",
reqinfo->mode);
return SNMP_ERR_GENERR;
}
return SNMP_ERR_NOERROR;
}3 静态库方法：
（1）将.c 和.h文件移动到源码目录下，我的是/opt/net-snmp-5.7.3/agent/mibgroup。
（2）在/opt/net-snmp-5.7.3下运行的configure命令，并且添加参数—with-mib-modules=Test，进行MIB库文件的配置。
（3）在/opt/net-snmp-5.7.3下运行make&makeinstall编译安装
（4）执行命令snmpget -v2c -c public localhost Test-MIB:GetTime.0测试
结果：Test-MIB::GetTime.0 = STRING: Mon Jun 6 21:17:54 2016
4 生成共享库的方法：
(1) 在Test.c增加main函数，修改后如下：
/*
* Note: this file originally auto-generated by mib2c using
* $
*/
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/agent/net-snmp-agent-includes.h>
#include "Test.h"
#include <time.h>
/** Initializes the Test module */
void
init_Test(void)
{
const oid GetTime_oid[] = { 1, 3, 6, 1, 4, 1, 16535, 1, 1 };
DEBUGMSGTL(("Test", "Initializing\n"));
netsnmp_register_scalar(netsnmp_create_handler_registration
("GetTime", handle_GetTime, GetTime_oid,
OID_LENGTH(GetTime_oid), HANDLER_CAN_RONLY));
}
int
handle_GetTime(netsnmp_mib_handler *handler,
netsnmp_handler_registration *reginfo,
netsnmp_agent_request_info *reqinfo,
netsnmp_request_info *requests)
{
/*
* We are never called for a GETNEXT if it's registered as a
* "instance", as it's "magically" handled for us.
*/
/*
* a instance handler also only hands us one request at a time, so
* we don't need to loop over a list of requests; we'll only get one.
*/
time_t t;
switch (reqinfo->mode) {
case MODE_GET:
time(&t);
char szTime[100];
snprintf(szTime,100,"%s",ctime(&t));
snmp_set_var_typed_value(requests->requestvb, ASN_OCTET_STR,
/*
* XXX: a pointer to the scalar's data
*/ szTime,
/*
* XXX: the length of the data in bytes
*/ strlen(szTime));
break;
default:
/*
* we should never get here, so this is a really bad error
*/
snmp_log(LOG_ERR, "unknown mode (%d) in handle_GetTime\n",
reqinfo->mode);
return SNMP_ERR_GENERR;
}
return SNMP_ERR_NOERROR;
}
static int keep_running;
RETSIGTYPE stop_server(int __attribute__((unused)) a) {
keep_running = 0;
}
int main()
{
const char *app_name = "Test";
/* we are a subagent */
netsnmp_ds_set_boolean(NETSNMP_DS_APPLICATION_ID, NETSNMP_DS_AGENT_ROLE, 1);
/* initialize the agent library */
init_agent(app_name);
/* initialize your mib code here */
init_Test();
/* Test will be used to read Test.conf files. */
init_snmp("Test");
keep_running = 1;
while(keep_running)
{
agent_check_and_process(1);/* block every 1 second */
}
/* at shutdown time */
snmp_shutdown(app_name);
/* deinitialize your mib code here */
/* shutdown the agent library */
shutdown_agent();
return 0;
}（2）编译和部署基于root用户，具体编译命令为：
gcc -I `net-snmp-config --cflags` -fPIC -shared -g -O0 -o Test.so Test.c `net-snmp-config --libs`
（3）修改/etc/snmp/snmpd.conf，添加一行：
dlmod Test /home/net-snmp/sbin/Test.so
其中/home/net-snmp/sbin/test.so为test.so的具体路径，测试完成后可以注掉这行即可
并添加：
view systemview included .1.3.6.1.4.1.16535.1.1
这样我们自定义的OID才可以访问，接下来执行：
service snmpd start
若snmpd已启动则执行restart，服务正确启动后在本机输入：
snmpget -v2c -c public localhost .1.3.6.1.4.1.16535.1.1
输出结果应为：
SNMPv2-SMI::enterprises.16535.1.1.0 = STRING: "Mon Jun 6 20:56:13 2016
"
5 扩展一个子代理的方法：
（1）执行命令让我们的子代理生成可执行程序，
net-snmp-config --compile-subagent Test Test.c，生成了Test程序
（2）现在Test程序已经生成了，我们先执行主代理（service snmpd start）,再执行子代理./Test,再ps -ef | grep Test，看一下，可以看到Test程序自动在后台启动了，本文在一台装有snmpd和子代理的linux server机器上直接测试。执行命令如下：
snmpget -v2c -c public localhost 1.3.6.1.4.1.16535.1.1.0
运行结果：SNMPv2-SMI::enterprises.16535.1.1.0 = STRING: "Mon Jun 6 21:07:06 2016
"
可以看到，我们开发的子代理成功工作了。如果自定义的MIB库已经加入到snmpd指定的目录中，我们可以执行
snmpget -v2c -c public localhost Test-MIB:GetTime.0
结果：Test-MIB::GetTime.0 = STRING: Mon Jun 6 21:08:24 2016
snmpget会自动在所有的MIB库中查找Test-MIB库，并把Test-MIB:GetTime.0转换成1.3.6.1.4.1.16535.1.1.0并发送get请求。
现在子代理是已经开发成功了，我们实际上只写了两三行代码就开发了net-snmp子代理，是不是很简单呢？
现在有个问题，怎么加入到我们自己的项目中呢？因为Test.c中并没有main函数，在一个项目中直接调用init_Test()能不能让子代理work呢？那就要研究一下netsnmptmp.12373.c。
（3）我们再次执行net-snmp-config --compile-subagent Test Test.c，然后立刻Ctrl+c,时间要控制好，让net-snmp-config程序产生了临时的C文件，却没有删除它。打开netsnmptmp.12373.c，我们来看一下代码，200多行，我们不管上面代码那些看不懂的函数，知道大概意思就行，现在我们来加入到自己的项目中，找到项目中的main函数，在main函数中添加初始化Test子代理的代码，本文为了方便理解，就在Test.c中添加main函数，就地取材，写个超简单函数，不考虑传入参数。修改后的Test.c文件如下：
/*
* Note: this file originally auto-generated by mib2c using
* $
*/
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/agent/net-snmp-agent-includes.h>
#include "Test.h"
#include <time.h>
/** Initializes the Test module */
void
init_Test(void)
{
const oid GetTime_oid[] = { 1, 3, 6, 1, 4, 1, 16535, 1, 1 };
DEBUGMSGTL(("Test", "Initializing\n"));
netsnmp_register_scalar(netsnmp_create_handler_registration
("GetTime", handle_GetTime, GetTime_oid,
OID_LENGTH(GetTime_oid), HANDLER_CAN_RONLY));
}
int
handle_GetTime(netsnmp_mib_handler *handler,
netsnmp_handler_registration *reginfo,
netsnmp_agent_request_info *reqinfo,
netsnmp_request_info *requests)
{
/*
* We are never called for a GETNEXT if it's registered as a
* "instance", as it's "magically" handled for us.
*/
/*
* a instance handler also only hands us one request at a time, so
* we don't need to loop over a list of requests; we'll only get one.
*/
time_t t;
switch (reqinfo->mode) {
case MODE_GET:
time(&t);
char szTime[100];
snprintf(szTime,100,"%s",ctime(&t));
snmp_set_var_typed_value(requests->requestvb, ASN_OCTET_STR,
/*
* XXX: a pointer to the scalar's data
*/ szTime,
/*
* XXX: the length of the data in bytes
*/ strlen(szTime));
break;
default:
/*
* we should never get here, so this is a really bad error
*/
snmp_log(LOG_ERR, "unknown mode (%d) in handle_GetTime\n",
reqinfo->mode);
return SNMP_ERR_GENERR;
}
return SNMP_ERR_NOERROR;
}
static int keep_running;
RETSIGTYPE stop_server(int __attribute__((unused)) a) {
keep_running = 0;
}
int main()
{
const char *app_name = "Test";
/* we are a subagent */
netsnmp_ds_set_boolean(NETSNMP_DS_APPLICATION_ID, NETSNMP_DS_AGENT_ROLE, 1);
/* initialize the agent library */
init_agent(app_name);
/* initialize your mib code here */
init_Test();
/* Test will be used to read Test.conf files. */
init_snmp("Test");
keep_running = 1;
while(keep_running)
{
agent_check_and_process(1);/* block every 1 second */
}
/* at shutdown time */
snmp_shutdown(app_name);
/* deinitialize your mib code here */
/* shutdown the agent library */
shutdown_agent();
return 0;
}当snmpd stop的时候会调用stop_server,也就会注销我们的子代理。编译一下，这里我们不用net-snmp-config编译，因为是要加入到自己的项目中，所以推荐写入到Makefile中，本文就不写Makefile了，直接调用gcc命令生成（直接用net-snmp-config的参数就可以），如下：
[root@localhost hepeng]# gcc -fno-strict-aliasing -g -O2 -Ulinux -Dlinux=linux -D_REENTRANT -D_GNU_SOURCE -fno-strict-aliasing -pipe -Wdeclaration-after-statement -I/usr/local/include -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64 -I/usr/include/gdbm -I/usr/lib64/perl5/5.8.8/x86_64-linux-thread-multi/CORE -I. -I/usr/local/include -o Test Test.c -L/usr/local/lib -lnetsnmpmibs -lnetsnmpagent -lnetsnmp -lnetsnmpmibs -lpci -lrpm -lrpmio -lnetsnmpagent -Wl,-E -Wl,-rpath,/usr/lib64/perl5/5.8.8/x86_64-linux-thread-multi/CORE -lnetsnmp -lcrypto
Test.c: In function ‘handle_GetTime’:
Test.c:45: warning: ISO C90 forbids mixed declarations and code[root@video6 ~]# ./Test
NET-SNMP version 5.7.2 AgentX subagent connected我们再调用snmpget来测试结果：
[root@localhost hepeng]# snmpget -v2c -c public localhost 1.3.6.1.4.1.16535.1.1.0
SNMPv2-SMI::enterprises.16535.1.1.0 = STRING: "Mon Jun 6 21:07:06 2016
"