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Debugging the Web with Fiddler. Ido Flatow Senior Architect Microsoft ASP.NET/IIS MVP SELA Group @idoflatow. About Me. Senior Architect, Sela Group Co-author of courses and books Microsoft ASP.NET/IIS MVP Focus on server, web, cloud, and DevOps

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debugging the web with fiddler

Debugging the Web with Fiddler

Ido Flatow

Senior Architect


SELA Group



about me
About Me
  • Senior Architect, Sela Group
  • Co-author of courses and books
  • Microsoft ASP.NET/IIS MVP
  • Focus on server, web, cloud, and DevOps
  • Manager of the Israeli WebDev User Group
fiddler facts
Fiddler Facts
  • A proxy server
  • An HTTP/HTTPS Sniffer
  • An HTTP Debugger
  • Written in .NET / Mono
  • A free utility (for now, cross your fingers)
  • An extensible application
  • A must tool for web developers
  • Unfortunately, it doesn’t make coffee!
fiddler by scenarios
Fiddler by Scenarios







the sessions list
The Sessions List



Manual comments

Response size

Sequence and color coding

Caching information

Origin process

fiddler to the rescue
Fiddler to the Rescue
  • Browsers (Static sites, ASP.NET, J2EE, PHP)
  • Desktop applications that use HTTP
  • Web services
  • Windows Phone Emulator
  • Any device that supports a proxy server ( )
filter by
Filter by…

By specific process

By host

By process name/PID/svchost


By status code

By MIME or size

By process type


fiddler and https
Fiddler and HTTPS
  • HTTPS is secured - between two machines
  • Fiddler acts as a “machine-in-the-middle”
  • Generates certificates for web sites on-the-fly
  • Fiddler supports client certificates for authentication
  • Supports excludingproblematic HTTPS sites
understanding the connection
Understanding the Connection

First connection

From client to Fiddler

Second connection

From Fiddler to the server

visualize the traffic with timeline
Visualize the Traffic with Timeline



Bar = begin receive

Disk = cached response

Red = new connection

Green = reused connection

Bar color = MIME

Stripes = buffered by Fiddler

Prefer stream over buffer. Use buffer when editing responses

statistics are important
Statistics are Important
  • Single page statistics give info on performance
    • Client processing
    • Server processing
    • Network latency
  • Server time: ServerBeginResponse – ServerGotRequest
  • Upload time: ServerGotRequest – ClientDoneRequest
  • Download time: ServerDoneResponse – ServerBeginResponse
  • Watch out for misleading connection reuse
  • Add timer columns instead of calculating
performance related inspectors
Performance-related inspectors
  • Transformer
  • Image view
  • Caching
  • Cookies
  • HTML Inspector (3rd party)
drawing conclusions
Drawing Conclusions
  • Reduce number of requests
  • Reduce traffic roundtrips
  • Reduce the size of requests and responses
  • Applying compression where needed
  • Identify non-cached responses
  • Use Fiddler to improve performance, not to measure it!
manipulating traffic with rules
Manipulating Traffic with Rules
  • Simulate user-agents
  • Disable sending cache headers
  • HOSTS file remapping
  • Custom rules
play it again sam
Play it Again, Sam
  • Why run the client scenario again, just to resend the request?
  • Simply click “Replay”
    • Reissue a single request
    • Reissue a set of requests
    • Reissue unconditionally (no cache headers)
    • Reissue multiple times
changing requests with composer
Changing Requests with Composer
  • Create any request from scratch
  • Use previous requests with drag-n-drop
  • Use Scratchpad to store common requests

breakpoints human in the middle
Breakpoints – Human in the Middle
  • Manual control over request & response
  • Stop on any direction
  • Manipulate headers and body
  • Use built-in responses or file
debugging localhost
Debugging Localhost
  • In some cases, proxies are bypassed when using localhost ( addresses
  • What to do?
  • Use the machine’s name
  • Use fictitious DNS names
    • localhost. or localhost.fiddler(converted to localhost)
    • ipv4.fiddler or ipv6.fiddler (converted to / [::1])
debugging services
Debugging Services
  • Fiddler hooks to the interactive user
  • IIS App Pools and Windows Services runs using built-in users
  • Use network sniffers and export as PCAP
  • Download PsExec and run:

PsExec.exe -i -u "nt authority\network service" "%ProgramFiles(x86)%\Fiddler2\Fiddler.exe"

if you see this don t panic
If You See This, Don’t Panic

When Fiddler crashes proxy setting are still in effect

Before rebooting your machinetry running Fiddler again

my code is perfect
“My Code is Perfect”
  • Save traffic to file
  • Save sessions as .SAZ (Zip) archive
  • .SAZ stores content and session info
  • Other archiving options:
    • HTTP Archive (HAR)
    • Visual Studio Web Test
auto responder an in memory website
Auto-Responder - an In-Memory Website
  • If URI matches… then respond with…
  • Respond with file/redirection/breakpoint/drop
  • Not only that, you can also:
    • Import sessions as responses
    • Edit stored responses
    • Use original latency time (updatable)
    • Export and import rules
fiddler is more than a sniffer
Fiddler is More Than a Sniffer
  • Monitors traffic, yes, but also…
  • Controls traffic
  • Modifies traffic
  • Generates traffic
  • Easy to use
  • Extensible

Enhance your web debugging with Fiddler today!


Fiddler Website

Fiddler Forum

Fiddler Blog (older blog)

My Info

This Presentation

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