test: add tests for DNSResolver

app/util/dns_resolver.rb

@@ -5,7 +5,7 @@ class DNSResolver
   attr_reader :nameservers
   attr_reader :timeout
 
-  def initialize(nameservers: nil, timeout: 5)
+  def initialize(nameservers: [], timeout: 5)
     @nameservers = nameservers
     @timeout = timeout
   end
@@ -115,7 +115,7 @@ def ip_to_hostname(ip_address)
   private
 
   def dns
-    Resolv::DNS.open(nameserver: @nameservers || []) do |dns|
+    Resolv::DNS.open(nameserver: @nameservers) do |dns|
       dns.timeouts = [@timeout, @timeout / 2]
       yield dns
     end

spec/util/dns_resolver_spec.rb

@@ -0,0 +1,85 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+RSpec.describe DNSResolver do
+  subject(:resolver) { described_class.new }
+
+  # Now, we could mock everything in here which would give us some comfort
+  # but I do think that we'll benefit more from having a full E2E test here
+  # so we'll test this using values which we know to be fairly static and
+  # that are within our control.
+
+  describe "#a" do
+    it "returns a list of IP addresses" do
+      expect(resolver.a("www.test.postalserver.io").sort).to eq ["1.2.3.4", "2.3.4.5"]
+    end
+  end
+
+  describe "#aaaa" do
+    it "returns a list of IP addresses" do
+      expect(resolver.aaaa("www.test.postalserver.io").sort).to eq ["2a00:67a0:a::1", "2a00:67a0:a::2"]
+    end
+  end
+
+  describe "#txt" do
+    it "returns a list of TXT records" do
+      expect(resolver.txt("test.postalserver.io").sort).to eq [
+        "an example txt record",
+        "another example"
+      ]
+    end
+  end
+
+  describe "#cname" do
+    it "returns a list of CNAME records" do
+      expect(resolver.cname("cname.test.postalserver.io")).to eq ["www.test.postalserver.io"]
+    end
+  end
+
+  describe "#mx" do
+    it "returns a list of MX records" do
+      expect(resolver.mx("test.postalserver.io")).to eq [
+        [10, "mx1.test.postalserver.io"],
+        [20, "mx2.test.postalserver.io"]
+      ]
+    end
+  end
+
+  describe "#effective_ns" do
+    it "returns the nameserver names that are authoritative for the given domain" do
+      expect(resolver.effective_ns("postalserver.io").sort).to eq [
+        "prestigious-honeybadger.katapultdns.com",
+        "the-cake-is-a-lie.katapultdns.com"
+      ]
+    end
+  end
+
+  describe "#ip_to_hostname" do
+    it "returns the hostname for the given IP" do
+      expect(resolver.ip_to_hostname("151.252.1.100")).to eq "ns1.katapultdns.com"
+    end
+  end
+
+  describe ".for_domain" do
+    it "finds the effective nameservers for a given domain and returns them" do
+      resolver = described_class.for_domain("test.postalserver.io")
+      expect(resolver.nameservers.sort).to eq ["151.252.1.100", "151.252.2.100"]
+    end
+  end
+
+  describe ".local" do
+    it "returns a resolver with no nameservers" do
+      resolver = described_class.local
+      expect(resolver.nameservers).to eq []
+    end
+  end
+
+  context "when using a resolver for a domain" do
+    subject(:resolver) { described_class.for_domain("test.postalserver.io") }
+
+    it "will not return domains that are not hosted on that server" do
+      expect(resolver.a("example.com")).to eq []
+    end
+  end
+end