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I'd love if there was a single command line tool for packet sniffing a single command in Linux. something like sniff dumpfile command so that you could just run the command you want to packet sniff in the terminal and get a dump of the packets somewhere else.

I'd like to dump / save / see only the network traffic of the single command that I enter, not all the TCP traffic on my single network interface. So if I was logged into my computer and had IRC running in the background, and I did sniff somefile wget http://www.google.com, I'd want to see all the network traffic that the wget command did to download http://www.google.com. I don't want 'somefile' to have the IRC network traffic confusing things.

There are lots of linux/unix commands that accept a different command and do something different. From sudo (run as superuser), nice change nice level, trickle (limit the bandwidth of a command)

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What is it, exactly, that you want to do? Do you want to check whether a certain command is run? Do you want to sniff network traffic? Because using the phrase 'packet sniffing a command' makes no sense to me at all... –  wzzrd Jun 22 '09 at 15:14
    
wzzrd, see expanded question –  Rory Jun 22 '09 at 15:18
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10 Answers

up vote 8 down vote accepted

There isn't any that I know of, but it theoretically shouldn't be hard to get something similar. Strace can be used to intercept networking syscalls.

# strace -f -e trace=network -s 10000 /usr/bin/command arguments

This will give you information about the data sent between the kernel and the process. The output of strace isn't exactly what you'd want. However, strace uses the ptrace syscall to intercept system calls. It might be possible to write a program to output the data a little more usefully.

Alternatively, you can also intercept the nice useful socket, bind and listen syscalls. It might be possible to write a small program that used ptrace on these calls and libpcap to dynamically change the capture filter every time a new socket is opened.

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It would be great if it output something that Wireshark could read. –  Brad Gilbert Jun 22 '09 at 18:10
    
Bear in mind that the strace described above is accurate for Solaris (and possibly other commercial Unices). On Linux, strace is a system call tracer, similar in operation and output to truss on Solaris or tusc on HP-UX. –  James F Jun 22 '09 at 18:20
    
I described strace on Linux. –  David Pashley Jun 22 '09 at 18:56
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Tracedump

Tracedump is a single application IP packet sniffer, which captures all TCP and UDP packets of a single Linux process.

Download and description here: http://mutrics.iitis.pl/tracedump

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1  
This should be the correct answer now. –  h0tw1r3 Jan 11 '13 at 16:01
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Try Wireshark -- the command will be tshark

  • tshark allows you to select filters on the traffic captured from an interface
  • use other utilities like the debian lsof to identify the kind of communication done by your application of interest.

Or, Do you really just want LSOF?


I don't think there is a tool that will dynamically keep filtering for all communication associated to a process. However, you could try tracking the process communications with tools like lsof and once you have a good filter that can isolate communication of that process from all other traffic running on your system, you could get the correct capture.

For, example, with wget usually the destination IP address is distinct from other process related traffic. Even if you take something like skype the destination port range is usually fixed for an instance.


This is a bit like the uncertainty principle. You can usually know what is going through a set of communication paths (with a sniffer filtering on identified group of flows), or where different communication links are being made (with lsof).

I would really like to know if both can be done for an application. I think it should be feasible. But, have not seen any tool do that yet.

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lsof is awesome, but I'd rather see what it is doing / sending / recieving. –  Rory Jun 22 '09 at 15:26
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Learn to use filter expressions.

Whilst this won't do the fancy trace stuff that you are asking for.

It will allow you to remove nearly all of the "confusing stuff like IRC" from the capture.

Additionally it's very useful to know the filter syntax for quick reference in the future.

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Specifically for a web browser / web page something like the Firebug plug-in for Firefox may give you some of the information you are looking for: http://getfirebug.com/net.html

For more general applications, you may need to use netstat to identify the port(s) used by the application and then Wireshark/tshark/dtrace with a filter to capture just that traffic. Not the one line answer you were looking for though...

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One idea, try VMWare

-setup a vm
-configure that vm to use a particular interface
-sniff on that interface from the host (its like a man in the middle attack)

If you isolate what network applications are running on that vm you might have your answer

A more ideal solution, I suppose, is to do what VMWare does in terms of how it specifies how it chooses an interface to talk over. I think its magic comes from the kernel modules it uses, in this case, probably vmnet kernel module.

To my knowledge applications are not aware for what interface they talk over and I believe this is by design; they shouldn't have to worry about such things.

Furthermore,
Perhaps a program already exists, I don't know. But if one was written you could call it nettrace (for example) and Usage could be like

nettrace program interface

then sniff the interface it uses and add routes (maybe it does the automatically) to your real interface

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yes that would be possible. However it'd be great if there was a simple and quick tool to do it,. –  Rory Jun 22 '09 at 16:07
    
Maybe it is time for this simple tool? I wonder if it would be that difficult, probably vmnet mod could be a good starting point (though I don't know if there would be licensing issues). Perhaps (a hack) just use vmnet mod. I know I am interested in questions like this often. –  rev Jun 22 '09 at 17:13
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Assuming that you are the only person on the box attempting to connect to google at the time, I'd think something like this should do the trick:

tcpdump -w <outfile> -i <interface> tcp dst host www.google.com and dst port 80

If you are not the only connection attempting to connect to google on the box, then if you can identify the IP/port you are connecting from you can also specify the src port/src ip.

Identifying the src port may be a problem unless you can specify it on whatever client you use. I'm not sure if you can with wget.

I strongly suspect that you can specify both src and dst ports with netcat so if it was really google you were interested in, you could do your GET (manually) through netcat.

Of course the man page will give you specifics

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Maybe this script will do what you want with appropriate modifications to the tshark command:

#!/bin/bash

# Start tshark in the background before we run the commsnd to be sniffed.
# Add more options to tshark, as appropriate to your command....
setsid tshark  -w dump -i eth0 tcp port 8080 >/dev/null 2>&1  &

sleep 2

wget www.google.com

# tshark keeps running if you don't kill it. 
# This kills all other tsharks that may be running.
pkill -9 tshark

Read the dump file later:

tshark -r dump
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You're capturing port 8080 but generating traffic on port 80; this won't do quite what you're intending. –  James F Jun 22 '09 at 18:18
    
James: My proxy connects to 8080, thats how I ran the script. I should have changed it 80 for the purpose of this post. –  Not Now Jun 22 '09 at 18:30
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Dtrace should allow this, although I don't know whether it's quite made it into Linux yet.

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The tcpdump man page and many web sites give in-depth examples of filters, and there are even a few online repositories of tcpdump filter expressions. It should be able to do almost anything you can dream of, assuming that you know something about the network traffic (source, destination, ports, protocols, etc.) beyond just what program is generating it.

If you're working on a server or headless box, you can always have tcpdump write a dump file and then open it in Wireshark on your workstation and get advanced filtering and a graphical interface.

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Erm, which switch to tcpdump command would limit it to a single application that generates the traffic? –  Milan Babuškov Jun 22 '09 at 17:36
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