Chapter 8. Polling and Setting

8.1. Retrieving a Single MIB Value

Let's start by querying a router for the name of its administrative contact. This operation, called polling, is accomplished with the SNMP get command. The following Perl script, snmpget.pl, uses an SNMP Perl module to retrieve the information we want (Chapter 5, "Network-Management Software" contains the URL for this module):

#!/usr/local/bin/perl
#filename: /opt/local/perl_scripts/snmpget.pl
use BER;
use SNMP_util;
use SNMP_Session;
$MIB1 = ".1.3.6.1.2.1.1.4.0";
$HOST = "orarouter1";
($value) = &snmpget("public\@$HOST","$MIB1");
if ($value) { print "Results :$MIB1: :$value:\n"; }
else { warn "No response from host :$HOST:\n"; } 

This script is obviously very primitive, but it is also easy to understand, even if you're not an experienced Perl user. It's importance isn't in what it does, which is very little, but as a template you can use to insert SNMP operations into other programs. (If you are not used to writing quick Perl programs, or are unfamiliar with the language, a good starting point is the official Perl web site, http://www.perl.com.) The script starts with three use statements, which are similar to #include statements in C. The use statements load Perl modules containing functions and definitions for working with SNMP. The three modules we use are:

BER

Describes how to encode management data into bit patterns for transmission. Basic Encoding Rules (BER) is an ISO standard.

SNMP_util

Defines a set of functions that use the SNMP_Session module to make it much more programmer-friendly. SNMP_util itself uses BER and SNMP_Session, but in this first script we chose to reference these other modules explicitly. In future programs, we'll just use SNMP_util.

SNMP_Session

Provides Perl with core SNMP functionality.

The next two lines specify the data we want to get. We have hardcoded the object ID of a particular piece of data defined by the MIB and the hostname from which we want to retrieve this MIB data. In a more flexible program, you might want to get these values from the command line, or build a user interface to help users specify exactly what they are interested in retrieving. For the time being, however, this will get us started. It is easy enough to replace orarouter1 with the hostname or IP address of the device you want to poll. The OID we are requesting is stored in the variable $MIB1. The value .1.3.6.1.2.1.1.4.0 requests the device's administrative contact. Again, you can replace this with any OID of your choice. We used the numeric form of this object, but you can also use the textual form for the OID, which is .org.dod.internet.mgmt.mib-2.system.sysContact.0. You can abbreviate this further to sysContact because SNMP_util defines some parts of the OID string for us (for example, SNMP_util defines sysContact as 1.3.6.1.2.1.1.4.0), but it's often safer to be explicit and use the entire OID. Don't forget to include the .0, which states that we want the first (0) and only instance of iso.org.dod.internetmgmt.mib-2.system.sysContact.0, at the end of your OID.

The next line polls the device. The snmpget function retrieves the data from the device specified by the variable $HOST. Notice the two arguments to the function. The first is the device we want to poll, preceded by the community name public. (If you need to use another community name -- you did change the community names when you configured the device, didn't you? -- you'll have to modify this line and insert your community name in place of it.) The second argument to snmpget is the OID in which we are interested. If you type the code in yourself, do not forget the parentheses around $value. If you omit the parentheses, $value will be set to the number of items in the array snmpget returns.

Once we have polled the device, we print either the output or an error message. I put a colon before and after any output that I print; this makes it easy to see if there are any hidden characters in the output. The decimal integer "16" is very different from "16\n", which is the decimal integer 16 followed by a newline character.

Now let's run the program:

$ /opt/local/perl_scripts/snmpget.pl
Results :.1.3.6.1.2.1.1.4.0: :ORA IT Group:

snmpget.pl prints the OID we requested, followed by the actual value of that object, which is ORA IT Group. Don't worry if the return value for sysContact is wrong or blank. (The trick of putting colons before and after the output will make it clear if sysContact is blank or empty.) This probably means that no one has configured an administrative contact, or that it was configured incorrectly. We'll show you how to fix that when we discuss the set operation. If you get an error, skip to the end of this chapter to see a list of some errors and their appropriate fixes.

We will now modify snmpget.pl to poll any host and any OID we want. This is accomplished by passing the host and OID as command-line arguments to the Perl script:

#!/usr/local/bin/perl
#filename: /opt/local/perl_scripts/snmpget.pl
use SNMP_util;
$MIB1 = shift;
$HOST = shift;
($MIB1) && ($HOST) || die "Usage: $0 MIB_OID HOSTNAME";
($value) = &snmpget("$HOST","$MIB1");
if ($value) { print "Results :$MIB1: :$value:\n"; }
else { warn "No response from host :$HOST:\n"; }

Now that this program is a little more flexible, it is possible to look up different kinds of information on different hosts. We even left out the community string, which allows us to poll hosts with different community names. Here's how to run the new version of snmpget.pl:

$ /opt/local/perl_scripts/snmpget.pl .1.3.6.1.2.1.1.1.0 public@orarouter1
Results :.1.3.6.1.2.1.1.1.0: :Cisco Internetwork Operating System Software
IOS (tm) 3000 Software (IGS-I-L), Version 11.0(16), RELEASE SOFTWARE (fc1)
Copyright (c) 1986-1997 by cisco Systems, Inc.
Compiled Tue 24-Jun-97 12:20 by jaturner:

In this example, we asked the router to describe itself by looking up the OID .1.3.6.1.2.1.1.1.0 (system.sysDesc.0). The result tells us that orarouter1 is a Cisco router running Version 11.0(16) of the IOS operating system, along with some other useful information.