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List:       apache-modgzip
Subject:    [Mod_gzip] Sending Header based upon Protocol
From:       Eli White <ewhite () stsci ! edu>
Date:       2002-03-14 16:53:30
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Is it not possible?

I've decided from talking to a few people that the easiest/simplist/safest 
way to implement mod_gzip and not have problems ...

Is to only allow HTTP/1.1 requests.  This will ignore bad proxies, and bad 
browsers.  A bit harsh, but does the job (60% of our requests are HTTP/1.1 
right now, so it helps a significant portion of the populace).

However.  From what I understand, I still need to send a 'Vary' header 
saying : "Vary: Accept-Encoding", right?  to make a HTTP/1.1 proxy 
understand that different content is coming back at them.

Problem is, I need to only send it for HTTP/1.1 requests.

Is this possible?  At all?

I can do the following:
  SetEnvIf Request_Protocol "^HTTP/1.1$" http11

But now how can I have a conditional section (like <Directory> or 
<Location>) based upon an Environment Variable??  I don't see a way.

The only possible way I see to accomplish this, is QUITE convoluted, Using 
mod_rewrite to 'change' the URL (either adding a suffix, or a 
subdirectory).  Then being able to use <Files> or <Location> to map this 
URL (which happens to be the original, and return the 'Vary' Header)

Is this the only way?

Something like:
...
DocumentRoot /var/html/htdocs
...
Alias /http11 /var/html/htdocs
<Location /http11>
   Header set Vary Accept-Encoding
</Location>
...
RewriteEngine on
RewriteCond %{THE_REQUEST} HTTP/1\.1$
RewriteCond %{REQUEST_URI} !^/http11
RewriteRule (.*) /http11$1 [PT]

This works, setting the Vary only for HTTP/1.1 browsers, but is QUITE 
convoluted for such a 'simple' task.  (And actually, the above should be a 
little more complicated, to only do this for files that aren't compressed 
in the first place, and not to rewrite .gifs/.jpgs/etc.)

So, what about the simple solutions?  How 'bad' is it to NOT return a Vary, 
if you are only responding to 1.1 responses anyway?  Should a 1.1 Proxy 
understand the compressed data, and therefore serve it correctly 'down below?'.

How 'bad' is it to just ALWAYS send a Vary, even perhaps for 
images/etc.  Sure, it may not actually be 'Vary-ing', but it gets the point 
across, even at the slight increased cost to the proxies of storing 
different versions for no reason.

Thanks,
Eli

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