Tcpdump usage examples
In most cases you will need root permission to be able to capture packets on an interface. Using tcpdump (with root) to capture the packets and saving them to a file to analyze with Wireshark (using a regular account) is recommended over using Wireshark with a root account to capture packets on an «untrusted» interface. See the Wireshark security advisories for reasons why.
See the list of interfaces on which tcpdump can listen:
Listen on interface eth0:
tcpdump -i eth0