Capture only TCP SYN packets tcpdump
Catch only connection-initiating SYNs (no SYN-ACK) to spot scans or backlog floods
tcpdump -nn 'tcp[tcpflags] & (tcp-syn|tcp-ack) == tcp-syn'
more tcpdump
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Capture SYN and FIN flags
tcpdump -nn 'tcp[tcpflags] & (tcp-syn|tcp-fin) != 0'
Host filter excluding SSH noise
tcpdump -nn -i any host <ip> and not port 22
Full payload hex+ascii dump
tcpdump -nn -s 0 -X -i any port <port>
Ring buffer capture by size
tcpdump -nn -i any -s 0 -C 100 -W 5 -w /tmp/cap.pcap
Time-rotated capture files
tcpdump -nn -i any -G 3600 -w '/tmp/cap-%Y%m%d-%H%M%S.pcap'