Passive scanning is used when the target can not be aware of the information collected, attention of experienced engineers will spend more than half the penetration time throughout the testing process on the information collected, the amount of information is too large, you need to automate information gathering tool.
First, the reference "kali linux2 Network Penetration Testing Practice Guidelines" Li Huafeng People Post Press, 2018
1.Maltego
https://www.cnblogs.com/youcanch/articles/5671227.html It is appropriate introduction, the main sign up
2.Recon-NG
3. Search engine ZoomEye