The Net-SNMP PERL Module

The Net-SNMP distribution comes complete with a PERL module. You'll notice that several SNMP modules exist in CPAN but these are deprecated and should not be used.

The module is loaded in the usual way, with a "use" statement. It is recommended that you supply a module version number to avoid any possible problems.

Three main statements are used in a basic application: SNMP::Session(), SNMP::VarList, and SNMP::getnext(). The SNMP::Session method connects to the target agent and returns a session handle. Once a handle is open, a varlist can be populated by passing a number of OIDs (typically just the unique key name). The VarList method builds a list and stores it using it's own internal format. We can use the GetNext method to actually fetch and then output all the returned values into a normal flat array to be used by other parts of our script.

Here is an example of our previous APC monitoring tool re-written using the Net-SNMP PERL module.

#!/usr/local/bin/perl

use SNMP '5.0.2.pre1' || die("Cannot load module\n");

$ENV{'MIBS'}="ALL";  #Load all available MIBs
$SNMP_TARGET = "10.10.1.224";
$SNMP_COMMUNITY = "public";

$SESSION = new SNMP::Session (DestHost => $SNMP_TARGET, 
                                Community => $SNMP_COMMUNITY,
                                Version => 1);

# Populate a VarList with OID values.
$APC_VLIST =  new SNMP::VarList(['upsBasicIdentModel'],         #0      
                                ['upsAdvIdentSerialNumber'],    #1
                                ['upsAdvBatteryCapacity'],      #2
                                ['upsAdvBatteryTemperature'],   #3
                                ['upsAdvOutputLoad'],           #4
                                ['upsAdvOutputVoltage'],        #5
                                ['upsAdvOutputFrequency'],      #6
                                ['upsBasicOutputStatus']);      #7

# Pass the VarList to getnext building an array of the output
@APC_INFO = $SESSION->getnext($APC_VLIST);

$APC_INFO[0] =~ s/\"//g;        # Ditch the quotes.
$APC_INFO[1] =~ s/\"//g;

# Output the results.
print <<END;
APC UPS                         ${SNMP_TARGET}
Model: ${APC_INFO[0]}       Serial No: ${APC_INFO[1]}

Battery Capacity: ${APC_INFO[2]}        
Battery Temp(F): ${APC_INFO[3]}

Output Status: ${APC_INFO[7]}
Output Load: ${APC_INFO[4]}

Output: ${APC_INFO[5]}VAC @ ${APC_INFO[6]}Hz  
END

Notice that in the header of our script we're defining the MIBS environmental variable to "ALL", which loads all of the MIBs in the Net-SNMP system MIBs directory.

Several varlists can be created from a single session to provide clear organization. When I create varlists I find it is helpful to put commented reference numbers next to each element of the list so that when I later use the populated array I can quickly check which OID is in which element.

The output looks the same as the previous versions did:

$ ./apc_status_mod.pl
APC UPS                         10.10.1.224
Model: Silcon DP340E        Serial No: SE0010000515    

Battery Capacity: 100   
Battery Temp(F): 33

Output Status: onLine
Output Load: 53

Output: 118VAC @ 60Hz