一、編寫MIB檔案

MIB檔案是用 ASN.1 語法來描述的，所以為了精確定義MIB中各管理物件，使用者不得不參考一些ASN.1語法的有關文件如RFC1155、RFC1212等等來定義裝置自己的MIB。ASN.1是抽象句法表示法一 （Abstract SyntaxNotation One） 的簡稱，對於每個管理物件它都用文字來描述，一般檔案的字尾名都用“.mib”,在net-snmp中字尾名是“.mib.txt”。

關於MIB檔案示例，可以見編譯安裝後的net-snmp目錄，一般是 /usr/local/net-snmp/share/snmp/mibs/。  

ls/usr/local/net-snmp/share/snmp/mibs/

AGENTX-MIB.txt                       IPV6-TCP-MIB.txt                     SNMP-NOTIFICATION-MIB.txt

...

IPV6-MIB.txt                         SNMP-MPD-MIB.txt                     UDP-MIB.txt

這裡我們建立一個mib檔案，命名為TEST-GET-MIB.txt，放在/usr/local/net-snmp/share/snmp/mibs/目錄下因為這個目錄是snmpd的預設目錄，只要把MIB庫放入該目錄就可以自動載入MIB庫，否則需要修改snmpd.conf檔案，自定義的MIB檔案如下：

--開始
TEST-GET-MIB DEFINITIONS ::= BEGIN

--引入部分
IMPORTS
    enterprises
        FROM RFC1155-SMI            
    Integer32,OBJECT-TYPE
        FROM SNMPv2-SMI            
    DisplayString
        FROM SNMPv2-TC
    TEXTUAL-CONVENTION
        FROM SNMPv2-TC; --引用結束，用分號


--定義節點
--enterprises的OID是1.3.6.1.4
testGet    OBJECT IDENTIFIER ::= { enterprises 77695 }

GetTime     OBJECT IDENTIFIER ::= { testGet   1}

GetTime OBJECT-TYPE       --物件名稱
SYNTAX DisplayString      --型別
MAX-ACCESS read-only      --訪問方式
STATUS current            --狀態
DESCRIPTION               --描述
"get current time"   
::= { testGet  1 }       --父節點

--結束定義
END 
 

寫完後我們測一個MIB庫有沒有問題，在linux機器上用snmptranslate-Tp -IR TEST-GET-MIB::testGet顯示結果如下：（這個測試不需要啟動snmpd程序）

[[email protected]]# ./snmptranslate -Tp -IR TEST-GET-MIB::testGet

+--testGet(77695)
   |
   +-- -R-- String    GetTime(1)
            Textual Convention: DisplayString
            Size: 0..255

OK,snmp自動發現了這個MIB庫， 有了自定義的OID，接下來開始新增處理程式。

二、生成原始碼

我們可以先來獲取一下前面定義的 testGet 節點的值試試。 因為 enterprises 的OID是 1.3.6.1.4 ，而 testGet是 enterprises 的葉子(77695)，而 GetTime 又是 testGet 的葉子節點(1)。所以其OID為 1.3.6.1.4.77695.1 。 
下面使用snmpget來測試一下（測試之前要先啟動snmpd程序）

[[email protected]]# ./snmpget -c public -v 2c localhost 1.3.6.1.4.1.77695.1.0

SNMPv2-SMI::enterprises.77695.1= No Such Object available on this agent at this OID  

結果是No Such Object available on this agent at this OID，我們需要用mib2c程式生成所需要的.c和.h檔案。

執行envMIBS="+/usr/local/net-snmp/share/snmp/mibs/TEST-GET-MIB.txt" ./mib2ctestGet，會引導你逐漸生成testGet.h和testGet.c,先選2再選1，過程如下：

[[email protected] bin]# env MIBS="+/usr/local/net-snmp/share/snmp/mibs/TEST-GET-MIB.txt" ./mib2c  testGet
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:                              testGet
  numeric translation:              .1.3.6.1.4.1.77695
  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 testGet")

  2) If you want to magically "tie" integer variables to integer
     scalars
     (by hand use: "mib2c -c mib2c.int_watch.conf testGet")

  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 testGet.h
writing to testGet.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 testGet.c
running indent on testGet.h

mib2c已經統計出我們的mib庫包含1個scalar變數，0個table變數，0個通知變數，Scalar就是包含我們常用的整型，字串，時間等等資料型別。table就是scalar的一種集合，有一個和多個列組成，類似於資料庫中的表。它必須具有索引項，用來按一定順序檢索表項，當然我們只寫了一個標量的OID，不是表結構也不是通知結構

生成的testGet.h如下

/*
 * Note: this file originally auto-generated by mib2c using
 *        $
 */
#ifndef TESTGET_H
#define TESTGET_H

/*
 * function declarations
 */
void            init_testGet(void);
Netsnmp_Node_Handler handle_GetTime;

#endif                          /* TESTGET_H */

生成的testGet.c檔案如下：

/*
 * 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 "testGet.h"

/** Initializes the testGet module */
void
init_testGet(void)
{
/*
 * 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 "testGet.h"

/** Initializes the testGet module */
void
init_testGet(void)
{
    const oid       GetTime_oid[] = { 1, 3, 6, 1, 4, 1, 77695, 1 };

    DEBUGMSGTL(("testGet", "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.
     */

    switch (reqinfo->mode) {

    case MODE_GET:
        snmp_set_var_typed_value(requests->requestvb, ASN_OCTET_STR,
                                 /*
                                  * XXX: a pointer to the scalar's data
                                  */ ,
                                 /*
                                  * XXX: the length of the data in bytes
                                  */ );
        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;
}
     

以上的程式碼都是自動生成的，我們沒有寫一行程式碼，到了這一步，我們需要把testGet.c裡面的XXX改成自己的值，也就兩行，修改後testGet.c檔案程式碼如下：

/*
 * 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 "testGet.h"

/** Initializes the testGet module */
void
init_testGet(void)
{
    const oid       GetTime_oid[] = { 1, 3, 6, 1, 4, 1, 77695, 1 };

    DEBUGMSGTL(("testGet", "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;
}

三、配置編譯

把testGet.c和testGet.h複製到/net-snmp-5.7.3/agent/mibgroups/路徑下

設定編譯引數（紅色部分即為加上我們自己的mib模組）

[[email protected]]# ./configure  --prefix=/usr/local/net-snmp   --with-mib-modules="testGet"

檢視檔案net-snmp-5.7.3/agent/mibgroup/mib_module_inits.h，發現已經新增到初始化中。

/*This file is automatically generated by configure.  Do not modify by hand. */

  if (should_init("testGet"))init_testGet();

編譯並安裝

[[email protected]]# make

[[email protected]]# make install

四、測試新加的MIB

啟動snmpd服務

[[email protected]]# ./snmpd -f -Le

Turning on AgentXmaster support.

NET-SNMP version5.7.3

No Shmem line in/proc/meminfo

我們再呼叫snmpget來測試結果：

[[email protected]]# ./snmpget  -v2c -c publiclocalhost  1.3.6.1.4.1.77695.1.0

SNMPv2-SMI::enterprises.77695.1.0= STRING: "Fri Nov  3 14:02:13 2017

"

顯示出了當前的時間，說明我們新增自定義MIB get操作成功！