3.5. Custom Object Variables
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## 3.5. Custom Object Variables

### 3.5.1. Introduction

Users often request that new variables be added to host, service, and contact definitions. These include variables for SNMP community, MAC address, AIM username, Skype number, and street address. The list is endless. The problem is that it makes Icinga less generic and more infrastructure-specific. Icinga was intended to be flexible, which meant things needed to be designed in a generic manner. Host definitions in Icinga, for example, have a generic "address" variable that can contain anything from an IP address to human-readable driving directions - whatever is appropriate for the user's setup.

Still, there needs to be a method for admins to store information about their infrastructure components in their Icinga configuration without imposing a set of specific variables on others. Icinga attempts to solve this problem by allowing users to define custom variables in their object definitions. Custom variables allow users to define additional properties in their host, service, and contact definitions, and use their values in notifications, event handlers, and host and service checks.

### 3.5.2. Custom Variable Basics

There are a few important things that you should note about custom variables:

- Custom variable names must begin with an underscore (_) to prevent name collision with standard variables

- Custom variable names are converted to all uppercase before use

- Custom variables are inherited from object templates like normal variables

- Scripts can reference custom variable values with macros and environment variables

| Important

Custom variables are replaced during runtime, neither during the verification phase (icinga -v icinga.cfg) nor during the Icinga start up process so placing them in host names, service descriptions, and several other places of the object definition will lead to an error message during verification like "`<object> contains one or more illegal characters.`". Changing the directive "illegal_object_name_chars" will not solve this issue.

### 3.5.3. Examples

Here's an example of how custom variables can be defined in different types of object definitions:

    define host{
            host_name       linuxserver
            _mac_address    00:06:5B:A6:AD:AA       ; <-- Custom MAC_ADDRESS variable
            _rack_number    R32                     ; <-- Custom RACK_NUMBER variable
            ...
            }

    define service{
            host_name       linuxserver
            description     Memory Usage
            _SNMP_community public                  ; <-- Custom SNMP_COMMUNITY variable
            _TechContact    Jane Doe                ; <-- Custom TECHCONTACT variable
            ....
            }

    define contact{
            contact_name    john
            _AIM_username   john16                  ; <-- Custom AIM_USERNAME variable
            _YahooID        john32                  ; <-- Custom YAHOOID variable
            ...
            }

### 3.5.4. Custom Variables As Macros

Custom variable values can be referenced in scripts and executables that Icinga runs for checks, notifications, etc. by using macros or environment variables.

In order to prevent name collision among custom variables from different object types, Icinga prepends "_HOST", "_SERVICE", or "_CONTACT" to the beginning of custom host, service, or contact variables, respectively, in macro and environment variable names. The table below shows the corresponding macro and environment variable names for the custom variables that were defined in the example above.

Object Type | Variable Name | Macro Name | Environment Variable
Host | MAC_ADDRESS | $_HOSTMAC_ADDRESS$ | ICINGA__HOSTMAC_ADDRESS
Host | RACK_NUMBER | $_HOSTRACK_NUMBER$ | ICINGA__HOSTRACK_NUMBER
Service | SNMP_COMMUNITY | $_SERVICESNMP_COMMUNITY$ | ICINGA__SERVICESNMP_COMMUNITY
Service | TECHCONTACT | $_SERVICETECHCONTACT$ | ICINGA__SERVICETECHCONTACT
Contact | AIM_USERNAME | $_CONTACTAIM_USERNAME$ | ICINGA__CONTACTAIM_USERNAME
Contact | YAHOOID | $_CONTACTYAHOOID$ | ICINGA__CONTACTYAHOOID

### 3.5.5. Custom Variables And Inheritance

Custom object variables are inherited just like standard host, service, or contact variables.

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