Caching DNS queries in Ruby

Question

We are using the rest-client gem in Ruby for automated testing of our REST API. However I noticed that on every single request it makes, it also does a DNS lookup for the host name. On the local environment if "localhost" is used the tests run fast, but if the proper hostname is used they take 2.5x the time, performing a huge number of DNS lookups.

I believe this issue is not related to rest-client in particular, but the base Ruby networking. I tried requiring 'resolv' and 'resolv-replace' but they did not help. 'dig' reports that the DNS query has a TTL of 1 hour.

Is there a way to make Ruby cache DNS requests? I could change the code to explicitly use the IP address, but that's the wrong place to fix the issue.

I'm running Ubuntu 12.04 and Ruby 1.9.3.

Answer 1

You can use the dnsruby gem to resolve the name to an address, and then use the address in your calls.

#! /usr/bin/env ruby

# Gets the IP address of a host.

require 'dnsruby'  # gem install dnsruby first, of course

def hostname_to_ip_addr(host_name)
  query = Dnsruby::Message.new(host_name)
  response = Dnsruby::Resolver.new.send_message(query)
  response.answer[1].address
end

host_name = 'cnn.com'
ip_addr = hostname_to_ip_addr(host_name)
puts("Host name: #{host_name}, IP address: #{ip_addr}")

^{original code from this Gist}

Answer 2

I got to this question looking for ruby dns caching and how resolv.rb might use the TTL to cache dns requests.

Discovering the TTL of a dns record is a bit hidden in the api of resolv.rb but it looks a little something like this:

def get_ip(hostname)
  dns = Resolv.new
  redis = Redis.new # storing in redis
  ip = redis.get("ip:#{hostname}") 
  return ip unless ip.nil?
  begin
    resource = dns.getresource(hostname, Resolv::DNS::Resource::IN::A)
  rescue Resolv::ResolvError
    return false
  end
  # storing in redis for only as long as the TTL allows
  redis.setex("ip:#{hostname}", resource.address.ttl, resource.address.to_s)
  resource.address.to_s # IP address as string
end

Gist

Note: uses Redis as the cache.