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/domain.rb

@@ -77,7 +77,9 @@ class Domain < ApplicationRecord
     verified_at.present?
   end
 
-  def verify
+  def mark_as_verified
+    return false if verified?
+
     self.verified_at = Time.now
     save!
   end
@@ -94,6 +96,8 @@ class Domain < ApplicationRecord
   end
 
   def dkim_key
+    return nil unless dkim_private_key
+
     @dkim_key ||= OpenSSL::PKey::RSA.new(dkim_private_key)
   end
 
@@ -114,28 +118,37 @@ class Domain < ApplicationRecord
   end
 
   def dkim_record
+    return if dkim_key.nil?
+
     public_key = dkim_key.public_key.to_s.gsub(/-+[A-Z ]+-+\n/, "").gsub(/\n/, "")
     "v=DKIM1; t=s; h=sha256; p=#{public_key};"
   end
 
   def dkim_identifier
+    return nil unless dkim_identifier_string
+
     Postal.config.dns.dkim_identifier + "-#{dkim_identifier_string}"
   end
 
   def dkim_record_name
-    "#{dkim_identifier}._domainkey"
+    identifier = dkim_identifier
+    return if identifier.nil?
+
+    "#{identifier}._domainkey"
   end
 
   def return_path_domain
     "#{Postal.config.dns.custom_return_path_prefix}.#{name}"
   end
 
-  def nameservers
-    @nameservers ||= get_nameservers
-  end
-
+  # Returns a DNSResolver instance that can be used to perform DNS lookups needed for
+  # the verification and DNS checking for this domain.
+  #
+  # @return [DNSResolver]
   def resolver
-    @resolver ||= Postal.config.general.use_local_ns_for_domains? ? Resolv::DNS.new : Resolv::DNS.new(nameserver: nameservers)
+    return DNSResolver.local if Postal.config.general.use_local_ns_for_domains?
+
+    @resolver ||= DNSResolver.for_domain(name)
   end
 
   def dns_verification_string
@@ -145,32 +158,14 @@ class Domain < ApplicationRecord
   def verify_with_dns
     return false unless verification_method == "DNS"
 
-    result = resolver.getresources(name, Resolv::DNS::Resource::IN::TXT)
-    if result.map { |d| d.data.to_s.strip }.include?(dns_verification_string)
+    result = resolver.txt(name)
+
+    if result.include?(dns_verification_string)
       self.verified_at = Time.now
-      save
-    else
-      false
+      return save
     end
-  end
 
-  private
-
-  def get_nameservers
-    local_resolver = Resolv::DNS.new
-    ns_records = []
-    parts = name.split(".")
-    (parts.size - 1).times do |n|
-      d = parts[n, parts.size - n + 1].join(".")
-      ns_records = local_resolver.getresources(d, Resolv::DNS::Resource::IN::NS).map { |s| s.name.to_s }
-      break if ns_records.present?
-    end
-    return [] if ns_records.blank?
-
-    ns_records = ns_records.map { |r| local_resolver.getresources(r, Resolv::DNS::Resource::IN::A).map { |s| s.address.to_s } }.flatten
-    return [] if ns_records.blank?
-
-    ns_records
+    false
   end
 
 end