Remote Proxy corrupting data - Classic no longer supported?

[Peter]

I'm configuring a printer to use Fiddler Classic as a remote proxy on a Windows 11 machine to monitor the printer communication with a cloud service.  The printer downloads a print document (several MB) then immediately returns an error to the cloud service saying there's some problem with the document.  It works fine with Fiddler Everywhere.  This will not be fixed on Classic?  Telerik says that Fiddler Classic is no longer being maintained yet new releases (including bug fixes) still seem to be coming out on a regular cadence.

[Nick Iliev]

Hello Peter,

 

There might be several reasons why your request ends up with an error in Fiddler Classic (but goes as expected with Fiddler Everywhere).

For example:

- Check if you have an issue related to the certificate (handshake errors, SSL/TLS errors). If that is the case, try changing the Fiddler's certificate generator to the BouncyCastle certificate generator (available as the CartMaker add-on here).

- Enable the support for TLS 1.1 and TLS 1.2 through Tools > Options > HTTPS > Protocol by adding the following values

<client>;ssl3;tls1.0;tls1.1;tls1.2

- Disable the streaming mode from the Fiddler's toolbar

If the issue persists, consider providing the error you are receiving so you can receive tailored suggestions.

Regarding the releases of Fiddler Classic. As noted on our product page, the Fiddler Classic application is not in active development, so the team can't commit to creating and releasing new features. However, we are still maintaining the application and releasing versions to address critical issues and security patches.

 

 

Regards,
Nick Iliev
Progress Telerik

[Peter]

Hi Nick, Thanks for your very detailed response.  Regarding your points -

1. The printer is configured to ignore certificate errors.  Without that setting it doesn't work on either Fiddler version since the printer wouldn't normally accept self-signed certificates.
2. Those TLS settings were set in my Fiddler configuration
3. Streaming mode was disabled.

Fiddler doesn't indicate any error and from the trace it appears that the whole document was downloaded, but the printer complains about it - either it was truncated or corrupted, it doesn't say which.  If I disable the proxy on the printer or use Fiddler Everywhere instead of Classic and change nothing else, then it works

[Nick Iliev]

One difference between Fiddler Classic and Fiddler Everywhere is that the latter supports HTTP/2 and TLS 1.3. So, if your cloud service has been configured to explicitly use one of these technologies, that would explain the differences in the behavior. You can use the Fiddler Everywhere and the Fiddler Classic capture to investigate further and compare the differences in the captured sessions.