>
> i want to capture ftp download from a server to a client. what is the capture filter to be used at both server and client so i can get only traffic from/to port 20 and port 21?
>
> i tried this --> tcp port 20 and tcp port 21 but no traffic is captured.
>
The correct syntax for what you thought of would be: tcp port 20 or tcp port 21
However, As the data port will often be negotiated (aka varies from transfer to transfer), you'll often won't be able to use a capture filter for this as you won't capture the data portion, see: http://wiki.wireshark.org/FTP
Regards, ULFL
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