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Operating Systems Linux Debian tcpdump filter (mis)behaviour Post 302505349 by Hollinch on Wednesday 16th of March 2011 06:33:14 PM
Old 03-16-2011
tcpdump filter (mis)behaviour

Hello, I am a bit puzzled. I am trying to capture data using tcpdump on a bonded interface, which works fine until I add a filter, then nothing is seen nor captured by libpcap/tcpump.

I have interfaces eth3 and eth4 bonded to bond0 because I am using a tap in a firewall connection to monitor all in and outgoing traffic.

All is fine if I just run tcpdump on the bond0 interface - all traffic is correctly shown. As soon as I specify a filter nothing seems to be seen nor captured anymore.

Some examples of command lines I tried:
Code:
/usr/local/sbin/tcpdump -i bond0 -nn tcp port 80
/usr/local/sbin/tcpdump -i bond0 -nn tcp port http



For example, without filters
output is shown as here (and http traffic was seen to be present):

Code:
root@lins01:~# /usr/local/sbin/tcpdump -i bond0 -nn
tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
listening on bond0, link-type EN10MB (Ethernet), capture size 96 bytes
23:07:30.871053 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 2019718192, win 382, options [nop,nop,TS val 1091628665 ecr 724203], length 112
23:07:30.871291 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 1, win 382, options [nop,nop,TS val 1091628665 ecr 724203], length 112
23:07:30.871540 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 1, win 382, options [nop,nop,TS val 1091628665 ecr 724203], length 192
23:07:30.871553 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 1, win 382, options [nop,nop,TS val 1091628665 ecr 724203], length 192
23:07:30.892570 IP 192.168.192.1.58229 > 192.168.192.239.22: Flags [.], ack 112, win 6032, options [nop,nop,TS val 724205 ecr 1091628665], length 0
23:07:30.892773 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 1, win 382, options [nop,nop,TS val 1091628670 ecr 724205], length 384
23:07:30.892780 IP 192.168.192.239.22 > 192.168.192.1.58229: Flags [P.], ack 1, win 382, options [nop,nop,TS val 1091628670 ecr 724205], length 192

With a valid filter specified it sits forever, and finally when I terminate tcpdump it shows 0 packets captured, 0 packets received by filter and 0 packets dropped by kernel. As if no traffic of the specified nature was present, while I am certain it is.

The OS is Debian 5.0.8, with tcpdump version 3.9.8 and libpcap version 0.9.8. I also tried to upgrade to tcpdump 4.0.0 and libpcap 1.0.0, but the result is the same.

I tried to remove the bond and just monitor on one of the individual interfaces, but this still did not produce any results with filters specified (without filters I correctly see half of my traffic, send or receive, depending on which interface I select).

Anyone has any suggestions what to try to get filters working?

Many thanks in advance for your help.


 

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DRIFTNET(1)						      General Commands Manual						       DRIFTNET(1)

NAME
driftnet - capture images from network traffic and display them in an X window; optionally, capture audio streams and play them. SYNOPSIS
driftnet [options] [filter code] DESCRIPTION
Driftnet watches network traffic, and picks out and displays JPEG and GIF images for display. It is an horrific invasion of privacy and shouldn't be used by anyone anywhere. It has been described as `a graphical tcpdump(8)', `EtherPeg for Unix', and called all sorts of nasty names by people on Freshmeat. It is also possible to use driftnet to capture MPEG audio data from the network and play it through a player such as mpg123(1). Images may be saved by clicking on them. OPTIONS
-h Print a summary of usage. -v Print additional details of packets captured to the terminal. -b Beep when a new image is displayed. -i interface Listen to packets on interface. By default, driftnet will try to pick up traffic on all interfaces, but this does not work with all versions of pcap(3); on such systems, an interface must be specified. On some systems, driftnet can only use promiscuous mode if an interface is specified. -f file Instead of listening on an interface, read captured packets from a pcap(3); dump file; file can be a named pipe for use with Kismet or similar. -p Do not put the interface into promiscuous mode. -a Operate in `adjunct mode', where driftnet gathers images for use by another program, such as Jamie Zawinski's webcollage. In this mode, no window is displayed; images are captured and saved in a temporary directory, and their names written on standard output. -m number In adjunct mode, silently drop images if there are more than number in the temporary directory. It is assumed that another process will delete images which it has processed. -x prefix The filename prefix to use when saving images, by default `driftnet-'. -d directory Use directory to store temporary files. Driftnet will clear this directory of its own temporary files on exit, but will not delete the directory or any other files. -s Attempt to capture streamed audio data from the network, and either play it or, in adjunct mode, save it in files. At present this only works with MPEG data. -S Capture streamed audio data only, ignoring images. -M command Use the named command to play MPEG audio data. The command, which is executed using the shell, should accept MPEG frames on standard input. The default is `mpg123 -'. filter code Additional filter code to restrict the packets captured, in the libpcap syntax. User filter code is evaluated as `tcp and (filter code)'. SEE ALSO
tcpdump(8), pcap(3), webcollage(1), mpg123(1), http://www.etherpeg.org/, http://freshmeat.net/projects/driftnet/, http://www.ex-parrot.com/~chris/driftnet/. AUTHOR
Chris Lightfoot <chris@ex-parrot.com> VERSION
Driftnet, version 0.1.6. $Id: driftnet.1.in,v 1.7 2004/04/26 14:42:36 chris Exp $ COPYING
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MER- CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. DRIFTNET(1)
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