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windows下使用net-snmp实现agent扩展(二)

刚刚实现了int型的get命令,可能更多的情况下,我们更需要的是字符串类型的。在实现int型的时候,用到了netsnmp_register_int_instance这个函数,很自然想到如果是string型的,用类似的netsnmp_register_string_instance,或者netsnmp_register_char_instance不就行了?很可惜的是:net-snmp并没有提供这两个函数。通过查找,在net-snmp5.7.1/agent/mibgroup/examples下面,有个watched.c文件,这里提供了对字符串类型的操作。实际上,net-snmp将string型归到了scalar类型里,做了统一的处理。好,看代码:

watched.h文件:

#ifndef EXAMPLES_WATCHED_H
#define EXAMPLES_WATCHED_H

#ifdef __cplusplus
extern "C" {
#endif

void init_watched(void);

#ifdef __cplusplus
}
#endif

#endif /* EXAMPLES_WATCHED_H */

watched.c文件:

/*
 * start by including the appropriate header files
 */
#include <net-snmp/net-snmp-config.h>
#include <net-snmp/net-snmp-includes.h>
#include <net-snmp/agent/net-snmp-agent-includes.h>

void init_watched_string(void);

void init_watched(void)
{
    init_watched_string();
}

void init_watched_string(void)
{
    /*
     * the storage for our string. It must be static or allocated.
     * we use static here for simplicity.
     */
    static char my_string[256] = "welcome to vcsky.net!";

    /*
     * the OID we want to register our string at.  This should be a
     * fully qualified instance.  In our case, it's a scalar at:
     * NET-SNMP-EXAMPLES-MIB::netSnmpExampleString.0  (note the trailing
     *  0 which is required for any instantiation of any scalar object)
     */
    oid             my_registration_oid[] =
        { 1, 3, 6, 1, 4, 1, 8072, 2, 1, 3, 0 };

    /*
     * variables needed for registration
     */
    <a name="_a0"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__handler__registration__s.html">netsnmp_handler_registration</a> *reginfo;
    static <a name="_a1"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__watcher__info__s.html">netsnmp_watcher_info</a> watcher_info;
    int watcher_flags;

    /*
     * a debugging statement.  Run the agent with -Dexample_string_instance
     * to see the output of this debugging statement.
     */
    DEBUGMSGTL(("example_string_instance",
                "Initalizing example string instance.  Default value = %s\n",
                my_string));

    /*
     * If we wanted a callback when the value was retrieved or set
     * (even though the details of doing this are handled for you),
     * you could change the NULL pointer below to a valid handler
     * function.
     *
     * Change RWRITE to RONLY for a read-only string.
     */
    reginfo = <a name="a2"></a><a href="http://www.net-snmp.org/dev/agent/group__handler.html#ga537fac61fff7e112e121e5f629eded65">netsnmp_create_handler_registration</a>("my example string", NULL,
                                                  my_registration_oid,
                                                  OID_LENGTH(my_registration_oid),
                                                  HANDLER_CAN_RWRITE);

    /*
     * the three options for a string watcher are:
     *   fixed size string (length never changes)
     *   variable size (length can be 0 - MAX, for some MAX)
     *   c string (length can be 0 - MAX-1 for some max, \0 is not a valid
     *     character in the string, the length is provided by strlen)
     *
     * we'll use a variable length string.
     */
     watcher_flags =WATCHER_MAX_SIZE;
    /*
     * create the watcher info for our string.
     */
    netsnmp_init_watcher_info6(&amp;watcher_info, my_string, strlen(my_string),
                               ASN_OCTET_STR, watcher_flags,
                               sizeof(my_string), NULL);

    /*
     * the line below registers our "my_string" variable above as
     * accessible and makes it writable.
     */
    <a name="a4"></a><a href="http://www.net-snmp.org/dev/agent/group__watcher.html#ga32250edcf6f39dd7376a20699e1c7cd0">netsnmp_register_watched_instance</a>(reginfo, &amp;watcher_info);

    DEBUGMSGTL(("example_string_instance",
                "Done initalizing example string instance\n"));
}

编译运行(注意在入口函数中将init_nstAgentSubagentObject替换为init_watched)。
打开cmd,测试一下:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0
返回结果:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “welcome to vcsky.net!”
snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “havenzhao”
返回结果:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “havenzhao”
OK,没问题! 接下来,我们想改变my_string这个变量的值,这个值需要从程序的其它地方获得,这样才能起到监测的作用。先简单改变一下试试看,能否正确运行: 修改下watched.c文件,添加一行为my_string改变值的语句,标红的代码即是。

#include &lt;net-snmp/net-snmp-config.h&gt;
#include &lt;net-snmp/net-snmp-includes.h&gt;
#include &lt;net-snmp/agent/net-snmp-agent-includes.h&gt;

static char my_string[256] = “welcome to vcsky.net!”; //作为全局变量

/*change_string_value()模拟在其它代码中改变要监测的my_string值的函数,此函数可以放在定时器中,或者其它能够动态改变my_string值的代码段里,以保证获取的my_string值是变化的。别忘了在头文件watched.h里声明*/

void change_string_value()
{
    strcpy(my_string, "haven zhao");
}

void init_watched_string(void);

void init_watched(void)
{
    init_watched_string();
}

void init_watched_string(void)
{
    oid             my_registration_oid[] =
        { 1, 3, 6, 1, 4, 1, 8072, 2, 1, 3, 0 };

    <a name="_a0"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__handler__registration__s.html">netsnmp_handler_registration</a> *reginfo;
    static <a name="_a1"></a><a href="http://www.net-snmp.org/dev/agent/structnetsnmp__watcher__info__s.html">netsnmp_watcher_info</a> watcher_info;
    int watcher_flags;

    DEBUGMSGTL(("example_string_instance",
                "Initalizing example string instance.  Default value = %s\n",
                my_string));

    reginfo = <a name="a2"></a><a href="http://www.net-snmp.org/dev/agent/group__handler.html#ga537fac61fff7e112e121e5f629eded65">netsnmp_create_handler_registration</a>("my example string", NULL,
                                                  my_registration_oid,
                                                  OID_LENGTH(my_registration_oid),
                                                  HANDLER_CAN_RWRITE);

watcher_flags = WATCHER_MAX_SIZE;

netsnmp_init_watcher_info6(&watcher_info, my_string, strlen(my_string),
ASN_OCTET_STR, watcher_flags,
sizeof(my_string), NULL);

netsnmp_register_watched_instance(reginfo, &watcher_info);

DEBUGMSGTL((“example_string_instance”,
“Done initalizing example string instance\n”));
}
为了方便,我们把change_string_value()函数放在入口函数文件(example-demon.c)的死循环里: while(keep_running) {

/* if you use select(), see snmp_select_info() in snmp_api(3) */
/*           — OR —        */
  change_string_value(); //放在这里了
agent_check_and_process(1); /* 0 == don’t block */
}

再次编译运行,看看有没有达到效果:

输入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = Hex-STRING: 68 61 76 65 6E 20 7A 68 61 6F 00 76

出现乱码,没有成功!

再试试set命令:snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “haven”

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven”

可见set成功!try again,再输入get命令:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven”

这次却成功了。

难道要先set,再get,才行?什么情况?再试一次!

输入:snmpset -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0 s “vcsky.net haven zhao”

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “vcsky.net haven zhao”

输入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = Hex-STRING: 68 61 76 65 6E 20 7A 68 61 6F 00 61

照样失败!

经过反复测试,好像和字符串的长度有关系。改变后的字符串与初始化时的字符串长度不一致时,总是达不到我们想要的效果。是的,问题就出在这里!

解决方案:

看这行代码:watcher_flags = WATCHER_MAX_SIZE;

就在这个小小的参数上,例子中的WATCHER_MAX_SIZE:variable size (length can be 0 – MAX, for some MAX),If set then the variable data_size_p points to is supposed to hold the current size of the watched object and will be updated on writes.

而我们则需要这个:c string (length can be 0 – MAX-1 for some max, \0 is not a valid  character in the string, the length is provided by strlen),也就是这个:WATCHER_SIZE_STRLEN

好,马上替换watcher_flags = WATCHER_MAX_SIZE;为watcher_flags = WATCHER_SIZE_STRLEN;

编译运行,测试:

输入:snmpget -v1 -c public localhost 1.3.6.1.4.1.8072.2.1.3.0

返回:NET-SNMP-MIB::netSnmp.2.1.3.0 = STRING: “haven zhao”

OK!到此为止,我们终于前进了一步,由get/set一个int型变量,到get/set一个字符串型变量,而且这个字符串变量,可由外部的代码对其进行设置,从而实现了监测变化的字符串变量的目的。

这只是单个的变量,可能要监测的变量不只一个,那怎么办呢?未完待续:http://vcsky.net

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