iosart blog

Tweaking with the Apache configuration, I encountered some pretty interesting problems.

I used mod_rewrite to serve different content based on the HTTP request. Basically, what I was trying to do is this: there are two domains, “one.com” and “two.com” both pointing to the same server. If the user used the first domain name in the address, serve “Page A”, else serve “Page B”. This can be accomplished quite easily by using the RewriteCond %{HTTP_HOST} ^one.com$ directive.

I used the above technique and all was well, until I noticed that the images served by the one.com domain are not being cached by the browser! To find out what was going on I used wget with the “-S” option to see the full header returned by the server. The server was returning a header which included a “Vary: Host” field, which means that the server didn’t serve a regular static page, but its reply depends on the “Host” field in the HTTP request. Browsers interpret this as “the content returned is dynamic, don’t cache it.

I needed to get rid of the “Vary” header. One option was to use the Header unset Vary directive, but that requires mod_header module, which is unfortunately isn’t included in my server. Another option is to use the SetEnv force-no-vary directive. This tells the server not to return the “Vary” field.

It seams like problem solved… Not exactly. SetEnv force-no-vary along with suppressing the “Vary” header, forces the response to be HTTP/1.0. Why is this a problem? When serving HTTP/1.0, some advanced features of HTTP/1.1 aren’t used. One of these features is “Keep-Alive”. This feature can increase the loading time of the page by up to 50% by using the same connection for more than one request. Turn this off and it takes much longer to load pages with many images – and this incidentally was exactly the type of pages I was serving!

So “SetEnv force-no-vary” is out too. What can be done to make the server stop sending the damn “Vary” header? I needed a way to fool the server so it doesn’t notice I’m checking the Host header field. A simple solution then came too mind – what if I save the HTTP_HOST value in an environment variable and use this variable in the condition directive. The server won’t have any way of knowing that this variable actually contains the HTTP_HOST and stop sending the “Vary” field. I used the following code:

RewriteRule .*  - [E=MY_HTTP_HOST:%{HTTP_HOST}]
RewriteCond %{ENV:MY_HTTP_HOST} ^one.com$

The first line sets a new environment variable “MY_HTTP_HOST” and initializes it with HTTP_HOST. The second line is the condition and it looks to the server as if I check a regular environment variable. This finally solved the problem.

This entry was posted on Wednesday, May 26th, 2004 at 11:37 pm and is filed under Bugs. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.