When you develop components in a server-client architecture, there is a moment when you deploy them and nothing works. This is when you want to see if packets are really travelling in the network:
tcpdump port 5432 -i lo -X-i is for specifing an interface, -X - ??? well, I need to check the man pages.
And also you would want to see if the ports are really open:
netstat -lpnt(install it with apt install net-tools)