[mod_python] Getting Hostname

Graham Dumpleton grahamd at dscpl.com.au
Wed Nov 15 16:12:12 EST 2006

On 16/11/2006, at 5:05 AM, g.farina at pharaondev.com wrote:

> Hi,
> in my simple MVC framework I'd like to use a different directory  
> root for
> controllers based on the website requested by the user.
> For instance if someone connects to www.test.it/first I'd like to use
> /var/www/test as controllers root, and if someone connects to
> www.test2.it/first use /var/www/test2.
> I didn't have enought knowledge of how Apache works, so I don't  
> know if
> the value of req.server.server_hostname is reliable to do the  
> switch of
> the root.
> The solution used should work indipendently from Apache  
> configuration, and
> I don't want to rely on headers sent by the client.
> The directory switching should be configured from the application,  
> so I
> can't use Virtual Hosting or Apache configuration directly.

Why can't you use virtual hosting, this is what it is designed for?  
If it is because
your site is already running within a virtual host and you don't want  
to pay some
ISP extra money for running an extra virtual host, then it will be  
pretty well
impossible to do what you want.

The reason it would be impossible if you are already running within a  
host is that the way virtual hosting works is that HTTP/1.1 compliant  
will set a 'Host' header in the HTTP request to identify the actual  
name of the
site it wants to connect to. The web server uses the value of this  
header to
determine which of many virtual hosts it may be hosting to direct the  
request to.
This is done by looking in the Apache configuration for VirtualHost  
which lists that site:

   <VirtualHost www.test.it>

A second independent virtual host could be setup listing the second  
site, thus:

   <VirtualHost www.test.it>

   <VirtualHost www.test2.it>

If they need to share some but not all content, you'll need to use  
Alias directives
as well as setting the DocumentRoot to the main content.

Alternatively, if you want everything to be the same but do certain  
things differently
based on host connected to, you can also use:

   <VirtualHost www.test.it www.test2.it>

In either case, it is going to require a change to the Apache  
configuration. If you
can't get one of these done, you can't do it, as the request to the  
additional site
will not even be able to be routed to you.

If the second virtual hosting arrangement is used, or virtual hosting  
isn't used at
all, then you still have to use some trickery to have different parts  
of the filesystem
mapped if the desired host is different.

The cleanest way of doing this is to use a PythonTransHandler.  
Because it is
called prior to any URL mapping, it must be at the level of  
VirtualHost or in
main Apache configuration if virtual hosting is not used. It cannot  
be a Directory,
Location or .htaccess context as mapping is already done at that point.

The transhandler you use would need to use (untested) something like:

   from mod_python import apache

   def transhandler(req):
     if req.hostname == 'www.test2.it':
       req.filename = '/var/www/test2' + req.uri
       return apache.OK
     return apache.DECLINED

This assumes that default DocumentRoot is 'var/www/test'. It then  
sees if the target
host is 'www.test2.it' and overrides req.filename to cause mapping of  
against the alternate document root.

A problem with this is that it will potentially override any other  
mappings you might have defined, thus to accommodate that the trans  
may have to only do it for certain values of req.uri.

NOTE, the above code may have to cleanup req.uri and normalise it  
before it
uses it to avoid attacks whereby people use '..' in a URI to try and  
get access to
files outside of the document tree. It might also normalise the path  
after creating
the combined path and ensuring the result still resides within the  
tree. If Apache has cleaned up req.uri already, this will not be  
needed. I need
to check for myself what Apache has already done at that point as not  

In terms of other ways of doing it, you might be able to use  
mod_rewrite, but for
this to work, the additional document tree would need to be available  
as a subtree
of the first. You would then be essentially using a internal redirect  
to get the
request applied against the subtree. This though can cause lots of  
base url
rewriting issues for pages. The subtree would also be directly  
accessible as
well unless you protect it by a handler in some way to ensure that is  
accessed as part of a subrequest.

In summary, you can't probably do it cleanly without modifying the  
main Apache
configuration, as you either have to setup a virtual host, or a trans  
handler, both
of which have to be done in the main configuration.

Can you now explain why you can't modify the main Apache configuration.


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