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Full Discussion: Port flow capture
Homework and Emergencies Emergency UNIX and Linux Support Port flow capture Post 302943623 by otheus on Monday 11th of May 2015 07:29:16 AM
Old 05-11-2015
Traditionally, you must specify a device. Therefore

Code:
tcpdump -i eth0 -w /tmp/syn.pcap 'tcp[tcpflags] & (tcp-syn) != 0'

But that assumes you want it from eth0. Maybe you want it from wlan0. But maybe your system currently has no interfaces that are up. That would explain why you're not seeing any tcp packets and are resorting to tcpdump in the first place.

Also, the output will be to that file so you won't know it's working right away. Try it without -w... until you get some output.
 

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tcp-env(1)                                                    General Commands Manual                                                   tcp-env(1)

NAME
tcp-env - set up TCP-related environment variables SYNOPSIS
tcp-env [ -rR ] [ -ttimeout ] program [ arg ... ] DESCRIPTION
The input for tcp-env must be a TCP connection. tcp-env finds out information about that connection, puts the information into several environment variables as described in tcp-environ(5), and runs program with the given arguments. Usually tcp-env is run from inetd. It might instead be run from another server that already sets up the right environment variables; if PROTO is set to TCP when tcp-env is invoked, tcp-env assumes that all the other variables are set up properly, and it does not check whether the input is a TCP connection. OPTIONS
-r (Default.) Attempt to obtain TCPREMOTEINFO from the remote host. -R Do not attempt to obtain TCPREMOTEINFO from the remote host. -ttimeout Give up on the TCPREMOTEINFO connection attempt after timeout seconds. Default: 30. SEE ALSO
tcp-environ(5), inetd(8) tcp-env(1)
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