On Daylight Savings Time

In Fiddler, the Caching tab will attempt to calculate the cache freshness lifetime for responses that lack an explicit Expires or Cache-Control: max-age directive. The standard suggests clients use (0.1 * (DateTime.Now – Last-Modified)) as a heuristic freshness lifetime. An alert Fiddler user noticed that the values he was seeing were slightly off what he expected: sometimes the valuesContinue reading “On Daylight Savings Time”

Automatically Evaluating Compressibility

Fiddler’s Transformer tab has long been a simple way to examine the use of HTTP compression of web assets, especially as new compression engines (like Zopfli) and compression formats (like Brotli) arose. However, the one-Session-at-a-time design of the Transformer tab means it is cumbersome to use to evaluate the compressibility of an entire page orContinue reading “Automatically Evaluating Compressibility”

What’s New in Fiddler 4.6.2

TLDR? – Get the newest Fiddler here. It’s been just over two months since the last significant release, and Fiddler 4.6.2.0 (and v2.6.2.0) are now available. As always, the latest build includes a slew of bugfixes and minor tweaks, as well as a number of features described in this post. Default Certificate Generator Changed ChangesContinue reading “What’s New in Fiddler 4.6.2”

Book-Writing: Just Do It!

Sadly, you’re unlikely to get wealthy by writing a book. You should definitely write one anyway. My Background People I respect suggest you shouldn’t write (or buy) books on specific technologies, going so far as to say that writing a book was on their top-10 lists of life regrets. Top-10… whoa! As a consequence, whenContinue reading “Book-Writing: Just Do It!”

Repairing Corrupt ZIP Files

Fiddler’s default file format is the SAZ Format, which is just a ZIP file with a particular structure. Unfortunately, sometimes users’ SAZ files get corrupted due to failing disks or incomplete downloads, and when this happens, Fiddler can no longer open them. Because Fiddler uses a standard ZIP file, surely a good ZIP reader willContinue reading “Repairing Corrupt ZIP Files”

Reset Fiddler’s HTTPS certificates

I’ve made changes to the latest versions of Fiddler to improve the performance of certificate creation, and to avoid problems with new certificate validation logic coming to Chrome and Firefox. The biggest of the Fiddler changes is that CertEnroll is now the default certificate generator on Windows 7 and later. Unfortunately, this change can causeContinue reading “Reset Fiddler’s HTTPS certificates”

What’s New in Fiddler 4.6.0.7

TLDR? – Get the newest Fiddler here. We’re performing a staged rollout of this build; it won’t be on autoupdate until next week. Under the Hood As mentioned in our notes about the Fiddler 4.6 release, we’ve started taking a very close look at Fiddler’s performance. Fiddler’s use of the CPU, system memory, and theContinue reading “What’s New in Fiddler 4.6.0.7”

Viewing HTTPS Handshakes in Fiddler

You can easily use Fiddler to evaluate what algorithms a client is using to connect to a HTTPS server in Fiddler. First, adjust Fiddler’s configuration using Tools > Fiddler Options to enable capture of CONNECT tunnels but disable decryption: Disabling decryption is necessary because Fiddler decrypts traffic using a HTTPS man-in-the-middle technique, which means thatContinue reading “Viewing HTTPS Handshakes in Fiddler”

Fiddler and Brotli

Regular readers of my blog know how excited I am about Google’s new open compression engine, Brotli. Support is in Firefox 44 Nightly already and is expected for other browsers. Because Brotli is used inside the WOFF2 font format, many clients already have the compression code and just need to expose it in a newContinue reading “Fiddler and Brotli”