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Why does fiddler listen on port 8888?

Posted on 2015-02-13
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Fiddler Web Debugging Proxy by default listens on port 8888.  

1) What is the significance of port 8888?  
2) It seems that as a rule, internet browsers use port 8888 to send and receive traffic, correct?
3) If so, then it seems like this is a majorly important port, so why wouldn't this be indicated when I search on port 8888 e.g. under "Well known ports"?
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Question by:SAbboushi
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by:David Johnson, CD, MVP
David Johnson, CD, MVP earned 150 total points
ID: 40608862
it is an unregistered port that they 'fiddler' may pay for and get it registered to them
https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.txt
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by:Dave Baldwin
ID: 40608942
Fiddler works by intercepting traffic from the web browser.  They do that by redirecting the web browser to some other port, 8888 in this case, reading the packets and then passing the traffic on thru the standard ports.  It's the same technique used by your anti-virus also.
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Author Comment

by:SAbboushi
ID: 40609104
it is an unregistered port that they 'fiddler' may pay for and get it registered to them
Thanks David.  The link I provided shows 2 "official" registrants for port 8888 (one of them is also listed in your link).  Since Fiddler has been around a long time and hasn't registered the port, it seems to me that software (e.g. Fiddler) which has need of a port for a non-standard use will likely select a rarely used port as default.  Is that the likely answer to my question?

Fiddler works by intercepting traffic from the web browser.  They do that by redirecting the web browser to some other port, 8888 in this case, reading the packets and then passing the traffic on thru the standard ports.
Thanks David.  So I was mistaken: web browsers don't use port 8888 as a rule to send/receive traffic?  What port do they use?

So you're saying that fiddler reroutes the http/https traffic from the default browser ports to port 8888 which they are listening to and then reroutes back to the default browser ports?

In my case, I'm monitoring SoapUI traffic (not browser).  Fiddler didn't see its traffic until I enabled a proxy within SoapUI's preferences for Host: localhost, Port: 8888.  But I don't understand what I did (i.e. the mechanics of what is going on), hence my questions.

Appreciate you guys taking the time to help.  Thanks--
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Author Comment

by:SAbboushi
ID: 40609120
Ah - I think I get it: somehow (in a manner I don't understand) Fiddler reroutes the foreign address for the http/https requests generated by my browsers to port 8888; then after Fiddler intercepts the packets, it forwards them to the originally designated foreign address.  Correct?
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Expert Comment

by:David Johnson, CD, MVP
ID: 40609121
yes it is a man-in-the-middle proxy
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Dave Baldwin earned 350 total points
ID: 40609134
HTML pages using HTTP protocol are normally communicated over port 80.  Fiddler is setup to create a proxy on some other port in your web browser.  You had to redirect SoapUI traffic because the automatic proxy wasn't created in SoapUI since it is not a browser with the Fiddler plugin / add-on.

Fiddler might still work if your page is on some other port in the browser but I don't know.

HTML pages using HTTPS protocol are normally communicated over port 443.  Fiddler has to create a temporary SSL certificate to monitor that traffic.  It can do it but does not do it unless you tell it to.
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Author Closing Comment

by:SAbboushi
ID: 40609212
Thanks Guys--
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