SNMP request lets program crash
Michael Jäger
I'm currently working on an snmp application, written in C, that requests
certain information from a device.
These requests contain OIDs of the LLDBMIB.
In order to request for different information I've created a function that
wraps
the Net-SNMP function 'snmp_synch_response(...)' called buildAndSend (see at
the bottom of this post).
This function allows for specifiying a textual representation of an
OID(char*) and also the net-snmp specific internal format (oid [])
and takes care of the rest that is necessary to send a request to the
network.
Recently, this function let the program crash, which seems to occur while
considering a specific device and a specific MIB entry.
While debugging the source code, I found out that the program crashes when
executing the function snmp_add_null_var.
After debugging deeper inside this function, the program crashes when the
function SNMP_MALLOC_TYPEDEF is called. (I can not
get any deeper when debugging.)
At that time I was specifying the internal oid format (oid[]) all the time
when sending an SNMP request.
The exact function call looked like this:
oid aulLSN[] = {1,0,8802,1,1,2,1,3,3,0};
netsnmp_pdu *ptResponse = buildAndSend(NULL, SNMP_MSG_GET, ptSess_handle,
aulLSN, 10);
After this problem occurred, I tried to specify the char* oid format.
netsnmp_pdu *ptResponse = buildAndSend("1.0.8802.1.1.2.1.3.3.0",
SNMP_MSG_GET, ptSess_handle, NULL, 0);
This time the program crashes when the function 'read_objid' is called,
which made me try adding the LLDPMIB
by using the functions '' add_mibdir' and 'read_mib', which unfortunately
did not work out.
Both times the program crashes when considering a certain device, but
obviously before the specific pdu is sent.
The same function calls work when considering other network devices; just
for the sake of clarification.
I've read that I can avoid using the char* oid representation when
specifying the internal oid format(oid []) directly, thats why my
'buildAndSend' function is structured like I explained.
Maybe I undstood something completely wrong when using the Net-snmp library
and maybe someone can point out some flaws
in my thought process which makes me find my error. :)
Thanks in advance, for any reply.
/**
* This function builds an SNMP pdu and sends it to the devices
based on the parameter *ptSess_handle.
* Sending an SNMP message is realized by using the Net-SNMP
library (http://www.net-snmp.org/).
* In order to determine what information is requested
by an SNMP request, an identifier, called OID, is used.
* The user is able to specify an OID in two ways.
Either as a char* version, or as an Net-SNMP library specific
* internal version.
* The char* version of an OID (provided via the
parameter *pchOID) is translated into internal format by
* the function read_objid(...). It is also possible to
pass the OID as the Net-SNMP library specific internal
* format via the parameter aulOidInternal[]. In that
case the parameter *pchOIDVarBind should be NULL.
*
*
* \param *pchOidVarBind The OID for an SNMP request
in char* format.
*
* \param iSnmpType Determines the type
of the snmp request, e.g. get, set, etc.
*
* \param ptSess_handle corresponds to the
previously established session with the station we want to talk to.
*
* \param aulOidInternal[] Net-SNMP library internal
format of oid which would be created through read_objid.
* This
can be avoided by directly providing an valid internal format of an oid.
* \param uOidInternal_length Length of the internal OID
format array.
*
* \return The response pdu received from the station we
communicated with.
*
*/
netsnmp_pdu *buildAndSend(const char *pchOid, int iSnmpType, netsnmp_session
*ptSess_handle, oid aulOidInternal[], size_t uOidInternal_length){
#ifdef FUNCCALLS
printf("sendMsg started\n");
printf("\n !! !! !! Request Data !! !! !!\n");
printf("Destination: %s\n", ptSess_handle->peername);
if(NULL == pchOid){
printf("pchOid == NULL\n");
printf("Current OID = ");
int iForIdx = 0;
for(iForIdx = 0; iForIdx < uOidInternal_length;
++iForIdx){
if(iForIdx != uOidInternal_length - 1){
printf("%i.", (int)
aulOidInternal[iForIdx]);
}else{
printf("%i\n", (int)
aulOidInternal[iForIdx]);
}
}
}else{
printf("pchOid != NULL\n");
printf("Current OID = %s", pchOid);
}
#endif
oid aulId_oid[MAX_OID_LEN]; // type = unsigned long
size_t uId_len = MAX_OID_LEN; //type = unsigned int
netsnmp_pdu *ptPduReq = NULL;
netsnmp_pdu *ptResponse = NULL;
ptPduReq = snmp_pdu_create(iSnmpType);
/*read_objid translates an oid in char format into an internal
format. If this is unnecessary, the function call read_objid will be
omitted.*/
if(NULL == pchOid){
int iForCnt = 0;
for(iForCnt = 0; iForCnt < uOidInternal_length; ++iForCnt){
aulId_oid[iForCnt] = aulOidInternal[iForCnt];
}
uId_len = uOidInternal_length;
}
else{read_objid(pchOid, aulId_oid, &uId_len);} //translating oid
into internal oid format (id_oid)
http://fixunix.com/snmp/324746-re-problems-read_objid.html
snmp_add_null_var(ptPduReq, aulId_oid, uId_len);
uId_len = MAX_OID_LEN; //reset id_len
/*PDU will be sent*/
int iStatus = snmp_synch_response(ptSess_handle, ptPduReq,
&ptResponse);
if(iStatus == 0){ //response received in ptResponse
#ifdef FUNCCALLS
printf("sendMsg finished\n");
#endif
return ptResponse;
}
/*Error handling*/
else{printf("Connection Error; status = %i\n", iStatus);}
return NULL;
}
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