Index: src/asyncdns.cr
==================================================================
--- src/asyncdns.cr
+++ src/asyncdns.cr
@@ -1,7 +1,8 @@
 require "./query"
 require "./resolver"
 
 # AsyncDNS is an asynchronous DNS resolver.
+# It is a port of ObjFW's <https://objfw.nil.im> OFDNSResolver & friends.
 module AsyncDNS
   VERSION = "0.1.0"
 end

Index: src/query.cr
==================================================================
--- src/query.cr
+++ src/query.cr
@@ -1,12 +1,11 @@
 require "./dns_class"
 require "./rr_type"
 
 module AsyncDNS
   class Query
-    getter :domain_name, :dns_class, :rr_type
+    getter :name, :class, :type
 
-    def initialize(@domain_name : String, @dns_class : DNSClass,
-                   @rr_type : RRType)
+    def initialize(@name : String, @class : DNSClass, @type : RRType)
     end
   end
 end

Index: src/resolver.cr
==================================================================
--- src/resolver.cr
+++ src/resolver.cr
@@ -1,13 +1,50 @@
 require "./rr"
 require "./settings"
+require "./response"
 
 module AsyncDNS
   class Resolver
     getter :settings
-    setter :settings
+
+    enum Error
+      NO_NAME_SERVER
+    end
+
+    private struct Context
+      def initialize(@id : UInt16, @settings : Settings,
+                     @block : Response | Error ->)
+      end
+    end
 
     def initialize
       @settings = Settings.new
+      @queries = Hash(UInt16, Context).new
+      @tcp_queries = Hash(Socket, Context).new
+    end
+
+    def resolve(query, &block : Response | Error ->)
+      id : UInt16
+      while true
+        id = Random::Secure.rand(UInt16::MIN..UInt16::MAX)
+        break unless @queries.has_key?(id)
+      end
+
+      if @settings.nameservers.empty?
+        yield Error::NO_NAME_SERVER
+        return
+      end
+
+      send(Context.new(id, settings.dup, block))
+    end
+
+    def send(context : Context)
+      @queries[context.@id] = context
+
+      # TODO
+    end
+
+    def stop
+      # TODO
     end
   end
 end

ADDED   src/response.cr
Index: src/response.cr
==================================================================
--- src/response.cr
+++ src/response.cr
@@ -0,0 +1,11 @@
+require "./rr"
+
+module AsyncDNS
+  class Response
+    property :name, :answer, :authority, :additional
+
+    def initialize(@name : String, @answer : Array(RR), @authority : Array(RR),
+                   @additional : Array(RR))
+    end
+  end
+end

Index: src/settings.cr
==================================================================
--- src/settings.cr
+++ src/settings.cr
@@ -1,14 +1,17 @@
 module AsyncDNS
   class Settings
     @local_domain : String?
 
-    property static_hosts, nameservers, local_domain, search_domains, uses_tcp
-    getter timeout, max_attempts, abs_num_dots
+    property :static_hosts, :nameservers, :local_domain, :search_domains,
+      :uses_tcp, :reload_period
+    getter :timeout, :max_attempts, :abs_num_dots
 
     def timeout=(timeout)
-      raise ArgumentError.new("timeout must be positive") if timeout < 0
+      if timeout < Time::Span.zero
+        raise ArgumentError.new("timeout must be positive")
+      end
 
       @timeout = timeout
     end
 
     def max_attempts=(max_attempts)

Index: tests/example.cr
==================================================================
--- tests/example.cr
+++ tests/example.cr
@@ -1,9 +1,10 @@
 require "../src/asyncdns"
 
-AsyncDNS::Query.new("crystal-lang.org", AsyncDNS::DNSClass::IN,
+AsyncDNS::RR::A.new("crystal-lang.org", Socket::IPAddress.new("127.0.0.1", 0),
+  1234)
+
+query = AsyncDNS::Query.new("crystal-lang.org", AsyncDNS::DNSClass::IN,
   AsyncDNS::RRType::A)
 
 resolver = AsyncDNS::Resolver.new
-
-AsyncDNS::RR::A.new("crystal-lang.org", Socket::IPAddress.new("127.0.0.1", 0),
-  1234)
+resolver.resolve(query) { |response| p response }