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.

app/models/concerns/has_dns_checks.rb

@@ -43,8 +43,8 @@ module HasDNSChecks
   #
 
   def check_spf_record
-    result = resolver.getresources(name, Resolv::DNS::Resource::IN::TXT)
-    spf_records = result.map(&:data).grep(/\Av=spf1/)
+    result = resolver.txt(name)
+    spf_records = result.grep(/\Av=spf1/)
     if spf_records.empty?
       self.spf_status = "Missing"
       self.spf_error = "No SPF record exists for this domain"
@@ -73,8 +73,7 @@ module HasDNSChecks
 
   def check_dkim_record
     domain = "#{dkim_record_name}.#{name}"
-    result = resolver.getresources(domain, Resolv::DNS::Resource::IN::TXT)
-    records = result.map(&:data)
+    records = resolver.txt(domain)
     if records.empty?
       self.dkim_status = "Missing"
       self.dkim_error = "No TXT records were returned for #{domain}"
@@ -104,8 +103,7 @@ module HasDNSChecks
   #
 
   def check_mx_records
-    result = resolver.getresources(name, Resolv::DNS::Resource::IN::MX)
-    records = result.map(&:exchange)
+    records = resolver.mx(name).map(&:last)
     if records.empty?
       self.mx_status = "Missing"
       self.mx_error = "There are no MX records for #{name}"
@@ -134,8 +132,7 @@ module HasDNSChecks
   #
 
   def check_return_path_record
-    result = resolver.getresources(return_path_domain, Resolv::DNS::Resource::IN::CNAME)
-    records = result.map { |r| r.name.to_s.downcase }
+    records = resolver.cname(return_path_domain)
     if records.empty?
       self.return_path_status = "Missing"
       self.return_path_error = "There is no return path record at #{return_path_domain}"