Document that request.get_host() fails when behind multiple reverse proxies

[tomevans222]

We run django behind several layers of proxies (please excuse ASCII art):

    Client
       |
       |
Apache (handling SSL connections, mod_proxy)
       |
       |
Apache (handling static content, load balancing, mod_proxy, mod_proxy_balancer)
    |                           | 
    |                           |
Apache (mod_fastcgi)       Apache (mod_fastcgi)
    |                           |
    |                           |
django instance             django instance

Each apache instance that proxies a request appends the ServerName of the proxy server into the X-Forwarded-For and X-Forwarded-Host headers. In our infrastructure, all of the apache instances respond to the same host name (which is preserved through the proxying using ProxyPreserveHost directive), so it ends up looking like this in request.META:

'HTTP_X_FORWARDED_FOR: '10.0.0.1, 10.0.0.2'
'HTTP_X_FORWARDED_HOST': 'portal.example.com, portal.example.com'
'HTTP_X_FORWARDED_SERVER': 'portal.example.com, portal.example.com'

This then breaks request.get_host(), which does the following:

   def get_host(self):
        """Returns the HTTP host using the environment or request headers."""
        # We try three options, in order of decreasing preference.
        if 'HTTP_X_FORWARDED_HOST' in self.META:
            host = self.META['HTTP_X_FORWARDED_HOST']
        elif 'HTTP_HOST' in self.META:
            host = self.META['HTTP_HOST']
        else:
            # Reconstruct the host using the algorithm from PEP 333.
            host = self.META['SERVER_NAME']
            server_port = str(self.META['SERVER_PORT'])
            if server_port != (self.is_secure() and '443' or '80'):
                host = '%s:%s' % (host, server_port)
        return host

IE, it completely ignores the possibility of more than one host in X-Forwarded-Host.

Breaking request.get_host() breaks request.build_absolute_uri(), which in turn breaks HttpResponseRedirect*, leading to redirects with location headers like:

  Location: http://portal.example.com, portal.example.com/foo/

I've not attached a patch, because I can think of a number of ways this could be fixed:

1) Change request parsing to notice multi value headers and correctly parse into arrays.
2) Change request.get_host() to look for multi value versions of X-Forwarded-Host and pull out the correct value.
3) Add separate middleware to rewrite request.META with just the 'closest' version of these headers.
... any number of other solutions.

This obviously is not a 'normal' deployment of django... We will probably work around with custom middleware for now, it is the least intrusive.

This is with 1.0, but the code for request.get_host() is the same in trunk.

[tomevans222]

Middleware to work around the issue

[Jacob]

As you say, this is not a "normal" way of deploying Django, and I don't think there'll be a general solution here. See the back and forth about handling X-Forwarded-For for some background: the basic problem is that Django can't magically know which of a list of values is "correct" here. I think that your solution of handling this problem in middleware is as good as it's gonna get.

It would, however, be good to have this documented, so I'm hijacking this ticket and turning it into a doc request. We could add a note to the docs for request.get_host() and include your little bit of middleware as an example. Feel free to work up a doc patch if you wanna grease the wheels a bit.

[tomevans222]

That sounds perfectly reasonable Jacob. I'll try to get a doc patch up as soon as $JOB allows :)

[arnav]

patch with changes to documentation including example

[arnav]

documentation patch added

[Gabriel Hurley]

(In [14493]) Fixed #11877 -- Documented that HttpRequest.get_host() fails behind multiple reverse proxies, and added an example middleware solution. Thanks to Tom Evans for the report, and arnav for the patch.

[Gabriel Hurley]

(In [14494]) [1.2.X] Fixed #11877 -- Documented that HttpRequest.get_host() fails behind multiple reverse proxies, and added an example middleware solution. Thanks to Tom Evans for the report, and arnav for the patch.

Backport of [14493] from trunk.

[John Borwick <john_borwick@…>]

Note for other readers: when using the included MultipleProxyMiddleware, consider whether you want line 30 to pull the *last* element [-1] or the *first* element [0].

The right-most/last is supposed to be the most recent proxy (per <​http://www.squid-cache.org/Doc/config/follow_x_forwarded_for/>). The left-most is the furthest upstream proxy.