Internal DNS on Amazon EC2

While looking for a way to name instances and update the /etc/hosts file, I came across Tim Dysinger’s blog entry.  It’s a ruby script that parses AWS metadata to get the internal IP address of the running instances in your AWS account and its respective keypair – perfect for /etc/hosts entries.  Obviously, the caveat here is that you need to generate 1 unique ssh private key for every instance you are running because key == internal dns name.  You’ll be managing a lot of ssh keys in the end.

This is perfect for situations where your instances interact with each other (load balancers in front of app servers with database servers at the back?).  It’s very handy calling your instances lb0, app23  or db3 instead of something like ec2-xxx-xxx-xxx-xxx.compute-1.amazonaws.com.  Automatic, too.  If you’re running the script as a cron job, your /etc/hosts file gets updated if you run new instances.

#!/usr/bin/env ruby
%w(optparse rubygems EC2 resolv pp).each { l require l }
options = {}
parser = OptionParser.new do p
  p.banner = "Usage: hosts [options]"
  p.on("-a", "--access-key USER", "The user's AWS access key ID.") do aki
    options[:access_key_id] = aki
  end
  p.on("-s",
       "--secret-key PASSWORD",
       "The user's AWS secret access key.") do sak
    options[:secret_access_key] = sak
  end
  p.on_tail("-h", "--help", "Show this message") {
    puts(p)
    exit
  }
  p.parse!(ARGV) rescue puts(p)
end
if options.key?(:access_key_id) and options.key?(:secret_access_key)
  puts "127.0.0.1 localhost"
  EC2::Base.new(options).describe_instances.reservationSet.item.each do r
    r.instancesSet.item.each do i
      if i.instanceState.name =~ /running/
        puts(Resolv::DNS.new.getaddress(i.privateDnsName).to_s +
             " #{i.keyName}.ec2 #{i.keyName}")
      end
    end
  end
else
  puts(parser)
  exit(1)
end

Just pass on your AWS access key and secret key as parameters, pipe it to /etc/hosts and you’re good to go.