Using iproute2 you can force replies to come back from the same interface that they are received. [by simon_matthews]
I want to be able to send IP packets from a userspace program and receive responses on a TUN interface.
While reading nmap man pages, I read for the -sn option:
The default host discovery done with -sn consists of an ICMP echo request,
TCP SYN to port 443, TCP ACK to port 80, and an ICMP timestamp request
I don't understand well the purpose of performing a TCP ACK on port 80.
Since the TCP stack of the targeted server would not process any SYN, it would just drop the received ACK pack