netstat

To install netstat on Ubuntu/Debian, run

apt install net-tools

To see all connections, run

netstat -antp

To see all listening connections, run

netstat -lntp

To see routing table

netstat -r

To monitor connections to port 80 and 443

netstat -ctupne | grep ESTABLISHED | cut -c45-1000 | grep -E ':443|:80' > connections.txt
cat connections.txt | cut -c1-50 | sort | uniq -c | sort -n > uniq.txt

To see open ports on Windows

netstat -aon

• How to find out what service is using a certain port?