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/util/dns_resolver.rb

@@ -0,0 +1,148 @@
+# frozen_string_literal: true
+
+class DNSResolver
+
+  attr_reader :nameservers
+  attr_reader :timeout
+
+  def initialize(nameservers: nil, timeout: 5)
+    @nameservers = nameservers
+    @timeout = timeout
+  end
+
+  # Return all A records for the given name
+  #
+  # @param [String] name
+  # @return [Array<String>]
+  def a(name)
+    dns do |dns|
+      dns.getresources(name, Resolv::DNS::Resource::IN::A).map do |s|
+        s.address.to_s
+      end
+    end
+  end
+
+  # Return all AAAA records for the given name
+  #
+  # @param [String] name
+  # @return [Array<String>]
+  def aaaa(name)
+    dns do |dns|
+      dns.getresources(name, Resolv::DNS::Resource::IN::AAAA).map do |s|
+        s.address.to_s
+      end
+    end
+  end
+
+  # Return all TXT records for the given name
+  #
+  # @param [String] name
+  # @return [Array<String>]
+  def txt(name)
+    dns do |dns|
+      dns.getresources(name, Resolv::DNS::Resource::IN::TXT).map do |s|
+        s.data.to_s.strip
+      end
+    end
+  end
+
+  # Return all CNAME records for the given name
+  #
+  # @param [String] name
+  # @return [Array<String>]
+  def cname(name)
+    dns do |dns|
+      dns.getresources(name, Resolv::DNS::Resource::IN::CNAME).map do |s|
+        s.name.to_s.downcase
+      end
+    end
+  end
+
+  # Return all MX records for the given name
+  #
+  # @param [String] name
+  # @return [Array<Array<Integer, String>>]
+  def mx(name)
+    dns do |dns|
+      records = dns.getresources(name, Resolv::DNS::Resource::IN::MX).map do |m|
+        [m.preference.to_i, m.exchange.to_s]
+      end
+      records.sort do |a, b|
+        if a[0] == b[0]
+          [-1, 1].sample
+        else
+          a[0] <=> b[0]
+        end
+      end
+    end
+  end
+
+  # Return the effective nameserver names for a given domain name.
+  #
+  # @param [String] name
+  # @return [Array<String>]
+  def effective_ns(name)
+    records = []
+    dns do |dns|
+      parts = name.split(".")
+      (parts.size - 1).times do |n|
+        d = parts[n, parts.size - n + 1].join(".")
+
+        records = dns.getresources(d, Resolv::DNS::Resource::IN::NS).map do |s|
+          s.name.to_s
+        end
+
+        break if records.present?
+      end
+    end
+
+    records
+  end
+
+  # Return the hostname for a given IP address.
+  # Returns the IP address itself if no hostname can be determined.
+  #
+  # @param [String] ip_address
+  # @return [String]
+  def ip_to_hostname(ip_address)
+    dns do |dns|
+      dns.getname(ip_address)&.to_s
+    end
+  rescue Resolv::ResolvError
+    ip_address
+  end
+
+  private
+
+  def dns
+    Resolv::DNS.open(nameserver: @nameservers || []) do |dns|
+      dns.timeouts = [@timeout, @timeout / 2]
+      yield dns
+    end
+  end
+
+  class << self
+
+    # Return a resolver which will use the nameservers for the given domain
+    #
+    # @param [String] name
+    # @return [DNSResolver]
+    def for_domain(name)
+      resolver = new
+      nameservers = resolver.effective_ns(name)
+      ips = nameservers.map do |ns|
+        resolver.a(ns)
+      end.flatten.uniq
+      new(nameservers: ips)
+    end
+
+    # Return a local resolver to use for lookups
+    #
+    # @return [DNSResolver]
+    def local
+      @local ||= new
+    end
+
+  end
+
+end