These three requests can come in one of these sequences:
The normal sequence is SET and then COMMIT. When we receive a SET request, we must make preparations to accept the new value. For example, check that it is for an existing object and instance, check the value type and contents to be valid, allocate memory, but we must not yet make the change.
If there are no SET errors, the next request we receive will be a COMMIT request. It is then that we must make the change, but we must also keep enough information such that we can UNDO the change later if we get a subsequent UNDO request. The latter may happen if the agent discovers any errors with other subagents while processing requests that belong to the same original SNMP SET packet. All the varBinds in the same SNMP request PDU must be processed "as if atomic".
When the DPI packet is parsed, the snmp_dpi_hdr structure shows in the packet_type that this is an SNMP_DPI_SET, SNMP_DPI_COMMIT, or SNMP_DPI_UNDO packet. In that case, the packet_body contains a pointer to a SET-varBind, represented in an snmp_dpi_get_packet structure. COMMIT and UNDO have same varBind data as SET upon which they follow:
struct dpi_set_packet { char *object_p; /* ptr to OIDstring */
char *group_p; /* ptr to sub-tree */
char *instance_p; /* ptr to rest of OID */
unsigned char value_type; /* SNMP_TYPE_xxxx */
unsigned short value_len; /* value length */
char *value_p; /* ptr to value itself */
struct dpi_set_packet *next_p; /* ptr to next in chain */
};
typedef struct dpi_set_packet snmp_dpi_set_packet;
#define snmp_dpi_set_packet_NULL_p ((snmp_dpi_set_packet *)0)
Assuming we have registered example sub-tree dpiSimpleMIB and a GET request comes in for one variable dpiSimpleString.0 so that is object 1 instance 0 in our sub-tree, and also assuming that the agent knows about our compiled dpiSimpleMIB so that it knows this is a DisplayString as opposed to just an arbitrary OCTET_STRING, the pointers in the snmp_dpi_set_packet structure would have pointers and values like:
object_p -> "1.3.6.1.4.1.2.2.1.5.2.0" group_p -> "1.3.6.1.4.1.2.2.1.5." instance_p -> "2.0" value_type -> SNMP_TYPE_DisplayString value_len -> 8 value_p -> pointer to the value to be set next_p -> snmp_dpi_get_packet_NULL_p
If there are multiple varBinds in a SET request, each one is represented in a snmp_dpi_set_packet structure and all the snmp_dpi_set_packet structures are chained via the next pointer. As long as the next pointer is not the snmp_dpi_set_packet_NULL_p pointer, there are more varBinds in the list.
Now we can analyze the varBind structure for whatever checking we want to do. Once we are ready to make a response that contains the value of the variable, we may prepare a new SET-varBind. However, by definition, the response to a successful SET is exactly the same as the SET request. So there is no need to return any varBinds. A response with SNMP_ERROR_noError and an index of zero will do. If there is an error, a response with the SNMP_ERROR_xxxx error code and an index pointing to the varBind in error (counting starts at 1) will do.
The following code example returns a response. We assume that there are no errors in the request, but proper code should do the checking for that. We also do not check if the varBind in the COMMIT and/or UNDO is the same as that in the SET request. A proper agent would make sure that is the case, but a proper subagent may want to verify that for itself. We only do one check that this is dpiSimpleString.0, and if it is not, we return a noCreation. This may not be correct, the mainline does not even return a response.
static int do_set(snmp_dpi_hdr *hdr_p, snmp_dpi_set_packet *pack_p)
{
unsigned char *packet_p;
int rc;
int index = 0;
int error = SNMP_ERROR_noError;
snmp_dpi_set_packet *varBind_p;
varBind_p = /* init the varBind chain */
snmp_dpi_set_packet_NULL_p; /* to a NULL pointer */
if (!pack_p->instance_p ||
(strcmp(pack_p->instance_p,"2.0") != 0))
{
if (pack_p->instance_p &&
(strncmp(pack_p->instance_p,"1.",2) == 0))
{
error = SNMP_ERROR_notWritable;
} else if (pack_p->instance_p &&
(strncmp(pack_p->instance_p,"2.",2) == 0))
{
error = SNMP_ERROR_noCreation;
} else if (pack_p->instance_p &&
(strncmp(pack_p->instance_p,"3.",2) == 0))
{
error = SNMP_ERROR_notWritable;
} else {
error = SNMP_ERROR_noCreation;
} /* endif */
packet_p = mkDPIresponse( /* Make DPIresponse packet */
hdr_p, /* ptr parsed request */
error, /* all is OK, no error */
1, /* index is 1, 1st varBind */
varBind_p); /* varBind response data */
if (!packet_p) return(-1); /* If it failed, return */
rc = DPIsend_packet_to_agent( /* send RESPONSE packet */
handle, /* on this connection */
packet_p, /* this is the packet */
DPI_PACKET_LEN(packet_p));/* and this is its length */
return(rc); /* return retcode */
}
switch (hdr_p->packet_type) {
case SNMP_DPI_SET:
if ((pack_p->value_type != SNMP_TYPE_DisplayString) &&
(pack_p->value_type != SNMP_TYPE_OCTET_STRING))
{ /* check octet string in case agent has no compiled MIB */
error = SNMP_ERROR_wrongType;
break; /* from switch */
} /* endif */
if (new_val_p) free(new_val_p); /* free these memory areas */
if (old_val_p) free(old_val_p); /* if we allocated any */
new_val_p = (char *)0;
old_val_p = (char *)0;
new_val_len = 0;
old_val_len = 0;
new_val_p = /* allocate memory for */
malloc(pack_p->value_len); /* new value to set */
if (new_val_p) { /* If success, then also */
memcpy(new_val_p, /* copy new value to our */
pack_p->value_p, /* own and newly allocated */
pack_p->value_len); /* memory area. */
new_val_len = pack_p->value_len;
} else { /* Else failed to malloc, */
error = SNMP_ERROR_genErr; /* so that is a genErr */
index = 1; /* at first varBind */
} /* endif */
break;
case SNMP_DPI_COMMIT:
old_val_p = cur_val_p; /* save old value for undo */
cur_val_p = new_val_p; /* make new value current */
new_val_p = (char *)0; /* keep only 1 ptr around */
old_val_len = cur_val_len; /* and keep lengths correct*/
cur_val_len = new_val_len;
new_val_len = 0;
/* may need to convert from ASCII to native if OCTET_STRING */
break;
case SNMP_DPI_UNDO:
if (new_val_p) { /* free allocated memory */
free(new_val_p);
new_val_p = (char *)0;
new_val_len = 0;
} /* endif */
if (old_val_p) {
if (cur_val_p) free(cur_val_p);
cur_val_p = old_val_p; /* reset to old value */
cur_val_len = old_val_len;
old_val_p = (char *)0;
old_val_len = 0;
} /* endif */
break;
} /* endswitch */
packet_p = mkDPIresponse( /* Make DPIresponse packet */
hdr_p, /* ptr parsed request */
error, /* all is OK, no error */
index, /* index is zero, no error */
varBind_p); /* varBind response data */
if (!packet_p) return(-1); /* If it failed, return */
rc = DPIsend_packet_to_agent( /* send RESPONSE packet */
handle, /* on this connection */
packet_p, /* this is the packet */
DPI_PACKET_LEN(packet_p));/* and this is its length */
return(rc); /* return retcode */
} /* end of do_set() */