How to Implement Browser Caching with Nginx's header Module on CentOS 7

Introduction

The faster a website loads, the more likely a visitor is to stay. When websites are full of images and interactive content run by scripts loaded in the background, opening a website is not a simple task. It consists of requesting many different files from the server one by one. Minimizing the quantity of these requests is one way to speed up your website.

This can be done in many ways, but one of the more important steps to take is to configure browser caching. This is telling the browser that files downloaded once can be reused from local copies instead of requesting the server for them again and again. To do this, new HTTP response headers telling the browser how to behave must be introduced.

This is where Nginx’s header module comes into play. This module can be used to add any arbitrary headers to the response, but its major role is to properly set caching headers. In this tutorial, we will look at how to use Nginx’s header module to implement browser caching.

Prerequisites

To follow this tutorial, you will need:

• One CentOS 7 server set up with this initial server setup tutorial, including a sudo non-root user.

• Nginx installed on your server by following the How To Install Nginx on CentOS 7 tutorial.

In addition to the header module, we’ll also be using Nginx’s map module in this article. To learn more about the map module, you can read How To Use Nginx’s map Module on CentOS 7.

Step 1 — Creating Test Files

In this step, we will create several test files in the default Nginx directory. We’ll use these files later to check Nginx’s default behavior and then to test that browser caching is working.

To make a decision about what kind of file is served over the network, Nginx does not analyze the file contents; that would be prohibitively slow. Instead, it just looks up the file extension to determine the file’s MIME type, which denotes the purpose of the file.

Because of this behavior, the content of our test files is irrelevant. By naming the files appropriately, we can trick Nginx into thinking that, for example, one entirely empty file is an image and another is a stylesheet.

Create a file named test.html in the default Nginx directory using truncate. This extension denotes that it’s an HTML page.

sudo truncate -s 1k /usr/share/nginx/html/test.html

Let’s create a few more test files in the same manner: one jpg image file, one css stylesheet, and one js JavaScript file.

sudo truncate -s 1k /usr/share/nginx/html/test.jpg
sudo truncate -s 1k /usr/share/nginx/html/test.css
sudo truncate -s 1k /usr/share/nginx/html/test.js

The next step is to check how Nginx behaves with respect to sending caching control headers on a fresh installation with the files we have just created.

Step 2 — Checking the Default Behavior

By default, all files will have the same default caching behavior. To explore this, we’ll use the HTML file we created in step 1, but you can run these tests with any of the example files.

So, let’s check if test.html is served with any information regarding how long the browser should cache the response. The following command requests a file from our local Nginx server and shows the response headers.

curl -I http://localhost/test.html

You should see several HTTP response headers:

Output: Nginx response headers
HTTP/1.1 200 OK
Server: nginx/1.10.1
Date: Thu, 06 Oct 2016 10:21:04 GMT
Content-Type: text/html
Content-Length: 1024
Last-Modified: Thu, 06 Oct 2016 10:20:44 GMT
Connection: keep-alive
ETag: "57f6257c-400"
Accept-Ranges: bytes

In the second to last line you can see the ETag header, which contains a unique identifier for this particular revision of the requested file. If you execute the previous curl command repeatedly, you will see the exact same ETag value.

When using a web browser, the ETag value is stored and sent back to the server with the If-None-Match request header when the browser wants to request the same file again — for example, when refreshing the page.

We can simulate this on the command line with the following command. Make sure you change the ETag value in this command to match the ETag value in your previous output.

curl -I -H 'If-None-Match: "57f6257c-400"' http://localhost/test.html

The response will now be different:

Output: Nginx response headers
HTTP/1.1 304 Not Modified
Server: nginx/1.10.1
Date: Thu, 06 Oct 2016 10:21:40 GMT
Last-Modified: Thu, 06 Oct 2016 10:20:44 GMT
Connection: keep-alive
ETag: "57f6257c-400"

This time, Nginx will respond with 304 Not Modified. It won’t send the file over the network again; instead, it will tell the browser that it can reuse the file it already has downloaded locally.

This is useful because it reduces network traffic, but it’s not quite good enough for achieving good caching performance. The problem with ETag is that browser always sends a request to the server asking if it can reuse its cached file. Even though the server responds with a 304 instead of sending the file again, it still takes time to make the request and receive the response.

In the next step, we will use the headers module to append caching control information. This will make the browser to cache some files locally without explicitly asking the server if its fine to do so.

Step 3 — Configuring Cache-Control and Expires Headers

In addition to the ETag file validation header, there are two caching control response headers: Cache-Control and Expires. Cache-Control is the newer version, which has more options than Expires and is generally more useful if you want finer control over your caching behavior.

If these headers are set, they can tell the browser that the requested file can be kept locally for a certain amount of time (including forever) without requesting it again. If the headers are not set, browsers will always request the file from the server, expecting either 200 OK or 304 Not Modified responses.

We can use the header module to set these HTTP headers. The header module is a core Nginx module, which means it doesn’t need to be installed separately to be used.

To add the header module, open the default server block Nginx configuration file in vi (here’s a short introduction to vi) or your favorite text editor.

sudo vi /etc/nginx/nginx.conf

Find the server configuration block, which looks like this:

. . .
server {
    listen 80 default_server;
    listen [::]:80 default_server;
. . .

Add the following two new sections here: one before the server block, to define how long to cache different file types, and one inside it, to set the caching headers appropriately.

. . .
# Expires map
map $sent_http_content_type $expires {
    default                    off;
    text/html                  epoch;
    text/css                   max;
    application/javascript     max;
    ~image/                    max;
}

server {
    listen 80 default_server;
    listen [::]:80 default_server;

    expires $expires;
. . .

The section before the server block is a new map block which defines the mapping between the file type and how long that kind of file should be cached.

We’re using several different settings in this map:

• The default value is set to off, which will not add any caching control headers. It’s a safe bet for content we have no particular requirements on how the cache should work.

• For text/html, we set the value to epoch. This is a special value which results explicitly in no caching, which forces the browser to always ask if the website itself is up to date.

• For text/css and application/javascript, which are stylesheets and Javascript files, we set the value to max. This means the browser will cache these files for as long as it can, reducing the amount of requests considerably given that there are typically many of these files.

• The last setting is for ~image/, which is a regular expression that will match all file types containing image/ in their MIME type name (like image/jpg and image/png). Like stylesheets, there are often a lot of images on websites that can be safely cached, so we set this to max as well.

Inside the server block, the expires directive (a part of the headers module) sets the caching control headers. It uses the value from the $expires variable set in the map. This way, the resulting headers will be different depending on the file type.

Save and close the file to exit.

To enable the new configuration, restart Nginx.

sudo systemctl restart nginx

Next, let’s make sure our new configuration works.

Step 4 — Testing Browser Caching

Execute the same request as before for the test HTML file.

curl -I http://localhost/test.html

This time the response will be different. You should see two additional HTTP response headers:

HTTP/1.1 200 OK
Server: nginx/1.10.1
Date: Thu, 06 Oct 2016 10:24:42 GMT
Content-Type: text/html
Content-Length: 1024
Last-Modified: Thu, 06 Oct 2016 10:20:44 GMT
Connection: keep-alive
ETag: "57f6257c-400"
Expires: Thu, 01 Jan 1970 00:00:01 GMT
Cache-Control: no-cache
Accept-Ranges: bytes

The Expires header shows a date in the past and Cache-Control is set with no-cache, which tells the browser to always ask the server if there is a newer version of the file (using the ETag header, like before).

You’ll see a difference response with the test image file.

curl -I http://localhost/test.jpg

HTTP/1.1 200 OK
Server: nginx/1.10.1
Date: Thu, 06 Oct 2016 10:25:02 GMT
Content-Type: image/jpeg
Content-Length: 1024
Last-Modified: Thu, 06 Oct 2016 10:20:46 GMT
Connection: keep-alive
ETag: "57f6257e-400"
Expires: Thu, 31 Dec 2037 23:55:55 GMT
Cache-Control: max-age=315360000
Accept-Ranges: bytes

In this case, Expires shows the date in the distant future, and Cache-Control contains max-age information, which tells the browser how long it can cache the file in seconds. This tells the browser to cache the downloaded image for as long as it can, so any subsequent appearances of this image will use local cache and not send a request to the server at all.

The result should be similar for both test.js and test.css, as both JavaScript and stylesheet files are set with caching headers too.

This means the cache control headers have been configured properly and your website will benefit from the performance gain and less server requests due to browser caching. You should customize the caching settings based on the content for your website, but the defaults in this article are a reasonable place to start.

Conclusion

The headers module can be used to add any arbitrary headers to the response, but properly setting caching control headers is one of its most useful applications. It increases performance for the website users, especially on networks with higher latency, like mobile carrier networks. It can also lead to better results on search engines that factor speed tests into their results. Setting browser caching headers is one of the major recommendations from Google’s PageSpeed testing tools.

More detailed information about the headers module can be found in Nginx’s official headers module documentation.

location ~* ^.+\.(ogg|ogv|svg|svgz|eot|otf|woff|woff2|mp4|ttf|zip|tgz|gz|rar|bz2|doc|xls|exe|ppt|tar|mid|midi|wav|bmp|rtf)$ {
       access_log off; 
       log_not_found off; 
       expires 1y;
       add_header Cache-Control "public, no-transform";    
}

location ~* \.(js|css|png|jpg|jpeg|gif|ico)$ {
    expires 1y;
    add_header Cache-Control "public, no-transform";
}

    location ~* \.(?:ico|css|js|gif|jpe?g|png|svg|woff)$ {
        expires 7d;
        add_header Pragma public;
        add_header Cache-Control "public";
        try_files $uri @rewrite;
    }
code

nginx: [emerg] "expires" directive invalid value in /etc/nginx/conf.d/tomcat-basic.conf:34