How can I monitor network traffic for potential DDoS attacks using command-line based free tools?

Command-line tools like `tcpdump` (Linux/macOS) or `tshark` (command-line version of Wireshark) are excellent for this. You can use them to capture traffic, filter it (e.g., `tcpdump -i eth0 'tcp[tcpflags] & tcp-syn != 0'`), and count packets or connections to identify abnormal spikes.