add support for ExternalName service type in dynamic mode

rootfs/etc/nginx/lua/util/dns.lua

@@ -0,0 +1,70 @@
+local resolver = require("resty.dns.resolver")
+local lrucache = require("resty.lrucache")
+local configuration = require("configuration")
+local util = require("util")
+
+local _M = {}
+local CACHE_SIZE = 10000
+local MAXIMUM_TTL_VALUE = 2147483647 -- maximum value according to https://tools.ietf.org/html/rfc2181
+
+local cache, err = lrucache.new(CACHE_SIZE)
+if not cache then
+  return error("failed to create the cache: " .. (err or "unknown"))
+end
+
+local function a_records_and_max_ttl(answers)
+  local addresses = {}
+  local ttl = MAXIMUM_TTL_VALUE -- maximum value according to https://tools.ietf.org/html/rfc2181
+
+  for _, ans in ipairs(answers) do
+    if ans.address then
+      table.insert(addresses, ans.address)
+      if ttl > ans.ttl then
+        ttl = ans.ttl
+      end
+    end
+  end
+
+  return addresses, ttl
+end
+
+function _M.resolve(host)
+  local cached_addresses = cache:get(host)
+  if cached_addresses then
+    ngx.log(ngx.INFO, string.format("addresses %s for host %s was resolved from cache", table.concat(cached_addresses, ", "), host))
+    return cached_addresses
+  end
+
+  local r, err = resolver:new{
+    nameservers = util.deepcopy(configuration.nameservers),
+    retrans = 5,
+    timeout = 2000,  -- 2 sec
+  }
+
+  if not r then
+    ngx.log(ngx.ERR, "failed to instantiate the resolver: " .. tostring(err))
+    return { host }
+  end
+
+  local answers, err, _tries = r:query(host, { qtype = r.TYPE_A }, {})
+  if not answers then
+    ngx.log(ngx.ERR, "failed to query the DNS server: " .. tostring(err))
+    return { host }
+  end
+
+  if answers.errcode then
+    ngx.log(ngx.ERR, string.format("server returned error code: %s: %s", answers.errcode, answers.errstr))
+    return { host }
+  end
+
+  local addresses, ttl = a_records_and_max_ttl(answers)
+  if #addresses == 0 then
+    ngx.log(ngx.ERR, "no A record resolved")
+    return { host }
+  end
+
+  cache:set(host, addresses, ttl)
+  return addresses
+end
+
+return _M