Have you ever found yourself wondering how browser interacts with your Web application? Have you encountered a strange performance bottleneck that you can't track down? Are you curious about which cookies are being sent, or what downloaded content is marked as cacheable?

In Web and Internet development you are unable to see directly what is being sent and received between your web browser / client and the server. Without this visibility it is difficult and time-consuming to determine exactly where the fault is.

Don’t give up, there are some applications to help you out.

On developers computer (client side) you will go for HTTP proxy like Fiddler or Charles.

For server debugging (in most situations) you can't use proxy. In that case you have to use network packet sniffer or network protocol analyzer like Wireshark (used to be known as Ethereal)

About Fiddler

Fiddler is a Web Debugging Proxy which logs all HTTP(S) traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP(S) traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler includes a powerful event-based scripting subsystem, can be extended using any .NET language and has ready powerful extensions.

Fiddler is freeware and can debug traffic from virtually any internet application, including Internet Explorer, Mozilla Firefox, Opera, and thousands more.

It will run only on Windows.

http://www.fiddler2.com/

About Charles

Charles is a web proxy (HTTP Proxy / HTTP Monitor) that runs on your own computer. Your web browser is then configured to access the Internet through Charles, and Charles is then able to record and display for you all of the data that is sent and received.

Charles makes debugging quick, reliable and advanced; saving you time and frustration!

Key Features

• SSL Proxying – view SSL requests and responses in plain text
• Bandwidth Throttling to simulate slower Internet connections including latency
• AJAX debugging – view XML and JSON requests and responses as a tree or as text
• AMF – view the contents of Flash Remoting / Flex Remoting messages as a tree
• Repeat requests to test back-end changes
• Edit requests to test different inputs
• Breakpoints to intercept and edit requests or responses
• Validate recorded HTML, CSS and RSS/atom responses using the W3C validator

It is shareware, and you may try Charles for 30 days.

It will run on Windows, Mac OS X and Linux/Unix.

http://www.charlesproxy.com/

About Wireshark

Wireshark is the world's foremost network protocol analyzer, and is the de facto (and often de jure) standard across many industries and educational institutions.

Network professionals, security experts, developers, and educators around the world use it regularly.

It is freely available as open source, and is released under the GNU General Public License version 2.

It is developed and maintained by a global team of protocol experts, and it is an example of a disruptive technology.

Wireshark used to be known as Ethereal. If you're still using Ethereal, it is strongly recommended that you upgrade to Wireshark.

Wireshark development thrives thanks to the contributions of networking experts across the globe. It is the continuation of a project that started in 1998.

Wireshark has a rich feature set which includes the following:

• Deep inspection of hundreds of protocols, with more being added all the time
• Live capture and offline analysis
• Standard three-pane packet browser
• Multi-platform: Runs on Windows, Linux, OS X, Solaris, FreeBSD, NetBSD, and many others
• Captured network data can be browsed via a GUI, or via the TTY-mode TShark utility
• The most powerful display filters in the industry
• Rich VoIP analysis
• Read/write many different capture file formats: tcpdump (libpcap), Pcap NG, Catapult DCT2000, Cisco Secure IDS iplog, Microsoft Network Monitor, Network General Sniffer® (compressed and uncompressed), Sniffer® Pro, and NetXray®, Network Instruments Observer, NetScreen snoop, Novell LANalyzer, RADCOM WAN/LAN Analyzer, Shomiti/Finisar Surveyor, Tektronix K12xx, Visual Networks Visual UpTime, WildPackets EtherPeek/TokenPeek/AiroPeek, and many others
• Capture files compressed with gzip can be decompressed on the fly
• Live data can be read from Ethernet, IEEE 802.11, PPP/HDLC, ATM, Bluetooth, USB, Token Ring, Frame Relay, FDDI, and others (depending on your platfrom)
• Decryption support for many protocols, including IPsec, ISAKMP, Kerberos, SNMPv3, SSL/TLS, WEP, and WPA/WPA2
• Coloring rules can be applied to the packet list for quick, intuitive analysis
• Output can be exported to XML, PostScript®, CSV, or plain text

It runs on Windows, OS X, Linux and UNIX

http://www.wireshark.org/