For Morgan: parses an ethers file to generate a dhcpd.conf file.

ethers.rb

#!/usr/bin/env ruby -wKU
# This program scans your ethers file to generate a valid dhcpd.conf file
# (or at least a segment of one). In addition, it will read your hosts file
# to see if there are any known aliases it can add. It then prints the 
# formatted results to standard out.
#
# Author::    Dana Merrick (mailto:letterkills+ethers@gmail.com)
# Copyright:: Copyright (c) 2009 Dana Merrick
# License::   Released under the MIT license

# This class holds the tools for parsing the ethers file. It is meant to be
# executed by calling the #parse_ethers_file method.
class EthersFile

  # accessors for class variables
  # TODO: change them to attr_writers?
  attr_accessor :ethers_file, :hosts_file, :regexps

  # Build the object with default options:
  # * /etc/ethers as the ethers file
  # * /etc/hosts as the hosts file
  def initialize(defaults)
    @ethers_file = "/etc/ethers"
    @hosts_file  = "/etc/hosts"
    @regexps = defaults
  end

  # Returns an array of aliases for the given host name.
  def find_aliases(host)
    # read each match into an array
    aliases = File.readlines(hosts_file).grep(/#{host}/)

    # remove line-long comments
    aliases.reject!{ |line| line =~ /^#.*/ }

    # remove comments at end of line
    aliases.each { |line| line.sub!(/#.*/,'') }

    # combine, split again on whitespace
    aliases = aliases.join.split(/\s+/)
    
    # sort and remove duplicates
    aliases.sort.uniq!

    # don't include the hostname as an alias
    aliases.reject! { |item| item == host }

    # finally, remove anything in the regexp array
    regexps.each {|re| aliases.reject! {|item| item =~ re }}

    return aliases

  end

  # Prints the information in the dhcpd.conf format. Including aliases
  # if applicable (the hostname is found in the hosts file).
  def format(mac, host)
    # find the aliases for the host
    aliases = find_aliases(host)

    # change here for formatting tweaks
    puts
    puts "host #{host} {"
    puts "  hardware ethernet #{mac}"
    puts "  aliases #{aliases.join(" ")}" unless aliases.empty?
    puts "}"

  end

  # Add an exclusion rule for the aliases section. For instance, you could
  # exclude IP addresses, "localhost," or any other alias you want ignored.
  def add_rule(rule)
    raise ArgumentError unless rule.is_a?(Regexp)
    regexps << rule
  end

  # The main method in this class. Will read the ethers file, dig out the
  # host and MAC address information, and call the #format method for each 
  # pair. 
  #--
  # TODO: more robust matching for valid lines (regexps)
  # TODO: can ethers files have end of line comments?
  def parse_ethers_file
    # open ethers file and scan each line
    File.open(ethers_file).each do |line|
      
      # ignore comments and blank lines
      unless line =~ /^#.*/ or line =~ /^\s*$/

        # format and print each line in the specified format
        format(*line.split(/\s+/))

      end
    end
  end

end

# Configuration options and execution process.
if $0 == __FILE__
  e = EthersFile.new([])

  # the name of the files we're accessing
  e.ethers_file = "ethers"
  #e.hosts_file  = "/etc/hosts"

  # add rules for items that you want removed from the aliases list
  e.add_rule(/\b\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}\b/) # IPv4 addresses
  e.add_rule(/::/) # IPv6 addresses (hacky!)

  # start the parsing and print
  e.parse_ethers_file

end