You can get tcpdump from http://www.tcpdump.org/ or as a standard package in most Linux distributions.
To capture full-sized packets:
$ tcpdump -i <interface> -s0 -w <some-file>
Specify the correct interface and the name of a file to save to. If you are unsure which interface to use, you can use the following tcpdump command to display the list of available interfaces:
$ tcpdump -D
Use ^C (Ctrl+C) to terminate the capture when you believe you have captured enough packets.
For more information, consult tcpdump's man page:
man tcpdump
Provide the tcpdump output to support.