tcpdump doesn’t capture traffic

Question

hi i,m new to tcpdump, i run it from linux terminal with this script but it only listens, it doesn't capture anything although it should, is there command like -k in wireshark , that makes tcpdump start capturing immediately ??

#!/bin/bash
tcpdump -i eth1 -s0 -c 700 -w ~/new.pcap "tcp"
sleep 3
iperf -c 192.168.1.2  &
sleep 6
killall tcpdump &
killall iperf

Answer 1

You don't send the tcpdump command to the background (no & at the end of the command), and thus the other commands will be executed only after tcpdump was stopped (I guess after you pressed CTRL-C)!

is there command like -k in wireshark , that makes tcpdump start capturing immediately ??

No, tcpdump starts to capture traffic, as soon as you start it.

BTW: If you add the & to the tcpdump command, and you still don't see any traffic, you are either listening on the wrong interface (eth1) or the iperf command does not generate any traffic.

Regards
Kurt