Default to system nameservers if a Resolver is initialized without any.

README.md

@@ -26,7 +26,7 @@ Here is a simple example showing how to use the resolver:
 
 ``` ruby
 Async::Reactor.run do
-	resolver = Async::DNS::Resolver.new([[:udp, "8.8.8.8", 53], [:tcp, "8.8.8.8", 53]])
+	resolver = Async::DNS::Resolver.new()
 
 	addresses = resolver.addresses_for("www.google.com.")
 
@@ -35,6 +35,16 @@ end
 # [#<Resolv::IPv4 202.124.127.240>, #<Resolv::IPv4 202.124.127.216>, #<Resolv::IPv4 202.124.127.223>, #<Resolv::IPv4 202.124.127.227>, #<Resolv::IPv4 202.124.127.234>, #<Resolv::IPv4 202.124.127.230>, #<Resolv::IPv4 202.124.127.208>, #<Resolv::IPv4 202.124.127.249>, #<Resolv::IPv4 202.124.127.219>, #<Resolv::IPv4 202.124.127.218>, #<Resolv::IPv4 202.124.127.212>, #<Resolv::IPv4 202.124.127.241>, #<Resolv::IPv4 202.124.127.238>, #<Resolv::IPv4 202.124.127.245>, #<Resolv::IPv4 202.124.127.251>, #<Resolv::IPv4 202.124.127.229>]
 ```
 
+You can also specify custom DNS servers:
+
+``` ruby
+resolver = Async::DNS::Resolver.new(Async::DNS::System.standard_connections(['8.8.8.8']))
+
+# or
+
+resolver = Async::DNS::Resolver.new([[:udp, "8.8.8.8", 53], [:tcp, "8.8.8.8", 53]])
+```
+
 ### Server
 
 Here is a simple example showing how to use the server:

lib/async/dns/resolver.rb

@@ -19,6 +19,7 @@
 # THE SOFTWARE.
 
 require_relative 'handler'
+require_relative 'system'
 
 require 'securerandom'
 require 'async'
@@ -46,8 +47,8 @@ class Resolver
 		# Servers are specified in the same manor as options[:listen], e.g.
 		#   [:tcp/:udp, address, port]
 		# In the case of multiple servers, they will be checked in sequence.
-		def initialize(endpoints, origin: nil, logger: Console.logger, timeout: DEFAULT_TIMEOUT)
-			@endpoints = endpoints
+		def initialize(endpoints = nil, origin: nil, logger: Console.logger, timeout: DEFAULT_TIMEOUT)
+			@endpoints = endpoints || System.nameservers
 
 			@origin = origin
 			@logger = logger

spec/async/dns/resolver_spec.rb

@@ -101,4 +101,13 @@
 
 		expect(addresses.size).to be > 0
 	end
+
+	it "should default to system resolvers" do
+		resolver = Async::DNS::Resolver.new()
+
+		response = resolver.query('google.com')
+
+		expect(response.class).to be == Async::DNS::Message
+		expect(response.answer.size).to be > 0
+	end
 end