fix(client) always inject localhost

Treat the localhost name special by always injecting it, according
to RFC 6761: Section 6.3.3

fixes #50

README.md

@@ -42,6 +42,7 @@ Versioning is strictly based on [Semantic Versioning](https://semver.org/)
 - Added: a new option `validTtl` that, if set, will forcefully override the
   `ttl` value of any valid answer received. [Issue 48](https://github.com/Kong/lua-resty-dns-client/issues/48).
 - Fix: remove multiline log entries, now encoded as single-line json. [Issue 52](https://github.com/Kong/lua-resty-dns-client/issues/52).
+- Fix: always inject a `localhost` value, even if not in `/etc/hosts`. [Issue 54](https://github.com/Kong/lua-resty-dns-client/issues/54).
 
 ### 2.1.0 (21-May-2018) Fixes
 

spec/client_spec.lua

@@ -118,6 +118,63 @@ describe("DNS client", function()
       assert.are.equal(#client.getcache(), 0) -- no hosts file record should have been imported
     end)
 
+    describe("inject localhost:", function()
+
+      it("if absent", function()
+        local result, err, record
+        result, err = assert(client.init({
+            nameservers = { "8.8.8.8:53" },
+            resolvConf = {},
+            hosts = {},
+          }))
+        assert.is.True(result)
+        assert.is.Nil(err)
+        record = client.getcache():get("28:localhost")
+        assert.equal("[::1]", record[1].address)
+        record = client.getcache():get("1:localhost")
+        assert.equal("127.0.0.1", record[1].address)
+      end)
+
+      it("not if ipv4 exists", function()
+        local result, err, record
+        result, err = assert(client.init({
+            nameservers = { "8.8.8.8:53" },
+            resolvConf = {},
+            hosts = {"1.2.3.4 localhost"},
+          }))
+        assert.is.True(result)
+        assert.is.Nil(err)
+
+        -- IPv6 is not defined
+        record = client.getcache():get("28:localhost")
+        assert.is_nil(record)
+
+        -- IPv4 is not overwritten
+        record = client.getcache():get("1:localhost")
+        assert.equal("1.2.3.4", record[1].address)
+      end)
+
+      it("not if ipv6 exists", function()
+        local result, err, record
+        result, err = assert(client.init({
+            nameservers = { "8.8.8.8:53" },
+            resolvConf = {},
+            hosts = {"::1:2:3:4 localhost"},
+          }))
+        assert.is.True(result)
+        assert.is.Nil(err)
+
+        -- IPv6 is not overwritten
+        record = client.getcache():get("28:localhost")
+        assert.equal("[::1:2:3:4]", record[1].address)
+
+        -- IPv4 is not defined
+        record = client.getcache():get("1:localhost")
+        assert.is_nil(record)
+      end)
+
+    end)
+
   end)
 
 

src/resty/dns/client.lua

@@ -485,6 +485,14 @@ _M.init = function(options)
     hosts = {}
   end
 
+  -- treat `localhost` special, by always defining it, RFC 6761: Section 6.3.3
+  if not hosts.localhost then
+    hosts.localhost = {
+      ipv4 = "127.0.0.1",
+      ipv6 = "[::1]",
+    }
+  end
+
   -- Populate the DNS cache with the hosts (and aliasses) from the hosts file.
   local ttl = 10*365*24*60*60  -- use ttl of 10 years for hostfile entries
   for name, address in pairs(hosts) do