Passive DNS is not new but remains a very interesting component to have in your hunting arsenal. As defined by CIRCL, a passive DNS is “a database storing historical DNS records from various resources. The historical data is indexed, which makes it searchable for incident handlers, security analysts or researchers”. There are plenty of existing passive DNS services: CIRCL, VirusTotal, RiskIQ, etc. I’m using them quite often but, sometimes, they simply don’t have any record for a domain or an IP address I’m interested in. If you’re working for a big organization or a juicy target (depending on your business), why not operate your own passive DNS? You’ll collect data from your network that will represent the traffic of your own users.