Using tcpdump and Wireshark
Run tcpdump on a remote machine
The -s option specifies capture of whole data packets.
You probably want to change the device "eth0" and expression "port 80" in the following command:
sudo tcpdump -w /tmp/tcpdump.pcap -s 0 -i eth0 port 80
The following example uses the pseudo-device "any" to collect data from all interfaces at the same time. I believe the warning about promiscuous mode can be ignored:
sudo tcpdump -w /tmp/tcpdump.pcap -s 0 -i any port 80
Here's an example which collects data from a range of ports:
sudo tcpdump -w /tmp/tcpdump.pcap -s 0 -i any 'portrange 5100-5200'
Analyze the output using Wireshark
The output file (tcpdump.pcap) may be analyzed on a different machine then it was collected on.