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See Also See Also: Object Configuration, Object Inheritance With Templates, Macros


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 that I see with doing this is that it makes Nagios Core less generic and more infrastructure-specific. Nagios Core was intended to be flexible, which meant things needed to be designed in a generic manner. Host definitions in Nagios Core, 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 Nagios Core configuration without imposing a set of specific variables on others. Nagios Core 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.

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


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

Custom Variables As Macros

Custom variable values can be referenced in scripts and executables that Nagios runs for checks, notifications, etc. by using macros or environment variables. Custom variable macros are trusted (because you define them) and therefore not cleaned/sanitized before they are made available to scripts.

In order to prevent name collision among custom variables from different object types, Nagios 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 TypeVariable NameMacro NameEnvironment Variable

Custom Variables And Inheritance

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