Adv CSS/JS Agg - If-Modified-Since warning

The Last-Modified header is needs to be set in order for it to work.

I should look into setting etags as well and testing; as that will give a 304 as well.

there is no Header unset Last-Modified line to remove from .htaccess.

That advice only is given because that string was found inside the .htaccess file. Code in question

        // Get path to advagg .htaccess file.
        $files = array(
          $file_path . '/.htaccess',
          DRUPAL_ROOT . '/.htaccess',
        );

        // Check for bad .htaccess files.
        $bad_config_found = FALSE;
        foreach ($files as $count => $file) {
          if (!file_exists($file)) {
            continue;
          }
          $contents = file_get_contents($file);
          if (strpos($contents, 'Header unset Last-Modified') !== FALSE) {
            $bad_config_found = TRUE;
            $requirements['advagg_' . $type . '_last-modified_' . $key . $count] = array(
              'title' => $t('Adv CSS/JS Agg - Last-Modified'),
              'severity' => REQUIREMENT_WARNING,
              'value' => $t('The Last-Modified header is not being sent out by your web server.'),
              'description' => $t('The web servers configuration will need to be adjusted. The server should have sent out a Last-Modified header. Remove "Header unset Last-Modified" inside this file to fix the issue: @file', array('@file' => $file)),
            );
          }
        }

Also see if any updates need to be ran. I have some updates that remove this from the htaccess file when coming from an older version of advagg.

Patch adds in etag support.

css htaccess

<FilesMatch "^css__[A-Za-z0-9-_]{43}__[A-Za-z0-9-_]{43}__[A-Za-z0-9-_]{43}.css(\.gz)?">
  # Use ETags.
  FileETag MTime Size
  
  # Use Expires Directive.
  <IfModule mod_expires.c>
    # Enable expirations.
    ExpiresActive On
    # Cache all aggregated css files for 52 weeks after access (A).
    ExpiresDefault A31449600
  </IfModule>
  
  <IfModule mod_headers.c>
    # Serve correct content type.
    Header set Content-Type text/css
    <IfModule !mod_expires.c>
      # Set a far future Cache-Control header to 52 weeks.
      Header set Cache-Control "max-age=31449600, no-transform, public"
    </IfModule>
    <IfModule mod_expires.c>
      Header append Cache-Control "no-transform, public"
    </IfModule>
  </IfModule>
  ForceType text/css
</FilesMatch>

js htaccess

<FilesMatch "^js__[A-Za-z0-9-_]{43}__[A-Za-z0-9-_]{43}__[A-Za-z0-9-_]{43}.js(\.gz)?">
  # Use ETags.
  FileETag MTime Size
  
  # Use Expires Directive.
  <IfModule mod_expires.c>
    # Enable expirations.
    ExpiresActive On
    # Cache all aggregated js files for 52 weeks after access (A).
    ExpiresDefault A31449600
  </IfModule>
  
  <IfModule mod_headers.c>
    # Serve correct content type.
    Header set Content-Type application/javascript
    <IfModule !mod_expires.c>
      # Set a far future Cache-Control header to 52 weeks.
      Header set Cache-Control "max-age=31449600, no-transform, public"
    </IfModule>
    <IfModule mod_expires.c>
      Header append Cache-Control "no-transform, public"
    </IfModule>
  </IfModule>
  ForceType application/javascript
</FilesMatch>

This will use Etag as well as Last-Modified. Hopefully one of them (Etag, Last-Modified) works for you.

I have (twice actually) the same warning on Status report, using Advagg 7.x-2.14, or last dev as of today.
My config D7.38 and Nginx/1.8.0 (a-la-perusio):
"Adv CSS/JS Agg - If-Modified-Since The If-Modified-Since (Last-Modified) header is being ignored by your web server.
The web servers configuration will need to be adjusted. The server should have responded with a 304, instead a 200 was returned."

However, when tested with https://varvy.com/tools/if-modified/ (Does your webpage support the 304 If Modified Since HTTP header?) the answer is YES.
This tool checked your HTTP headers and received this response ... Last-Modified: Wed, 05 Aug 2015 07:07:19 GMT
Tool then rechecked looking for if modified since support and found it... Server Response HTTP/1.1 304 Not Modified

Any clues ? Maybe the status report is not having this response asking from localhost instead of "external" ping ?
Thanks.

pedrosp
Looking into this more and can you open up a new support request for the issue you are having?

Done #2546564: Adv CSS/JS Agg - If-Modified-Since warning but 304 is delivered.

Thanks.