Applying an upstream proxy in mitmproxy

https://github.com/mitmproxy/mitmproxy/issues/6006#issuecomment-1482367935 This explains how to dynamically apply an upstream proxy in mitmproxy. There was a pa

GGAMI2024-11-28

https://github.com/mitmproxy/mitmproxy/issues/6006#issuecomment-1482367935

This explains how to dynamically apply an upstream proxy in mitmproxy.

There was a particularly tricky part in dynamically handling upstream proxy mode itself: it was the proxy authentication.

Even if you add Proxy-Authorization to the header value,

When connecting to the actual upstream proxy, the value is stripped before being sent.

Proxy-Authorization

As a result, authentication fails.

Using http_connect_upstream here lets you include proxy authentication when connecting upstream.

But the biggest problem.

To use it dynamically, you have to set the proxy address in request and configure proxy authentication in http_connect_upstream.

The problem is that once you select a dynamic proxy in request, http_connect_upstream has no way of knowing which proxy was used.

The GitHub issue I posted to solve this problem is about this very issue.

from mitmproxy.net.server_spec import ServerSpec

	def http_connect_upstream(self, flow: http.HTTPFlow):
		if hasattr(flow.client_conn, "proxy_auth"):
			username, password = getattr(flow.client_conn, "proxy_auth")
			auth = f"Basic {base64.b64encode(f"{username}:{password}".encode()).decode()}"
			flow.request.headers["Proxy-Authorization"] = auth

	def request(self, flow: http.HTTPFlow):
		proxy_info = { 'host': 'proxyHost', 'port': 7777, 'username': 'proxyUsername', 'password': 'proxyPassword' }
		setattr(flow.client_conn, "proxy_auth", (proxy_info['username'], proxy_info['password']))
		address = (proxy_info["host"], proxy_info["port"])
		flow.server_conn.via = ServerSpec(("http", address))
		

This way, you set the proxy address in request and store the proxyAuth information in the client connection info for flow.client_conn. Then, in http_connect_upstream, you retrieve the connection information stored in flow.client_conn and use it for authentication.

With this approach, you can implement dynamic proxies in mitmproxy.

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