Ruby DNS Testing - First Glance

DNS is one of those ‘small config change here, errors a long way over there later’ technologies that always leaves me a little worried about the knock on effect of my changes. As a simple, coarse, safeguard at work we use Nagios to check that a canary record in each zone can be resolved from each DNS server. It’s far from a perfect solution but it does catch some of the bigger errors and typos.

In order to beef up this safety net (and encourage me to spend time using a language other than Perl) I’ve been investigating some of the testing options available in ruby, namely RSpec and Cucumber. I have to say the testing libraries themselves are actually nice to use and easy to pick up even for a non-rubyists like me. On the other hand I quickly developed a strong dislike of the ruby resolv library that actually does the DNS queries. The lack of decent tutorials or documentation for anything beyond the very basic uses and the (to me) very awkward API nearly had me running back to the safety of Net::DNS , a mature and widely used perl module. There is a ruby port that I’ll have a look at in the future

For the testing itself I started writing RSpec stories against my own DNS and found the API easy to use. Testing existing configs against local policies is simple - for example all our domains should list at least three name servers -

it "should have at least 3 NS records" do
  @resolver =
  @nameservers = @resolver.getresources( domain, Resolv::DNS::Resource::IN::NS)
  @nameservers.should have_at_least(3).items

After doing some more scut work and testing other record types I moved onwards and upwards to Cucumber. I’m not sure I’d be able to gift someone else with writing the scenarios but they’ll be easier to show business people than raw rspec stories. They’ll also be very handy in migration meetings (an executable todo list), if it’s not on the page it’s not getting done.

Feature: Mass DNS Resolution
  In order to present a consistent brand image
  As a System Administrator
  I want to ensure no domains point away from our main IP

  Scenario Outline: Resolve a name to a number
    Given a hostname of <hostname>
    Then I should see the IP address

     | hostname        |
     |     |
     | |

Next time I work somewhere with ISO 27xxx compliance requirements I’ll see if the controls can be written like this and have automatic verification run from them. Cucumber is a little wordy for my tastes but I can see where that could be a strength when presenting to the right audience (such as compliance auditors).

Next time we have a set of DNS migrations I’ll be using at least one of these tools to write before and after test cases to ensure nothing gets missed or slips through the cracks. Once I’ve been through a couple I’ll write up in more detail what we end up with.