Debian + Apache VPS is caching files, how do I stop or manage that?

[AndyNewcastle]

We have a very simple VPS: Debian+Apache+PHP+MySQL. When we make some changes (for example make change in style.css) and open it in browser we can still see an old version of the file. Although when we look at file on server it is changed. But in browser we get old version. This issue always resolves itself within 5 minutes.

This server is currently used by 4 people: 2 people are in Europe, 1 in Asia and 1 in USA. So it is not a browser cache problem, router cache, internet provider cache ... it has to be something with the server itself. I'm suspecting Apache.

I tried to look into loaded modules (apache2ctl -M). But I don't see mod_cache or anything similar.

core_module (static)
 so_module (static)
 watchdog_module (static)
 http_module (static)
 log_config_module (static)
 logio_module (static)
 version_module (static)
 unixd_module (static)
 access_compat_module (shared)
 alias_module (shared)
 auth_basic_module (shared)
 authn_core_module (shared)
 authn_file_module (shared)
 authz_core_module (shared)
 authz_host_module (shared)
 authz_user_module (shared)
 autoindex_module (shared)
 deflate_module (shared)
 dir_module (shared)
 env_module (shared)
 filter_module (shared)
 headers_module (shared)
 mime_module (shared)
 mpm_prefork_module (shared)
 negotiation_module (shared)
 php_module (shared)
 reqtimeout_module (shared)
 rewrite_module (shared)
 setenvif_module (shared)
 socache_shmcb_module (shared)
 ssl_module (shared)
 status_module (shared)

[phranque]

perhaps there is a CDN or load balancer that is part of your hosting service.

i would look for evidence of proxy-related headers in the response, such as Via, Forwarded, or any of the X-Forwarded- headers.