Passive reconnaissance is the process of collecting information in a covert manner about an intended target without the target knowing what is occurring. Mainly is done searching information about the target on the Internet (google, linkedin, etc) and also searching for metadata (i.e. domain registers information, OSINT tools, etc).
Sniff network data (inside the target network):
tcpdump -X -i $IFACE
- Maltego https://www.paterva.com/web7/downloads.php
- theHarvester https://gbhackers.com/theharvester-information-gathering-tool/
theHarvester -d $DOMAIN -l 300 -b google