使用net-snmp做自己的agent

参见http://net-snmp.sourceforge.net/tutorial/tutorial-4/agent/04-basic-code.html

1. 注册mib

REGISTER_MIB( "example", example_variables, variable2,example_variables_oid );
参数分别为：

"example" : 注册的mib module的名称

example_variables: 相关的变量（之后详述）

variable2: 是example_variables的类型

example_variables_oid ： 这个module的root OID

2. 设置 相关变量

struct variable2 example_variables[] = {
{ EXAMPLESTRING, ASN_OCTET_STR, RONLY, var_example, 1, {1}}

};
各个分量的注释：

a magic number (the #defined integer constant described earlier) 这个magic就是用在routine里面，switch (vp->magic)用的

a type indicator (from the values listed in <snmplib/snmp_impl.h>)

an access indicator (essentially

RWRITE

or

RONLY

)

the name of the routine used to handle this entry，真正处理请求的地方

the length of the OID suffix used, and

an array of integers specifying this suffix (more on this in a moment) 表示sufix是什么

像上面这个例子， {1}, 就表示了 这个的oid就是 example_variables_oid，1

3. routine 处理请求的地方

unsigned char *
var_example(struct variable *vp,
oid *name,
size_t *length,
int exact,
size_t *var_len,
WriteMethod **set_method)

Four of these parameters are used for passing in information about the request, these being: 四个参数是传进来的

struct variable *vp;
// The entry in the [code]variable

N array from the
// header file, for the object under consideration.
// Note that the

name

field of this structure has been
// completed into a fully qualified OID, by prepending
// the prefix common to the whole array.
oid *name; // The OID from the request
int *length; // The length of this OID
int exact; // A flag to indicate whether this is an exact
// request (GET/SET) or an 'inexact' one (GETNEXT)
[/code]
Four of the parameters are used to return information about the answer. The function also returns a pointer to the actual data for the variable requested (or NULL if this data is not available for any reason). The other result parameters are: 四个参数是传出去的

oid *name;	// The OID being returned
int *length;	// The length of this OID
int *var_len;	// The length of the answer being returned
WriteMethod **write_method;
// A pointer to the SET function for this variable

这个函数进入后，先要判断这次的oid是否在范围之内。如果是get-next就要对oid做相应的处理。

vp->magic表示了实际要获取的位置。 最后根据这个magic值返回response

例如

switch(vp->magic) {

case IFNAME:

if ( strlen(ifXEntry.ifName) > IFNAME_DISPLAY_STRING_LEN)
*var_len = IFNAME_DISPLAY_STRING_LEN;
else
*var_len = strlen(ifXEntry.ifName);
return (unsigned char *) ifXEntry.ifName;
break;
default:
ERROR_MSG("");
}