Needed to get the IP address of a certain mac from the dhcpd leases file, wrote this, seems to work, albeit short. IANAP, YMMV. All of my programming comes from looking at examples, so any faults of mine are actually someone else’s. Blame fR and niblr!

#!/usr/bin/ruby -w
# getdhcpip.rb Bryan McLellan -- bryanm@widemile.com
# parse through dhcpd.leases in search of a mac to get it's current ip
# assume not malformed. remember that this is a log file and the most recent (bottom) is the most accurate

def lastdhcpip(ourmac)
  curLeaseIp = nil
  curLeaseMac = nil
  lastip = nil

  f = File.open("/var/lib/dhcp/dhcpd.leases")
  f.each do |line|
    case line
    when /lease (.*) \{/
      curLeaseIp = $1
    when /hardware ethernet (.*);/
      curLeaseMac = $1
      if ourmac == curLeaseMac
        lastip = curLeaseIp
      end
    end
  end

  f.close
  return lastip
end

if ARGV[0]
  puts lastdhcpip(ARGV[0])
else
  puts "Requires MAC address as argument: getdhcpip.rb 00:00:00:00:00:00"
end