One of the problems that I find annoying in puppet is that you can’t easily obtain the IP address of a specific network interface.

Let’s say that you want to define a class or type to which you wish to pass a interface name and use it’s IP address as a variable.

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class dns ($interface) {
    $address = [ insert magic here ]

    $message = "So you want to know the IP address for ${interface} huh?"
    $message += "Well, it's ${address}"
    notify {  "${message}": }
}

class { 'dns': interface => 'eth0' }

We could use facters $::ipaddress, but what happens if the network interface is not our main network interface? What if we want to use more than one network interface? There’s $::ipaddress_eth0, but how do we construct or reference a variable in a dynamic way (construct a variable based on another variable) ?

Luckily there’s a nifty trick involving inline templates and lookupvar:

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class dns ($interface) {
    $address = inline_template("<%= scope.lookupvar('::ipaddress_${interface}') -%>")

    $message = "So you want to know the IP address for ${interface} huh?"
    $message += "Well, it's ${address}"
    notify {  "${message}": }
}

class { 'dns': interface => 'eth0' }