A requests TransportAdapter allowing to force a specific IP for HTTPS connections.
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forcediphttpsadapter
README.md
example.py
setup.py Bump version Dec 15, 2017

README.md

This module implements a set of requests TransportAdapter, PoolManager, ConnectionPool and HTTPSConnection with one goal only:

  • to use a specific IP address when connecting via SSL to a web service without running into SNI trouble.

The usual technique to force an IP address on an HTTP connection with Requests is (assuming I want http://example.com/some/path on IP 1.2.3.4):

requests.get("http://1.2.3.4/some/path", headers={'Host': 'example.com'})

this is useful if I want to specifically test how 1.2.3.4 is responding; for instance, if example.com is DNS round-robined to several IP addresses and I want to hit one of them specifically.

This also works for https requests if using Python <2.7.9 because older versions don't do SNI and thus don't pass the requested hostname as part of the SSL handshake.

However, Python >=2.7.9 and >=3.4.x conveniently added SNI support, breaking this way of connecting to the IP, because the IP address embedded in the URL is passed as part of the SSL handshake, causing errors (mainly, the server returns a 400 Bad Request because the SNI host 1.2.3.4 doesn't match the one in the HTTP headers example.com).

The "easiest" way to achieve this is to force the IP address at the lowest possible level, namely when we do socket.create_connection. The rest of the "stack" is given the actual hostname. So the sequence is:

  1. Open a socket to 1.2.3.4
  2. SSL wrap this socket using the hostname.
  3. Do the rest of the HTTPS traffic, headers and all over this socket.

Unfortunately Requests hides the socket.create_connection call in the deep recesses of urllib3, so the specified chain of classes is needed to propagate the given dest_ip value all the way down the stack.

Because this applies to a very limited set of circumstances, the overridden code is very simplistic and eschews many of the nice checks Requests does for you.

Specifically:

  • It ONLY handles HTTPS.
  • It does NO certificate verification (which would be pointless)
  • Only tested with Requests 2.2.1 and 2.9.1.
  • Does NOT work with the ancient urllib3 (1.7.1) shipped with Ubuntu 14.04. Should not be an issue because Ubunt 14.04 has older Python which doesn't do SNI.

How to use it

It's like any other transport adapter. Just pass the IP address that connections to the given URL prefix should use.

session = requests.Session()
session.mount("https://example.com", ForcedIPHTTPSAdapter(dest_ip='1.2.3.4'))
response = session.get(
    '/some/path', headers={'Host': 'example.com'}, verify=False)

Note this module will ImportError if there's no sane requests/urllib combination available so the adapter won't work, and it's up to the caller to decide what to do. The caller can, for instance, check the Python version and if it's <2.7.9 decide to use the old "http://$IP/ technique. If Python is >=2.7.9 and the adapter doesn't work, unfortunately, there's nothing that can be done :(

An example.py script is provided, it illustrates how to import the module, how to decide whether to use the adapter or the old technique, and how to define the IP and mount the adapter on a session.