dns_error_test_stub.rb

require 'test/unit'

class Test < Test::Unit::TestCase
  def test_resolv_error
    force_resolv_error do
      # do something
    end
  end

  private
  def force_resolv_error &block
    original_resolver = Resolv::DefaultResolver
    replace_resolver ErrorResolv.new
    begin
      block.call
    ensure
      replace_resolver original_resolver
    end
  end

  def replace_resolver new_resolver
    current = $VERBOSE
    $VERBOSE = nil # to ignore const redefined warning
    Resolv.const_set(:DefaultResolver, new_resolver)
    $VERBOSE = current
  end
end

class ErrorResolv
  PublicAPIs = [:getaddresses, :getaddress, :each_address, :getname, :getnames, :each_name]
  def raise_error *args
    raise Resolv::ResolvError
  end

  PublicAPIs.each {|method| alias_method method, :raise_error }
  private :raise_error
end