Setting Fiddler alongside Python applications

Environment

Product	Fiddler Everywhere
Product Version	1.0.0 and above
Operating System	macOS, Windows, Linux
3rd-party tools	Python

Description

Q: How can I capture traffic from Python applications?

Q: When Fiddler Everywhere acts as a system proxy, I can't use python's pip library (or other Python library that makes HTTPS requests).

Q. My python library fails with CERTIFICATE_VERIFY_FAILED when Fiddler is an intermediate proxy.

Using the Fiddler's Terminal

The Fiddler Everywhere application allows starting a dedicated terminal that automatically proxies HTTPS requests through the Fiddler proxy. The Fiddler's terminal supports capturing from Python applications without additional configuration - Fiddler Everywhere sets everything.

FIddler Everywhere terminal supports Python version 3 and above, which was first introduced in 2009. Older versions of Python are unsupported.

Steps needed to capture HTTPS traffic from a Python application:

1. Start the Fiddler Everywhere application.

2. Click Open Terminal to start dedicated terminal instance.

3. Run the Python app within the dedicated terminal instance.

Learn more about the Open Browser option in Fiddler Everywhere here...

Setting Fiddler Proxy and CA through Environment Variables

As an alternative to the preconfigured terminal, you can configure your python applications to go through the Fiddler Everywhere proxy explicitly.

For Fiddler Everywhere to capture traffic from a Python application, the application must be configured to respect the Fiddler Everywhere proxy. You can achieve that by setting the environment variables http_proxy and https_proxy. The export command (for macOS and Linux) or the set command (for Windows). The export command will generate an environment variable in a child process environment. It does not affect other already-existing settings.

The following example demonstrates how to define the proxy settings through environmental variables on macOS and Linux. Note that you need to have the Fiddler root CA exported to the Desktop folder through Settings > HTTPS > Advanced > Export Root Certificate (PEM/ASCII).

**

export http_proxy=http://127.0.0.1:8866
export https_proxy=http://127.0.0.1:8866
export SSL_CERT_FILE=~/Desktop/Fiddler_Root_Certificate_Authority.pem
export REQUESTS_CA_BUNDLE=~/Desktop/Fiddler_Root_Certificate_Authority.pem

The following example demonstrates how to define the proxy settings through environmental variables on Windows. Note that you need to have the Fiddler root CA exported to the Desktop folder through Settings > HTTPS > Advanced > Export Root Certificate (PEM/ASCII).

set http_proxy=http://127.0.0.1:8866
set https_proxy=http://127.0.0.1:8866
set SSL_CERT_FILE=~/Desktop/Fiddler_Root_Certificate_Authority.pem
set REQUESTS_CA_BUNDLE=~/Desktop/Fiddler_Root_Certificate_Authority.pem

Once the Fiddler Everywhere proxy is set, you can immediately capture traffic through the Python application.

Resetting the Fiddler Proxy

Once debugging with Fiddler Everywhere, reset the current environment proxy by removing the Fiddler Everywhere proxy variables. You can use the unset command on macOS and Linux to achieve that. On Windows, you need to set an empty string.

The following example demonstrates how to unset the proxy on macOS and Linux.

unset http_proxy
unset https_proxy

The following example demonstrates how to unset the proxy on Windows.

set http_proxy=
set https_proxy=

Setting Fiddler Proxy through Python Code

You can set the Fiddler proxy explicitly through the Python application's code base.

import requests

proxies = {'http': '127.0.0.1:8866', 'https': '127.0.0.1:8866'}
r = requests.get('https://reqbin.com/echo', proxies=proxies)