Resolving Issues Caused by 'Header unset Vary' in Speed Optimizer

Users of the 'Speed Optimizer – The All-In-One Performance-Boosting Plugin' have reported encountering issues related to a specific rule the plugin adds to their .htaccess file. This article explains the problem, why it occurs, and provides the most common and effective solutions.

The Problem: 'Header unset Vary'

The plugin adds the following code block to the .htaccess file to improve caching efficiency:

# SGO Unset Vary
Header unset Vary
# SGO Unset Vary END

This directive is intended to remove the Vary: User-Agent HTTP header, which can sometimes prevent efficient caching of site resources. However, this has been linked to two primary issues:

1. Incorrect Caching Behavior: In some scenarios, this rule can cause browsers like Safari, Chrome, and Firefox to serve aggressively cached versions of webpages, even when dynamic caching is disabled or when there are other no-cache directives present. This can break functionality that relies on cookies or user-specific content for anonymous users.
2. Internal Server Errors: On some server configurations, particularly local development or staging environments that may not have the Apache mod_headers module enabled, the unconditional Header unset Vary directive can cause a 500 Internal Server error and prevent the site from loading entirely.

Why This Happens

The core of the issue is that the directive is written without a conditional wrapper to check if the required Apache module is present. While the plugin is designed for SiteGround's servers, which always have the mod_headers module active, users often sync their .htaccess files to other environments (like local development machines) where this module might not be loaded. The unconditional rule then fails, crashing the site.

Common Solutions

Solution 1: Modify the .htaccess Rule (Recommended)

The most robust fix is to wrap the directive in a conditional check for the mod_headers.c module. This prevents server errors on environments where the module is not available.

1. Access your website's root directory via FTP, SFTP, or your hosting file manager.
2. Locate and open the .htaccess file for editing.
3. Find the block of code that starts with # SGO Unset Vary.
4. Replace the existing code with the following wrapped version:

   # SGO Unset Vary
   <IfModule mod_headers.c>
     Header unset Vary
   </IfModule>
   # SGO Unset Vary END

5. Save the changes and upload the file back to the server.

This change ensures the command only runs in a compatible environment and is widely reported to resolve both the caching and server error issues.

Solution 2: Toggle the Browser-Specific Caching Setting

Based on user testing, the plugin's behavior is also tied to its 'Browser-Specific Caching' option. The interaction can be confusing:

• If 'Browser-Specific Caching' is DISABLED: The plugin adds the Header unset Vary rule.
• If 'Browser-Specific Caching' is ENABLED: The plugin removes the Header unset Vary rule entirely and does not add a Vary: User-Agent header.

Therefore, a simple workaround is to enable the 'Browser-Specific Caching' option within the Speed Optimizer plugin settings. This will trigger the removal of the problematic rule from your .htaccess file.

Conclusion

The unconditional Header unset Vary rule can cause significant caching problems and server errors. The most future-proof solution is to manually edit your .htaccess file to wrap the directive in an <IfModule> tag. As an alternative, enabling the 'Browser-Specific Caching' feature in the plugin's interface will also remove the rule. Users who frequently sync files from a production environment to a local development setup may find the first solution more permanent.