I need Linux sniffer
 

I need a sniffer like tool based on linux. can you help? where can I download a free version?

Ethereal
 

is a graphical sniffer, with versions available for both LINUX and Windows. If this is not satisfactory in some way, perhaps you could go into more detail as far as what your difficulties are/what in particular you wish to moniter?

I'm sorry. My question is so unclear. I want a tool to catch network IP packets. and analyse the packet content.

oh!! I found it, my godess tell me I can use "tethereal" to do the job, but not a fast traffic. My godness help me a lot.

ethereal (as mentioned before) IMO is an excellent network sniffer, it has a nice GUI to view all the captured traffic and you can have filters to help you sort through it

if you need more advanced features use ettercap, it can do almost anything on a network to sniff it, but it doesn't have quite as nice of an interface and you will probably want to use ethereal to view your captured packets