Debugging with Fiddler

Ido Flatow
Senior Architect
Microsoft MVP
SELA Group
Debugging the Web with Fiddler
@idoFLATOW
http://bit.ly/flatow-blog
This presentation:
http://sdrv.ms/13mgEP9

www.devconnections.com
DEBUGGING THE WEB WITH FIDDLER
MEET FIDDLER…

Application
Network
APIs
Proxy Website
WHAT IS FIDDLER?

HOW FIDDLER WORKS…
Internet
Explorer
WinINET
Office
CryptoAPI WinHTTP
Fiddler
Firefox
Upstream
Proxy
Web Site
Firewall

FIDDLER FACTS
 A proxy server
 An HTTP/HTTPS Sniffer
 An HTTP Debugger
 Written in .NET 3.5 / 4
 A free utility (for now, cross your fingers)
 An extensible application
 A must tool for web developers
 Unfortunately,
it doesn’t make coffee!

SCENARIO: INSPECTING TRAFFIC
 Web sessions
 Inspectors
 Filters
 QuickExec

THE SESSIONS LIST
9
Sequence and
color coding
Caching information
Origin process
Manual
commentsResponse size

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 ( )

THE RIGHT INSPECTOR FOR THE JOB

FILTER BY…
By process type
By specific process
By host
By process name/PID/svchost
By URL
By status code
By MIME or size
QuickExec

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 excluding
problematic HTTPS sites

IF YOU SEE THIS, DON’T PANIC

SCENARIO: PERFORMANCE TUNING
 Timeline
 Statistics
 More inspectors

REMINDER
UNDERSTANDING THE CONNECTION
Application
Network
APIs
Proxy Website
Second connection
From Fiddler to the server
First connection
From client to Fiddler

VISUALIZE THE TRAFFIC WITH TIMELINE
Start End
Bar color = MIME
Red = new connection
Green = reused connection
Disk = cached response
Bar = begin receive
Stripes = buffered by Fiddler
Prefer stream over buffer
Use buffer when editing responses

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

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!

SCENARIO: DEBUGGING
 Replay
 Composer
 Breakpoints

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
 Create any request from scratch
 Use previous requests with drag-n-drop
 Don’t bother calculating content length
www.bobthebuilder.com

DEBUGGING LOCALHOST
 In some cases, proxies are bypassed when
using localhost (127.0.0.1) addresses
 What to do?
 Use the machine’s name
 Use fictitious DNS names
 ipv4.fiddler or ipv6.fiddler
(converted to 127.0.0.1 / [::1])
 localhost. or localhost.fiddler
(converted to localhost)
 Force the use of a proxy with netsh

SCENARIO: TESTING
 Save / Export
 Auto responder

WHAT DID YOU TYPE IN WRONG
TO GET IT TO CRASH?
 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
 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

SCENARIO: CUSTOMERS
 Reverse proxy
 Fiddler Cap

CAN’T FIDDLE THE CLIENT?
FIDDLE THE SERVER
Local Network
Web ServerFiddler
8180
http://bit.ly/fiddler-reverse-proxy

MINI FIDDLER, CUSTOMER-ORIENTED
 Six simple steps
 Install and run
 Click 1. Start Capture
 Browse the web site
 [Take some screenshots]
 Click 2. Stop Capture
 Click 3. Save Capture
 Email me the .SAZ file
29

EXTENDING FIDDLER
 Customize rules script
 Create custom inspectors
 Create requests and responses auto-modifiers
 Create new exporter/importer
 Extend QuickExec with new commands
 Add menus and tabs

FIDDLER IS MORE THAN A SNIFFER
 Monitors traffic, yes, but also…
 Controls traffic
 Modifies traffic
 Generates traffic
 Easy to use
 Extensible

http://bit.ly/flatowblog
@IdoFlatow
idof@sela.co.il
My Info
Fiddler Website
http://www.fiddler2.com
http://groups.google.com/group/httpfiddler
Fiddler Forum
http://www.telerik.com/automated-testing-tools/blog/eric-lawrence.aspx
http://blogs.msdn.com/b/fiddler (older blog)
Fiddler Blog
http://sdrv.ms/13mgEP9
This Presentation
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Debugging with Fiddler

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