add example inline script for dns spoofing, refs #486

examples/README

@@ -1,6 +1,7 @@
 # inline script examples
 add_header.py             Simple script that just adds a header to every request.
 change_upstream_proxy.py  Dynamically change the upstream proxy
+dns_spoofing.py           Use mitmproxy in a DNS spoofing scenario.
 dup_and_replay.py         Duplicates each request, changes it, and then replays the modified request.
 iframe_injector.py        Inject configurable iframe into pages.
 modify_form.py            Modify all form submissions to add a parameter.

examples/dns_spoofing.py

@@ -0,0 +1,35 @@
+"""
+This inline scripts makes it possible to use mitmproxy in scenarios where IP spoofing has been used to redirect
+connections to mitmproxy. The way this works is that we rely on either the TLS Server Name Indication (SNI) or the
+Host header of the HTTP request.
+Of course, this is not foolproof - if an HTTPS connection comes without SNI, we don't
+know the actual target and cannot construct a certificate that looks valid.
+Similarly, if there's no Host header or a spoofed Host header, we're out of luck as well.
+Using transparent mode is the better option most of the time.
+
+Usage:
+    mitmproxy
+        -p 80
+        -R http://example.com/  // Used as the target location if no Host header is present
+    mitmproxy
+        -p 443
+        -R https://example.com/ // Used as the target locaction if neither SNI nor host header are present.
+
+mitmproxy will always connect to the default location first, so it must be reachable.
+As a workaround, you can spawn an arbitrary HTTP server and use that for both endpoints, e.g.
+mitmproxy -p  80 -R       http://localhost:8000
+mitmproxy -p 443 -R https2http://localhost:8000
+"""
+
+
+def request(context, flow):
+    if flow.client_conn.ssl_established:
+        # TLS SNI or Host header
+        flow.request.host = flow.client_conn.connection.get_servername() or flow.request.pretty_host(hostheader=True)
+
+        # If you use a https2http location as default destination, these attributes need to be corrected as well:
+        flow.request.port = 443
+        flow.request.scheme = "https"
+    else:
+        # Host header
+        flow.request.host = flow.request.pretty_host(hostheader=True)