fluentd source code for explain dns cache behavior.

out_forward.rb

      def resolved_host
        case @expire_dns_cache
        when 0
          # cache is disabled
          return resolve_dns!

        when nil
          # persistent cache
          return @resolved_host ||= resolve_dns!

        else
          now = Engine.now
          rh = @resolved_host
          if !rh || now - @resolved_time >= @expire_dns_cache
            rh = @resolved_host = resolve_dns!
            @resolved_time = now
          end
          return rh
        end
      end