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Graph a Single SNMP OID

When dealing with SNMP-enabled devices, there are often times when you want to graph the value of a single OID. This tutorial explains how to do this in Cacti. It also assumes that you have the “SNMP - Generic OID Template” graph template, which is now included in Cacti as of version 0.8.5. If you do not see this template listed under Graph Templates, download the template from the Cacti website in XML form and import it using the Import Templates menu item.

To start the process of creating a new graph for your OID, click the New Graphs menu item and select the host that contains the target OID from the dropdown. Under the Graph Templates box, you will see a dropdown on the last line that reads (Select a graph type to create). From this dropdown, choose “SNMP - Generic OID Template” and click the Create button at the bottom of the page.

You will be presented with several fields that will require input before the new graph can be created. They are described in more detail below.

Name Description
(Graph) Title The title that is to be used for the new graph. It is generally a good idea to keep host_description in the title, as to make the graph easier to identify later.
(Graph) Vertical Label The text that will be printed along the y-axis of the graph. It is generally used to describe units, such as 'bytes' or 'percent'.
(Graph Items) Legend Color The color that will be used to reprent the data on the graph.
(Graph Items) Legend Text The text that will be used to describe the data on the graph legend.
(Data Source) Name The title that is to be used for the new data source. It is generally a good idea to keep host_description in the title, as to make the data source easier to identify later.
(Data Source) Maximum Value [snmp_oid]The maximum value that will be accepted from the OID. Make sure you choose a value that is reasonable for the data you are trying to graph because anything larger than the maximum will be ignored. If you are graphing a percentage, you should use '100' as the value should never exceed this.
(Data Source) Data Source Type [snmp_oid]How the data from the OID should be stored by RRDTool and interpreted on the graph. If the value of the OID represents the actual data, you should use GAUGE for this field. If the OID value is a constantly incrimenting number, you should use COUNTER for this field. The two remaining field values, DERIVE and ABSOLUTE can be ignored in most situations.
(Custom Data) OIDThe actual SNMP OID to graph. It is typically a good idea to enter the number OID here as opposed to using MIB names. For instance, to get the number of open files on a Netware server, you would use ”.1.3.6.1.4.1.23.2.28.2.7.0” as the OID.


Data Input Methods

Data input methods allow Cacti to retrieve data to insert into data sources and ultimately put on a graph. There are different ways for Cacti to retrieve data, the most popular being through an external script or from SNMP.

Creating a Data Input Method

To create a new data input method, select the Data Input Methods option under the Management heading. Once on that screen, click Add on the right. You will be presented with a few fields to populate on the following screen.

Table 11-1. Field Description: Data Input Methods

Name Description
Name Give the data query a name that you will use to identify it. This name will be used throughout Cacti to identify the data input method.
Input Type Select the type of data input method you are trying to create. Valid options here are 'Script/Command', 'SNMP', 'SNMP Query', and 'Script Query'.
Input String This field is only used when the Input Type is set to 'Script/Command'. It specifies the full path to the script including any per data source variables inside <>'s from the user. For instance, if you are passing an IP address to a script, your input string might look something like:/path/to/script.pl <ip>When the user creates a data source based on this data input method, they will be prompted for an IP address to pass onto the script.
Output String This field is only here for legacy purposes and will probably go away in a future version of Cacti.

When you are finished filling in all necessary fields, click the Create button to continue. You will be redirected back to the same page, but this time with two new boxes, Input Fields and Output Fields. The Input Fields box is used to define any fields that require information from the user. Any input fields referenced to in the input string must be defined here. The Output Fields box is used to define each field that you expect back from the script. All data input methods must have at least one output field defined, but may have more for a script.

Data Input Fields

To define a new field, click Add next to the input or output field boxes. You will be presented with some or all of the fields below depending on whether you are adding an input or output field.

Table 11-2. Field Description: Data Input Fields

Name Description
Field/Field Name Either select or type in the name of the field you want to define. No spaces or other non-alphanumeric characters (except '-' or '_') should be used in this field.
Friendly Name Enter a more descriptive name for this field which will be used for identification throughout Cacti.
Regular Expression Match (Input Only) If you want to enforce a certain regular expression pattern when the user enters a value for this field, enter it here. The regular expression must follow POSIX syntax as it will be passed to PHP's ereg() function.
Allow Empty Input (Input Only) Select whether you want to allow the user to leave the value of this field blank or not.
Special Type Code (Input Only) Sometimes Cacti needs to reference a field internally, but needs to rely on more than just the field name. For instance, if your field requires an IP address from the user, you can enter 'management_ip' here and Cacti will fill this field in with the current IP address of the selected host. Valid values for this field are: 'hostname', 'management_ip', 'snmp_community', 'snmp_username', 'snmp_password', and 'snmp_version'.
Update RRD File (Output Only) Check this box if you want Cacti to insert the return value from this field into the RRD file. Obviously, this box needs to be checked for at least one output field per data input source, but can be left blank to have Cacti store the value in the database instead.

When you are finished filling in all necessary fields, click the Create button to continue. You will be redirected back to the data input method edit page. From here you can continue to add additional fields, or click Save on this screen when finished.

Making Your Scripts Work With Cacti

The simplest way to extend Cacti's data gathering functionality is through external scripts. Cacti comes with a number of scripts out of the box which are located in the scripts/ directory. These scripts are used by the data input methods that are present in a new installation of Cacti.

To have Cacti call an external script to gather data you must create a new data input method, making sure to specify Script/Command for the Input Type field. See the previous section, Creating a Data Input Method for more information about how to create a data input method. To gather data using your data input method, Cacti simply executes the shell command specified in the Input String field. Because of this, you can have Cacti run any shell command or call any script which can be written in almost any language.

What Cacti is concerned with is the output of the script. When you define your data input method, you are required to define one or more output fields. The number of output fields that you define here is important to your script's output. For a data input method with only one output field, your script should output its value in the following format:

<value_1>

So if I wrote a script that outputs the number of running processes, its output might look like the following:

Example 11-1. Example script output using 1 field
67

Data input methods with more than one output field are handled a bit differently when writing scripts. Scripts that output more than one value should be formatted like the following:

<fieldname_1>:<value_1> <fieldname_2>:<value_2> ... <fieldname_n>:<value_n>

Lets say that I write a script that outputs the 1, 5, and 10 minute load average of a Unix machine. In Cacti, I name the output fields '1min', '5min', and '10min', respectively. Based on these two things, the output of the script should look like the following:

Example 11-2. Example script output using 3 fields
1min:0.40 5min:0.32 10min:0.01

One last thing to keep in mind when writing scripts for Cacti is that they will be executed as the user the data gatherer runs as. Sometimes a script may work correctly when executed as root, but fails due to permissions problems when executed as a less privileged user.






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