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From André Warnier>
Subject Re: [users@httpd] Port-based vhosts
Date Wed, 11 Mar 2009 09:15:01 GMT
Charles Sprickman wrote:
   Under what
> conditions does Apache then get involved and alter the URL?  Just 
> redirects?  I understand a common redirect is just adding a trailing 
> slash when the user does not supply it.  What are some other common 
> cases? Who's call is it when a simple static site uses non-absolute URLs 
> for all the links?  Is the browser building the fully-qualified links or 
> apache (I suspect the former)?
If you suspect that it is the browser, you suspect correctly.  But the 
explanation is somewhat messy (and lengthy) unless you really understand 
the basics.  Let me try a not entirely correct but hopefully didactic 

Say the browser retrieves a first html page from a server, using the URL 
"".  This URL, from which the 
browser retrieved the current page, is now for the browser the "base 
URL" of the currently displayed document.
Now say that this page contains a relative link like <img 
src="images/myface.gif" />.
If the user clicks on this link, the browser will construct a new URL by
- removing the last component of the base URL (in this case 
"mypage.html"), leaving ""
- re-adding to that the relative link "images/myface.gif", giving 
- retrieving this new URL
Nothing of that happens at the server side.  It's all done at the 
browser level, any browser.

In reality, what happens is a bit different, because in a URL like 
"", there are several parts 
which are processed differently and independently, and a HTTP request is 
not really to "".  The real 
HTTP request sequence is more like this :

a) the browser opens a TCP connection to port 80 of the host which has 
the IP address corresponding to the DNS resolution of the hostname 

b) on that connection, the browser writes a HTTP request like
GET /mydir/mypage.html HTTP/1.1

then it switches to read mode and waits for the server's response to 
arrive on that same connection.

So in my first explanation above, you have to leave out the "protocol" 
and "host:port" from the current page's base URL, but the general idea 

Now about the redirects, re-using the above logic.
(This is what is called "external redirects", see later).

b) the browser sends a request to the server, like
GET /mydir HTTP/1.1

c) the server sends a response to the browser, like
301 (this thing has moved, definitely)
Location: /mydir/  (here is the new location)

d) now the browser, automatically, re-sends a new request on the same 
connection :
GET /mydir/ HTTP/1.1

e) and, presumably, the server now responds with the requested content.

In addition, if the browser is smart, it will remember that the URL 
"/mydir" has moved to "/mydir/", and the next time it will request it 
directly, even if the forgetful user would request "/mydir" again.  It 
will also show the "/mydir/" in the URL bar for that page, because that 
is the real URL it got the page from (and in the vain hope of educating 
the user about the fact that the URL "/mydir" is the wrong one and 
should not be used anymore).

So, the penalty of using a 301 re-direct is that there is one more 
round-trip server-browser-server (see c and d above).  But it is a 
relatively small one, because the content is very short, and because 
nowadays with keep-alive connections the same TCP connection 
browser-server can be used for all of it.
The benefit is that the browser has the correct idea of what the "base 
URL" is at all times, and thus that it can correctly interpret relative 
URLs and compose the correct follow-up requests.

"Internal" redirects :

These are things that the server does internally, without telling the 
browser about it.  mod_rewrite allows you to internally modify a request 
URL before the rest of the server will make an attempt at finding and 
serving the requested resource.  In that case thus, the browser sends a 
request like
GET /mydir HTTP/1.1

and the server, internally, modifies this "/mydir" to "/anotherdir/", 
then proceeds to immediately serve the content of "/anotherdir/", 
without sending a redirect to the browser, and without telling the 
browser about anything.  The browser gets a response :
200 OK
.. content of "/anotherdir/"

This is obviously faster, because you avoid a round-trip to the browser 
and back, through a potentially slow connection.

But now the browser does not know about the substitution, and genuinely 
believes that what it got was the content corresponding to the "/mydir" 
URL. So now if in this content it finds relative links like 
"images/myface.gif", it will interpret them relative to the base URL 
"/mydir", and that may cause further problems.
So by doing this, you may be saving one round-trip for the original 
"/mydir", but at best forcing subsequent round-trips for other links, at 
worst potentially confusing the browser into requesting further invalid 

Whether one or the other scenario is better in your case, depends on 
many factors, and you have to evaluate those yourself in function of 
your website and what is really going on there.

The official User-To-User support forum of the Apache HTTP Server Project.
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