loading snmp object name to OID mappings

[Martin Janzen]

> Cosmo, Patrick wrote:
>> I have an application that allows administrators to configure snmp 
>> object name=value parameters. The application itself must be able to 
>> lookup the OID for the objectname in order to set a value on the 
>> device. So, somehow, we need to get the OID to object name mappings 
>> into the snmp manager.

Frank Fock wrote:
 > SNMP++ does not contain any MIB compiler code.
 > Thus, if you need to parse MIBs at runtime, you
 > will have to use libSMI, which is a free C library
 > to access MIB information. If it is OK, to compile
 > your MIBs at runtime, then you could use AgenPro
 > to create appropriate C++ objects or data structures.
 > AgenPro is fully customizable by code generation
 > templates. See http://www.agentpp.com for more
 > details.

The approach I use is to use NET_SNMP (formerly UCD_SNMP) to parse the 
MIBs.  Yeah, it's kind of silly to link in both libraries, but the 
combination does the job pretty painlessly; see example below.

-- 
Martin Janzen
janzen at pixelmetrix dot com




// NET-SNMP includes, used only for parsing MIBs.
// These are all C functions, so hide them in a NetSnmp namespace.
//
// Some NET-SNMP header files do bad things with timeval, so we
// limit the damage by renaming it.
namespace NetSnmp
{
#define timeval net_snmp_timeval
#include "ucd-snmp/ucd-snmp-config.h"
#include "ucd-snmp/asn1.h"
#include "ucd-snmp/mib.h"
#include "ucd-snmp/parse.h"
#undef timeval

// Last error detected by NET-SNMP.
extern "C" { extern int snmp_errno; }
}

[...]

void
MyAgent::readMIBs()
{
     // Use the NET-SNMP library to translate symbolic OIDs, as described
     // in the variables(5) man page.

     // The NET-SNMP library will search MIB directories in the order
     // in which they appear in the MIBDIRS environment variable.
     // Later filenames override earlier ones, so add default
     // directories first, then more specific ones.
     //
     // The initial "+" tells the library to read these directories in
     // addition to those containing the standard MIB definitions
     // (eg. SNMPv2-MIB, TCP-MIB, etc.).
     string mibdirs("+");
     for ( /* each of your MIB directories */ )
         mibdirs += ... ;
     setenv("MIBDIRS", mibdirs.c_str(), 1);

     // Now build a list of your MIB module names, and add each name
     // to the MIBS environment variable, to tell the NET-SNMP library
     // that this module should be loaded on startup.
     // The initial "+" says that these modules should be loaded in
     // addition to the standard modules.
     std::set<string> moduleNames;
     for ( /* each of your MIB files */ )
     {
         FILE* fp = fopen( /* next MIB file */, "r");
         if (fp == 0) continue;
         char token[MAXTOKEN];
         if (NetSnmp::snmp_get_token(fp, token, MAXTOKEN) > 0)
             moduleNames.insert(token);
         fclose(fp);
     }

     string mibs("+");
     std::set<string>::const_iterator mit = moduleNames.begin();
     while (mit != moduleNames.end())
     {
         if (mibs.size() > 1) mibs += ':';
         mibs += *mit++;
     }
     setenv("MIBS", mibs.c_str(), 1);

     // Print warning messages (1=some, 2=lots) when parsing MIBs.
     NetSnmp::snmp_set_mib_warnings(1);

     // Now initialize the NET-SNMP library's MIB API.  Among other
     // things, this reads in all default MIBs, plus MIBs added above;
     // see mib_api(3).
     NetSnmp::init_mib();
}


/*static*/ Agentpp::Oidx
MyAgent::parseOID(const string& oidName)
{
     NetSnmp::oid subID[MAX_OID_LEN];
     size_t       subIDLen = MAX_OID_LEN;

     // Convert this object identifier from numeric or symbolic form
     // to a list of subidentifiers.
     if (NetSnmp::read_objid(oidName.c_str(), subID, &subIDLen))
         return Agentpp::Oidx(subID, subIDLen);

     // An empty OID indicates a failed parse.
     return Agentpp::Oidx();
}