refactor: refactor DNS resolution

This commit also adds some of tests for the Domain model. It was during the writing of these tests that the DNS resolution refactoring requirement became apparent.

lib/postal/mx_lookup.rb

@@ -1,36 +0,0 @@
-# frozen_string_literal: true
-
-module Postal
-  class MXLookup
-
-    class << self
-
-      def lookup(domain)
-        records = resolve(domain)
-        records = sort(records)
-        records.map { |m| m[1] }
-      end
-
-      private
-
-      def sort(records)
-        records.sort do |a, b|
-          if a[0] == b[0]
-            [-1, 1].sample
-          else
-            a[0] <=> b[0]
-          end
-        end
-      end
-
-      def resolve(domain)
-        Resolv::DNS.open do |dns|
-          dns.timeouts = [10, 5]
-          dns.getresources(domain, Resolv::DNS::Resource::IN::MX).map { |m| [m.preference.to_i, m.exchange.to_s] }
-        end
-      end
-
-    end
-
-  end
-end

lib/postal/received_header.rb

@@ -19,26 +19,13 @@ module Postal
         header = "by #{our_hostname} with #{method.to_s.upcase}; #{Time.now.utc.rfc2822}"
 
         if server.nil? || server.privacy_mode == false
-          hostname = resolve_hostname(ip_address)
+          hostname = DNSResolver.local.ip_to_hostname(ip_address)
           header = "from #{helo} (#{hostname} [#{ip_address}]) #{header}"
         end
 
         header
       end
 
-      private
-
-      def resolve_hostname(ip_address)
-        Resolv::DNS.open do |dns|
-          dns.timeouts = [10, 5]
-          begin
-            dns.getname(ip_address)
-          rescue StandardError
-            ip_address
-          end
-        end
-      end
-
     end
 
   end

lib/postal/smtp_sender.rb

@@ -225,7 +225,7 @@ module Postal
 
     def servers
       @options[:servers] || self.class.relay_hosts || @servers ||= begin
-        mx_servers = MXLookup.lookup(@domain)
+        mx_servers = DNSResolver.local.mx(@domain).map(&:last)
         if mx_servers.empty?
           mx_servers = [@domain] # This will be resolved to an A or AAAA record later
         end
@@ -243,16 +243,13 @@ module Postal
 
     def lookup_ip_address(type, hostname)
       records = []
-      Resolv::DNS.open do |dns|
-        dns.timeouts = [10, 5]
-        case type
-        when :a
-          records = dns.getresources(hostname, Resolv::DNS::Resource::IN::A)
-        when :aaaa
-          records = dns.getresources(hostname, Resolv::DNS::Resource::IN::AAAA)
-        end
+      case type
+      when :a
+        records = DNSResolver.local.a(hostname)
+      when :aaaa
+        records = DNSResolver.local.aaaa(hostname)
       end
-      records.first&.address&.to_s&.downcase
+      records.first&.to_s&.downcase
     end
 
     class << self