Linux – Tcpdump says “Illegal instruction”

linuxtcpdump

Following the tutorial on http://www.waitingforthefuture.org/2009/12/25/compiling-tcpdump-for-tomato-firmware/, I tried to compile tcpdump. I did the compilation on an Intel Pentium 4 2.26Ghz computer running Ubuntu 10.04.

My router uses a BCM3302 V2.9 CPU. However, when I run tcpdump on it, it says "Illegal instruction".

Why does this happen? What other configuration must I do before compilation?

Best Answer

You need to compile for the platform you plan to run things on. This might be possible with GCC Cross compilation, but only if you can get the descriptions and symbol tables for the target processor.

You're getting "illegal instruction" because the software is looking for an x86 instruction on a decidedly non x86 chip

BCM3302 appears to be a MIPS32 architecture chip. There are some toolchains available.

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Monitoring HTTP traffic using tcpdump

tcpdump prints complete packets. "Garbage" you see are actually TCP package headers.

you can certainly massage the output with i.e. a perl script, but why not use tshark, the textual version of wireshark instead?

tshark 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)'

it takes the same arguments as tcpdump (same library) but since its an analyzer it can do deep packet inspection so you can refine your filters even more, i.e.

tshark 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' -R'http.request.method == "GET" || http.request.method == "HEAD"'

Linux – How to make tcpdump not print the tcp headers

As Josh suggests, tcpflow can print just the TCP packet data to a file or STDOUT. You can pipe tcpdump to tcpflow like this:

tcpdump -i lo -l -w - port 23 | tcpflow -C -r -

To only view one side of the conversation, you can use filters for tcpdump, e.g. dst port 23.

Best Answer

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