Port Scan with Spoofed IP Addresses

The Nmap book[1] covers this and a lot of other really cool tricks it can do. I highly recommend reading it cover-to-end if you use nmap at all.

Related to spoofed IP scanning, check out Zombie Scans[2]

[1] https://nmap.org/book/toc.html

[2] https://nmap.org/book/idlescan.html#scan-methods-idle-scan-f...