Add script to enable use of pac server (#7397)

* Add upstream_pac script

* [autofix.ci] apply automated fixes

---------

Co-authored-by: jweigel <jweigel@ebay.com>
Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

examples/contrib/upstream_pac.py

@@ -0,0 +1,93 @@
+"""
+Mitmproxy add-on to support using a PAC file to determine the upstream proxy to use.
+Supports adding an alternate proxy to use for when the PAC decides the connection should be DIRECT.
+Adds two options to mitmproxy:
+
+pac_url - an url that will return a pac file to use for evaluating which upstream proxy to use.
+direct_upstream_proxy - an alternate proxy to be used if the PAC returns DIRECT.
+
+Requires pypac to be installed and available on the python path.
+
+This class is inspired by the user contributed add-on:
+https://github.com/mitmproxy/mitmproxy/blob/main/examples/contrib/change_upstream_proxy.py
+"""
+
+import logging
+
+import pypac
+
+from mitmproxy import ctx
+from mitmproxy import http
+from mitmproxy.net import server_spec
+
+
+class UpstreamPac:
+    pac_file: pypac.parser.PACFile | None
+
+    @staticmethod
+    def load(loader) -> None:
+        loader.add_option(
+            name="direct_upstream_proxy",
+            typespec=str | None,
+            default=None,
+            help="Alternate upstream proxy to use when PAC resolution returns direct (http://localhost:8081)",
+        )
+
+        loader.add_option(
+            name="pac_url",
+            typespec=str | None,
+            default=None,
+            help="Proxy autoconfig url used to retrieve the PAC file",
+        )
+
+    @staticmethod
+    def configure(updated) -> None:
+        if "pac_url" in updated:
+            if ctx.options.pac_url is None:
+                UpstreamPac.pac_file = None
+                logging.info("No pac file specified")
+            else:
+                UpstreamPac.pac_file = pypac.get_pac(
+                    url=ctx.options.pac_url,
+                    allowed_content_types=[
+                        "application/x-ns-proxy-autoconfig ",
+                        "application/x-javascript-config",
+                        "text/html",
+                        "text/plain",
+                    ],
+                )
+                if UpstreamPac.pac_file is None:
+                    logging.error(
+                        "Failed to load pac file from: %s", ctx.options.pac_url
+                    )
+
+    @staticmethod
+    def proxy_address(flow: http.HTTPFlow) -> tuple[str, tuple[str, int]] | None:
+        if UpstreamPac.pac_file:
+            proxy = UpstreamPac.pac_file.find_proxy_for_url(
+                flow.request.url, flow.request.host
+            )
+
+            if proxy == "DIRECT":
+                if ctx.options.direct_upstream_proxy is not None:
+                    return server_spec.parse(ctx.options.direct_upstream_proxy, "http")
+                else:
+                    return None
+            else:
+                proxy_url = pypac.parser.proxy_url(proxy)
+                return server_spec.parse(proxy_url, "http")
+        return None
+
+    @staticmethod
+    def request(flow: http.HTTPFlow) -> None:
+        address = UpstreamPac.proxy_address(flow)
+
+        if address is not None:
+            logging.info(
+                "Using proxy %s://%s:%s for %s"
+                % (address[0], address[1][0], address[1][1], flow.request.host)
+            )
+            flow.server_conn.via = address
+
+
+addons = [UpstreamPac()]