I'm accessing my localhost ftp server by Iceweasel or by the terminal (with the command ftp) while listening on the ftp port with sudo tcpdump -vv -A 'port 20'or sudo tcpdump -vv -A port ftpbut nothing is printed although the connection is well established.What am I doing wrong? 
Why don't I see packets when I listen on my ftp port with tcpdump on localhost?
ftp;tcpdump
You indicated you are connecting via localhost. You will probably need to specify the interface with -i lo (or lo0 on a Mac), so use:sudo tcpdump -i lo -vv -A port ftpor on a Mac:sudo tcpdump -i lo0 -vv -A port ftpThe you should see the traffic. The reason:-i     Listen on interface.  If unspecified, tcpdump searches the system interface list for the lowest numbered, configured up interface (excluding loopback), which may turn out to be, for example, eth0.