NET-SNMP-EXTEND-MIB
) that can be used to query arbitrary shell scripts. To specify the shell script to run, use the extend
directive in the /etc/snmp/snmpd.conf
file. Once defined, the Agent will provide the exit code and any output of the command over SNMP. The example below demonstrates this mechanism with a script which determines the number of httpd
processes in the process table.
Using the proc directive
proc
directive. See the snmpd.conf(5) manual page for more information.
httpd
processes running on the system at a given point in time:
#!/bin/sh NUMPIDS=`pgrep httpd | wc -l` exit $NUMPIDS
extend
directive to the /etc/snmp/snmpd.conf
file. The format of the extend
directive is the following:
extend
name prog args
/usr/local/bin/check_apache.sh
, the following directive will add the script to the SNMP tree:
extend httpd_pids /bin/sh /usr/local/bin/check_apache.sh
NET-SNMP-EXTEND-MIB::nsExtendObjects
:
~]$ snmpwalk localhost NET-SNMP-EXTEND-MIB::nsExtendObjects
NET-SNMP-EXTEND-MIB::nsExtendNumEntries.0 = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendCommand."httpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendArgs."httpd_pids" = STRING: /usr/local/bin/check_apache.sh
NET-SNMP-EXTEND-MIB::nsExtendInput."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendCacheTime."httpd_pids" = INTEGER: 5
NET-SNMP-EXTEND-MIB::nsExtendExecType."httpd_pids" = INTEGER: exec(1)
NET-SNMP-EXTEND-MIB::nsExtendRunType."httpd_pids" = INTEGER: run-on-read(1)
NET-SNMP-EXTEND-MIB::nsExtendStorage."httpd_pids" = INTEGER: permanent(4)
NET-SNMP-EXTEND-MIB::nsExtendStatus."httpd_pids" = INTEGER: active(1)
NET-SNMP-EXTEND-MIB::nsExtendOutput1Line."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendOutputFull."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendOutNumLines."httpd_pids" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids" = INTEGER: 8
NET-SNMP-EXTEND-MIB::nsExtendOutLine."httpd_pids".1 = STRING:
extend
directive. For example, the following shell script can be used to determine the number of processes matching an arbitrary string, and will also output a text string giving the number of processes:
#!/bin/sh PATTERN=$1 NUMPIDS=`pgrep $PATTERN | wc -l` echo "There are $NUMPIDS $PATTERN processes." exit $NUMPIDS
/etc/snmp/snmpd.conf
directives will give both the number of httpd
PIDs as well as the number of snmpd
PIDs when the above script is copied to /usr/local/bin/check_proc.sh
:
extend httpd_pids /bin/sh /usr/local/bin/check_proc.sh httpd extend snmpd_pids /bin/sh /usr/local/bin/check_proc.sh snmpd
snmpwalk
of the nsExtendObjects
OID:
~]$ snmpwalk localhost NET-SNMP-EXTEND-MIB::nsExtendObjects
NET-SNMP-EXTEND-MIB::nsExtendNumEntries.0 = INTEGER: 2
NET-SNMP-EXTEND-MIB::nsExtendCommand."httpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendCommand."snmpd_pids" = STRING: /bin/sh
NET-SNMP-EXTEND-MIB::nsExtendArgs."httpd_pids" = STRING: /usr/local/bin/check_proc.sh httpd
NET-SNMP-EXTEND-MIB::nsExtendArgs."snmpd_pids" = STRING: /usr/local/bin/check_proc.sh snmpd
NET-SNMP-EXTEND-MIB::nsExtendInput."httpd_pids" = STRING:
NET-SNMP-EXTEND-MIB::nsExtendInput."snmpd_pids" = STRING:
...
NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids" = INTEGER: 8
NET-SNMP-EXTEND-MIB::nsExtendResult."snmpd_pids" = INTEGER: 1
NET-SNMP-EXTEND-MIB::nsExtendOutLine."httpd_pids".1 = STRING: There are 8 httpd processes.
NET-SNMP-EXTEND-MIB::nsExtendOutLine."snmpd_pids".1 = STRING: There are 1 snmpd processes.
Integer exit codes are limited
httpd
processes. This query could be used during a performance test to determine the impact of the number of processes on memory pressure:
~]$snmpget localhost \
'NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids"' \
UCD-SNMP-MIB::memAvailReal.0
NET-SNMP-EXTEND-MIB::nsExtendResult."httpd_pids" = INTEGER: 8 UCD-SNMP-MIB::memAvailReal.0 = INTEGER: 799664 kB
extend
directive is a fairly limited method for exposing custom application metrics over SNMP. The Net-SNMP Agent also provides an embedded Perl interface for exposing custom objects. The net-snmp-perl package provides the NetSNMP::agent
Perl module that is used to write embedded Perl plug-ins on Fedora.
NetSNMP::agent
Perl module provides an agent
object which is used to handle requests for a part of the agent's OID tree. The agent
object's constructor has options for running the agent as a sub-agent of snmpd
or a standalone agent. No arguments are necessary to create an embedded agent:
use NetSNMP::agent (':all'); my $agent = new NetSNMP::agent();
agent
object has a register
method which is used to register a callback function with a particular OID. The register
function takes a name, OID, and pointer to the callback function. The following example will register a callback function named hello_handler
with the SNMP Agent which will handle requests under the OID .1.3.6.1.4.1.8072.9999.9999
:
$agent->register("hello_world", ".1.3.6.1.4.1.8072.9999.9999", \&hello_handler);
Obtaining a root OID
.1.3.6.1.4.1.8072.9999.9999
(NET-SNMP-MIB::netSnmpPlaypen
) is typically used for demonstration purposes only. If your organization does not already have a root OID, you can obtain one by contacting your Name Registration Authority (ANSI in the United States).
HANDLER
, REGISTRATION_INFO
, REQUEST_INFO
, and REQUESTS
. The REQUESTS
parameter contains a list of requests in the current call and should be iterated over and populated with data. The request
objects in the list have get and set methods which allow for manipulating the OID and value of the request. For example, the following call will set the value of a request object to the string “hello world”:
$request->setValue(ASN_OCTET_STR, "hello world");
getMode
method on the request_info
object passed as the third parameter to the handler function. If the request is a GET request, the caller will expect the handler to set the value of the request
object, depending on the OID of the request. If the request is a GETNEXT request, the caller will also expect the handler to set the OID of the request to the next available OID in the tree. This is illustrated in the following code example:
my $request; my $string_value = "hello world"; my $integer_value = "8675309"; for($request = $requests; $request; $request = $request->next()) { my $oid = $request->getOID(); if ($request_info->getMode() == MODE_GET) { if ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setValue(ASN_OCTET_STR, $string_value); } elsif ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.1")) { $request->setValue(ASN_INTEGER, $integer_value); } } elsif ($request_info->getMode() == MODE_GETNEXT) { if ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.1"); $request->setValue(ASN_INTEGER, $integer_value); } elsif ($oid < new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.0"); $request->setValue(ASN_OCTET_STR, $string_value); } } }
getMode
returns MODE_GET
, the handler analyzes the value of the getOID
call on the request
object. The value of the request
is set to either string_value
if the OID ends in “.1.0”, or set to integer_value
if the OID ends in “.1.1”. If the getMode
returns MODE_GETNEXT
, the handler determines whether the OID of the request is “.1.0”, and then sets the OID and value for “.1.1”. If the request is higher on the tree than “.1.0”, the OID and value for “.1.0” is set. This in effect returns the “next” value in the tree so that a program like snmpwalk
can traverse the tree without prior knowledge of the structure.
NetSNMP::ASN
. See the perldoc
for NetSNMP::ASN
for a full list of available constants.
#!/usr/bin/perl use NetSNMP::agent (':all'); use NetSNMP::ASN qw(ASN_OCTET_STR ASN_INTEGER); sub hello_handler { my ($handler, $registration_info, $request_info, $requests) = @_; my $request; my $string_value = "hello world"; my $integer_value = "8675309"; for($request = $requests; $request; $request = $request->next()) { my $oid = $request->getOID(); if ($request_info->getMode() == MODE_GET) { if ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setValue(ASN_OCTET_STR, $string_value); } elsif ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.1")) { $request->setValue(ASN_INTEGER, $integer_value); } } elsif ($request_info->getMode() == MODE_GETNEXT) { if ($oid == new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.1"); $request->setValue(ASN_INTEGER, $integer_value); } elsif ($oid < new NetSNMP::OID(".1.3.6.1.4.1.8072.9999.9999.1.0")) { $request->setOID(".1.3.6.1.4.1.8072.9999.9999.1.0"); $request->setValue(ASN_OCTET_STR, $string_value); } } } } my $agent = new NetSNMP::agent(); $agent->register("hello_world", ".1.3.6.1.4.1.8072.9999.9999", \&hello_handler);
/usr/share/snmp/hello_world.pl
and add the following line to the /etc/snmp/snmpd.conf
configuration file:
perl do "/usr/share/snmp/hello_world.pl"
snmpwalk
should return the new data:
~]$ snmpwalk localhost NET-SNMP-MIB::netSnmpPlaypen
NET-SNMP-MIB::netSnmpPlaypen.1.0 = STRING: "hello world"
NET-SNMP-MIB::netSnmpPlaypen.1.1 = INTEGER: 8675309
snmpget
should also be used to exercise the other mode of the handler:
~]$snmpget localhost \
NET-SNMP-MIB::netSnmpPlaypen.1.0 \
NET-SNMP-MIB::netSnmpPlaypen.1.1
NET-SNMP-MIB::netSnmpPlaypen.1.0 = STRING: "hello world" NET-SNMP-MIB::netSnmpPlaypen.1.1 = INTEGER: 8675309