Mozilla Firefox is unlike most Windows browsers in that it does not use the system’s proxy and certificate settings by default. That means that Firefox may require some additional configuration to work properly with Fiddler.

To get Firefox running with Fiddler, you need to:

• Configure Firefox to proxy its traffic to Fiddler.
• Configure Firefox to trust Fiddler’s root certificate.

Fortunately, both of these are simple tasks.

Configuring the Proxy

Firefox’s proxy settings are found by opening its Tools menu, clicking the Options item, and opening the Advanced settings. Select the Network tab. At the top of the tab, click the ...

Over the years, the most interesting class of support requests for Fiddler are of the form: “My application or website is failing, but when I try to capture a repro with Fiddler, the problem goes away completely! How is Fiddler fixing it?”

In general, Fiddler isn’t designed to automatically “fix” problems in web applications—it’s designed only to permit you to debug them. If you enable Fiddler’s Lint feature, it can flag problematic traffic to get your attention, but it still won’t fix the issues it finds.

However, in some cases the introduction of a proxy (e.g. Fiddler) into ...

A few weeks back, Jonathan Sampson noted that testing the HTML5 AppCache feature is a bit of a pain. One reason is that, in IE10, putting the browser into Work Offline mode is ineffective—the browser will detect that a network connection is available and exit the Work Offline mode automatically. Jonathan wondered whether Fiddler could help him test his HTML5 cache manifest, and I’m happy to say that, yes, Fiddler is quite useful for this sort of testing.

Background on AppCache

First, let’s start with some background. HTML5 introduces the concept of an AppCache, which allows web authors to declare, ...

A Fiddler user recently sent me the following question:

Eric-- I noticed a “minor” difference between Raw and Hex Inspectors when examining requests in Fiddler. When I access www.microsoft.com, the Raw view looks like this:

    GET http://www.microsoft.com/ HTTP/1.1
    ...
    Host: www.microsoft.com

While the Hex view looks like this:

    GET / HTTP/1.1
    ...
    Host: www.microsoft.com

Where is this difference  in the Request Line coming from? I’ve noticed that if I use a socket to directly send a GET for the absolute URL, some servers return an error.

When a client like a ...

Recently, a Fiddler user contacted me to note that they discovered a client/server application which failed when the TLS version 1.1 option is enabled in Internet Explorer’s Advanced settings. They were surprised to find that when they attempted to use Fiddler to debug the communication, the connection was instead successful.

Back in my old job, I’d encountered many such servers and services, and even wrote a post noting that many servers have compatibility problems with TLS 1.1 and TLS 1.2. Fiddler seems to magically “fix” such problems because, by default, it uses only SSL 3.0 and TLS 1.0 when ...

Yesterday, an updated Fiddler Beta was released, targeting .NET2 or .NET4. This update contains the usual set of bugfixes as well as a number of small features that I hope you’ll find useful.

Column Customization

Fiddler has always permitted you to customize the set of displayed columns to show information you’re interested in directly inside the Web Sessions list. However, adding new columns isn’t as easy as it could be, and this Beta includes the first steps toward simplifying this process. Simply right-click on the column headers and choose Customize columns…

A simple popup will open to allow you ...

Fiddler offers numerous ways to tag Web Sessions so you can easily identify them later. For instance, you can use the Comment button on the toolbar (or hit the M key) after selecting items in Web Sessions list to add notes about the traffic:

However, often you may find that you simply want to quickly mark some Sessions for later evaluation. To do so, you can use the context menu’s Mark submenu to choose a color:

The text of the selected entries will be bolded and colored as selected.

You may also press the CTRL+1 through CTRL+...

In my previous post, I discussed the recent enhancements to Fiddler’s ImageView extension that expose metadata about image files under inspection. My initial goal in exposing metadata was to help you optimize the size of images in order to build faster websites. However, in some cases the privacy implications of such metadata can be of far greater concern.

EXIF GPSInfo

As I started exploring the metadata found in real-world images, I found a surprising number of photos contain the GPS coordinates of the camera, stored within a GPSInfo structure in the EXIF metadata within the JPEG. Most of the ...

The latest Fiddler beta v2.4.2.5 (targeting .NET2 or .NET4) includes several improvements to the ImageView Response Inspector. While its appearance remains mostly unchanged, the new version provides much more insight into the metadata often included with image files. Read on for more details.

GIFs

GIF images are limited to 256 colors, are losslessly-compressed, and support animation and transparency. When a GIF file is selected in Fiddler’s Web Sessions list, the ImageView Inspector’s sidebar shows the size and dimensions of the image, as well some GIF-specific fields:

The Frames field shows the number of frames (images) contained within this GIF. ...

A key advantage Fiddler has over a traditional packet sniffer or in-browser network monitor is the ability to modify the traffic. This can be done manually (using breakpoints and Inspectors) or automatically, using FiddlerScript or extensions.

In today’s post, I’d like to introduce one such extension—ContentBlock. This extension allows you to block certain requests to simulate the behavior of users who are browsing with ad-blockers or other methods.

Using ContentBlock

After ContentBlock.dll is installed in Fiddler’s Scripts folder and Fiddler is restarted, a new top-level menu appears, titled ContentBlock. This menu exposes the following options:

The ...