fix dns_spoofing example, avoid connecting to itself

examples/dns_spoofing.py

@@ -8,30 +8,43 @@ Similarly, if there's no Host header or a spoofed Host header, we're out of luck
 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.
+        -s dns_spoofing.py
+        # Used as the target location if neither SNI nor host header are present.
+        -R http://example.com/
+    mitmdump
+        -p 80
+        -R http://localhost:443/
 
-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
+    (Setting up a single proxy instance and using iptables to redirect to it
+    works as well)
 """
+import re
+
+
+# This regex extracts splits the host header into host and port.
+# Handles the edge case of IPv6 addresses containing colons.
+# https://bugzilla.mozilla.org/show_bug.cgi?id=45891
+parse_host_header = re.compile(r"^(?P<host>[^:]+|\[.+\])(?::(?P<port>\d+))?$")
 
 
 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"
+        sni = flow.client_conn.connection.get_servername()
+        port = 443
     else:
-        # Host header
-        flow.request.host = flow.request.pretty_host(hostheader=True)
+        flow.request.scheme = "http"
+        sni = None
+        port = 80
+
+    host_header = flow.request.pretty_host(hostheader=True)
+    m = parse_host_header.match(host_header)
+    if m:
+        host_header = m.group("host").strip("[]")
+        if m.group("port"):
+            port = int(m.group("port"))
+
+    flow.request.host = sni or host_header
+    flow.request.port = port

libmproxy/models/connections.py

@@ -1,4 +1,4 @@
-from __future__ import absolute_import
+from __future__ import (absolute_import, print_function, division)
 
 import copy
 import os

libmproxy/models/flow.py

@@ -1,4 +1,4 @@
-from __future__ import absolute_import
+from __future__ import (absolute_import, print_function, division)
 import copy
 import uuid
 

libmproxy/protocol/base.py

@@ -103,6 +103,7 @@ class ServerConnectionMixin(object):
     def __init__(self, server_address=None):
         super(ServerConnectionMixin, self).__init__()
         self.server_conn = ServerConnection(server_address)
+        self._check_self_connect()
 
     def reconnect(self):
         address = self.server_conn.address
@@ -110,12 +111,30 @@ class ServerConnectionMixin(object):
         self.server_conn.address = address
         self.connect()
 
+    def _check_self_connect(self):
+        """
+        We try to protect the proxy from _accidentally_ connecting to itself,
+        e.g. because of a failed transparent lookup or an invalid configuration.
+        """
+        address = self.server_conn.address
+        if address:
+            self_connect = (
+                address.port == self.config.port and
+                address.host in ("localhost", "127.0.0.1", "::1")
+            )
+            if self_connect:
+                raise ProtocolException(
+                    "Invalid server address: {}\r\n"
+                    "The proxy shall not connect to itself.".format(repr(address))
+                )
+
     def set_server(self, address, server_tls=None, sni=None, depth=1):
         if depth == 1:
             if self.server_conn:
                 self._disconnect()
             self.log("Set new server address: " + repr(address), "debug")
             self.server_conn.address = address
+            self._check_self_connect()
             if server_tls:
                 raise ProtocolException(
                     "Cannot upgrade to TLS, no TLS layer on the protocol stack."
@@ -141,7 +160,7 @@ class ServerConnectionMixin(object):
             self.server_conn.connect()
         except tcp.NetLibError as e:
             raise ProtocolException(
-                "Server connection to '%s' failed: %s" % (self.server_conn.address, e), e)
+                "Server connection to %s failed: %s" % (repr(self.server_conn.address), e), e)
 
 
 class Log(object):

libmproxy/protocol/http_replay.py

@@ -1,3 +1,4 @@
+from __future__ import (absolute_import, print_function, division)
 import threading
 
 from netlib.http import HttpError

libmproxy/proxy/config.py

@@ -1,4 +1,4 @@
-from __future__ import absolute_import
+from __future__ import (absolute_import, print_function, division)
 import collections
 import os
 import re

libmproxy/proxy/server.py

@@ -1,4 +1,4 @@
-from __future__ import absolute_import, print_function
+from __future__ import (absolute_import, print_function, division)
 
 import traceback
 import sys