10 Killer Open Source Networking Tools
#7 Tcpdump and Wireshark
Tcpdump and Wireshark lets you capture the network traffic on a network card and view the packets and frames in real time.
If a browser can't find the Web server, you can fire up Tcpdump and see what's happening. And this tool is also useful to view the query and reply packets that help you determine if the DNS server is replying with NXDomain or if the user changed the DNS server settings or maybe the DNS queries and replies are fine, but the remote Web server is not responding. Then you would see the HTTP request packet leave the computer, but no replies from the Web server.
Tcpdump is a great tool by itself, but when paired with Wireshark, you have an unbeatable system for troubleshooting network application issues. You can save your Tcpdump packet captures to files and open them in Wireshark for easier analysis. Wireshark gives you a GUI to examine Tcpdump captures and sort the data for more thorough analysis. You can compare time stamps on individual packets to see how long it's taking for a reply to be returned after a request has been made.