Getting the interface that matches an IP in Puppet

January 14, 2020 – Samuli Seppänen

I was recently in a situation where I needed to figure out which interface name matched a particular, static IPv4 address. On a single or a handful of nodes this could have been hardcoded in Hiera, for example, but there were dozens of nodes. The way I solved this was by creating a custom fact that uses the IPv4 address and loops through the interfaces list to find the matching interface name:

# frozen_string_literal: true

# Get the name of the interface that is bound to 192.168.122.10
Facter.add(:kvm_iface) do

  confine kernel: 'Linux'

  desired_ip = '192.168.122.10'
  interfaces_fact = Facter.value(:networking)['interfaces']
  interfaces_list = interfaces_fact.keys
  interfaces_list.each do |interface|
    interface_ip = interfaces_fact[interface]['bindings'][0]['address']
    next unless interface_ip == desired_ip

    desired_interface = interface
    setcode do
      desired_interface
    end
  end
end

In the end the code did not get used because we could rely on the interface name being static, but hopefully this code is helpful for somebody else.

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