Travis Horn

I recently ran into a frustrating caching issue with Apache. I requested a page where the request headers included If-Modified-Since and the response headers included Last-Modified. Since the timestamps matched, the server should have responded with a 304 Not Modified (and no content). Instead, it kept responding with a 200 OK and the full content payload.

Here is how I went down the rabbit hole to fix it.

How Conditional Requests Work

Before diving into the specific error, it helps to understand the “dance” the browser and server perform to decide whether to download a file again or use the cached version. This mechanism is called a Conditional GET.

Ideally, if the browser already has the file, we want the server to respond with a 304 Not Modified (header only, zero bytes of content) rather than a 200 OK (which re-sends the entire file).

There are two primary ways validators handle this:

1. Time-Based Validation

• Server sends: Last-Modified (a timestamp of when the file changed).

• Client asks: If-Modified-Since (sending that timestamp back).

• Logic: If the file hasn’t changed since that date, return 304.

2. Content-Based Validation (ETags)

• Server sends: ETag (a unique hash or fingerprint of the file contents, e.g., "12345").

• Client asks: If-None-Match (sending that hash back).

• Logic: If the file’s current hash matches the one the client has, return 304.

Usually, browsers send both. However, HTTP specifications and server configurations often prioritize the ETag because it is considered a more accurate validator than a simple timestamp.

The Investigation

First, I discovered that Apache prioritizes If-None-Match over If-Modified-Since. My request headers did indeed include If-None-Match, and the response headers included an ETag. Even though the values seemed to match, I was still getting a 200 OK.

Then I remembered I had enabled compression (mod_deflate) using AddOutputFilterByType.

It turns out that when Gzip compression is enabled, Apache appends a -gzip suffix to the ETag. This suffix was present in both my If-None-Match sent by the client and the ETag returned by the server. So, why was the cache validation still failing?

The Root Cause

The issue lies in Apache’s order of operations. Apache calculates the ETag for the underlying file and compares it to the client’s header before it applies compression.

The flow looks like this:

1. Receive Request: Client sends If-None-Match: "12345-gzip".

2. Calculate ETag: Apache calculates the ETag for the uncompressed file: "12345".

3. Compare: Apache compares "12345-gzip" (Client) vs "12345" (Server).

4. Result: No match.

5. Process Response: Apache compresses the file and appends -gzip to the ETag.

6. Send: Apache sends 200 OK with ETag: "12345-gzip".

Because the suffix is added after the comparison, the client’s ETag will never match the server’s calculated ETag.

The Solution

The solution is to simply disable ETags when compression is active, forcing the browser and server to rely on the If-Modified-Since header instead.

Inside your <IfModule mod_deflate.c> block, add FileETag None:

<IfModule mod_deflate.c>
    # ... your compression settings ...
    AddOutputFilterByType DEFLATE text/plain text/html ...
    
    # Disable ETags so the server falls back to Last-Modified
    FileETag None
</IfModule>
  

With ETags removed, the client and server successfully fall back to If-Modified-Since and Last-Modified. Since those timestamps match, Apache finally respects the cache and returns the elusive 304 Not Modified.

Removing ETags is Usually Fine

Disabling a standard HTTP feature like ETags might feel like a drastic measure, but for most static sites, it is actually a safe and often recommended optimization.

The only real downside is that Last-Modified has a one-second resolution. It works by checking the file’s timestamp. Unless you are modifying the exact same file multiple times within a single second, the timestamp precision is more than enough for browsers to detect changes.