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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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IPRESEND(1)						      General Commands Manual						       IPRESEND(1)

NAME
ipresend - resend IP packets out to network SYNOPSIS
ipresend [ -EHPRSTX ] [ -d <device> ] [ -g <gateway> ] [ -m <MTU> ] [ -r <filename> ] DESCRIPTION
ipresend was designed to allow packets to be resent, once captured, back out onto the network for use in testing. ipresend supports a num- ber of different file formats as input, including saved snoop/tcpdump binary data. OPTIONS
-d <interface> Set the interface name to be the name supplied. This is useful with the -P, -S, -T and -E options, where it is not otherwise possi- ble to associate a packet with an interface. Normal "text packets" can override this setting. -g <gateway> Specify the hostname of the gateway through which to route packets. This is required whenever the destination host isn't directly attached to the same network as the host from which you're sending. -m <MTU> Specify the MTU to be used when sending out packets. This option allows you to set a fake MTU, allowing the simulation of network interfaces with small MTU's without setting them so. -r <filename> Specify the filename from which to take input. Default is stdin. -E The input file is to be text output from etherfind. The text formats which are currently supported are those which result from the following etherfind option combinations: etherfind -n etherfind -n -t -H The input file is to be hex digits, representing the binary makeup of the packet. No length correction is made, if an incorrect length is put in the IP header. -P The input file specified by -i is a binary file produced using libpcap (i.e., tcpdump version 3). Packets are read from this file as being input (for rule purposes). -R When sending packets out, send them out "raw" (the way they came in). The only real significance here is that it will expect the link layer (i.e. ethernet) headers to be prepended to the IP packet being output. -S The input file is to be in "snoop" format (see RFC 1761). Packets are read from this file and used as input from any interface. This is perhaps the most useful input type, currently. -T The input file is to be text output from tcpdump. The text formats which are currently supported are those which result from the following tcpdump option combinations: tcpdump -n tcpdump -nq tcpdump -nqt tcpdump -nqtt tcpdump -nqte -X The input file is composed of text descriptions of IP packets. SEE ALSO
snoop(1m), tcpdump(8), etherfind(8c), ipftest(1), ipresend(1), iptest(1), bpf(4), dlpi(7p) DIAGNOSTICS
Needs to be run as root. BUGS
Not all of the input formats are sufficiently capable of introducing a wide enough variety of packets for them to be all useful in testing. If you find any, please send email to me at darrenr@pobox.com IPRESEND(1)
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