A Ruby 1.9.3 snippet that can set IP and FQDN on a Debian/Ubuntu system

brainwash

#!/usr/bin/env ruby
require 'ipaddr'

module Brainwash

  module Help
    extend self

    def examples
      <<-END

  EXAMPLES
  brainwash dhcp                          # Use DHCP
  brainwash 172.20.0.99                   # Use static IP
  brainwash host.example.com              # Set FQDN
  brainwash dhcp host.example.com         # Use DHCP and set FQDN
  brainwash 172.20.0.99 host.example.com  # Use static IP and set FQDN

      END
    end

    def only_debian_derivates
      <<-END

  Are you sure this is a Debian/Ubuntu system?

      END
    end

    def sudo_needed
      <<-END

  Please run me with super-user privileges.

      END
    end

    def no_interface_found
      <<-END

  No Network Interface found.

      END
    end
  end

  module OS
    extend self

    def debian?
      File.file?('/etc/lsb-release')
    end
  end

  module Params
    extend self

    def ip
      return :dhcp if ARGV[0] == 'dhcp'
      ip? ARGV[0]
    end

    def fqdn
      candidate = ARGV.last
      return nil if ip?(candidate)
      return candidate if candidate.split('.').size > 1
    end

    private

    def ip?(string)
      IPAddr.new(string) rescue nil
    end
  end

  module Interface
    extend self

    def identify
      %w{ eth0 eth1 }.detect { |name| exists?(name) }
    end

    def filename
      '/etc/network/interfaces'
    end

    def set(ip)
      interface = identify
      content = []
      content << "auto lo"
      content << "iface lo inet loopback"
      content << "auto #{interface}"
      if ip.is_a?(IPAddr)
        puts "Enabling static IP #{ip}..."
        content << "iface #{interface} inet static"
        content << "  address #{ip}"
        content << "  netmask 255.255.0.0"
        content << "  gateway 172.20.0.1"
        content << "  dns-nameservers 172.20.0.19"
        kill_dhclient! if dhclient?
      else
        puts "Enabling DHCP..."
        content << "iface #{interface} inet dhcp"
      end
      content << ''
      store! content.join("\n")
      restart!
    end

    private

    def store!(content)
      File.open(filename, 'w') { |file| file.write(content) }
    end

    def dhclient?
      %x{ps aux | grep dhclient3 | grep -v grep &> /dev/null}
    end

    def kill_dhclient!
      puts "Killing DHClient..."
      %x{killall -9 dhclient3 &> /dev/null}
    end

    def restart!
      puts "Restarting network..."
      %x{/etc/init.d/networking restart &> /dev/null}
    end

    def exists?(name)
      return name if %x{ifconfig #{name} &> /dev/null}
    end
  end

  module Hostname
    extend self

    def set(fqdn)
      hostname = fqdn.split('.').first
      set_hostname!(hostname)
      set_fqdn!(fqdn)
    end

    private

    def set_hostname!(name)
      puts "Setting hostname #{name}..."
      store! '/etc/hostname', name
    end

    def set_fqdn!(fqdn)
      hostname = fqdn.split('.').first
      puts "Setting FQDN #{fqdn}..."
      content = []
      content << '### BRAINWASH START ###'
      content << '127.0.0.1 localhost'
      content << "127.0.0.1 #{fqdn} #{hostname}"
      content << '::1 localhost ip6-localhost ip6-loopback'
      content << 'fe00::0 ip6-localnet'
      content << 'ff00::0 ip6-mcastprefix'
      content << 'ff02::1 ip6-allnodes'
      content << 'ff02::2 ip6-allrouters'
      content << 'ff02::3 ip6-allhosts'
      content << '### BRAINWASH END ###'
      content << ''
      store! '/etc/hosts', content.join("\n")
      restart!
    end

    def store!(path, content)
      File.open(path, 'w') { |file| file.write(content) }
    end

    def restart!
      puts "Restarting hostname..."
      %x{/etc/init.d/hostname restart &> /dev/null}
    end
  end

end

include Brainwash

if ARGV.empty?
  puts Help.examples
  exit
end

unless OS.debian?
  puts Help.only_debian_derivates
  exit 1
end

unless File.writable?(Interface.filename)
  puts Help.sudo_needed
  exit 2
end

if Params.fqdn
  Hostname.set Params.fqdn
end

if Params.ip
  unless Interface.identify
    puts Help.no_interface_found
    exit 3
  end

  Interface.set Params.ip
end

puts "Finished."