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I'm using tcpdump on a linux ARM computer (Raspberry Pi). The command tcpdump host "ip" -v gives nothing when I go on the website "ip" with my laptop, but if I try tcpdump port 80 -v it works properly, I see packets.

Why doesn't the host filter work?

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Most web sites use more than one IP address to serve information to clients, are you sure you are filtering all of it's IPs? How did you obtain them? –  David Kohen Mar 20 '13 at 10:37
    
You can check that you're filtering on the correct IP address by doing tcpdump -n port 80 and looking at the source address. –  Flup Mar 20 '13 at 10:40
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How are the Raspberry and the laptop connected? Are they connected via a switch? Is "IP" an ip address or a dns hostname? –  Kotte Mar 20 '13 at 15:01
    
with tcpdump -n port 80 -v i see nothing when i call websites. –  matthieu Mar 20 '13 at 18:12
    
What's the laptop doing here? You mention both a laptop and a Pi but there's no connection between the two. Are proxies involved? Please clarify your question. –  Gilles Mar 20 '13 at 23:01

1 Answer 1

tcpdump can only show packets it sees. And unless you use your raspberry pi as a proxy (or you live in the 90's and use a hub) your raspberry pi will not see any packet that your laptop transmits to anything else than the raspberry pi.

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