Nginx Tip - Implement the ngx_http_sub_module for response filtering

Nginx is a powerful web server and reverse proxy server that is widely used for hosting websites and applications. It offers various modules that enhance its functionality and allow for advanced configurations. One such module is the ngx_http_sub_module, which enables response filtering and modification. In this article, we will explore how to implement this module and leverage its capabilities.

What is the ngx_http_sub_module?

The ngx_http_sub_module is an Nginx module that provides the ability to modify the response body of HTTP requests. It allows you to search for specific patterns in the response and replace them with different content. This can be useful for various purposes, such as rewriting URLs, modifying HTML content, or adding headers to the response.

Enabling the ngx_http_sub_module

To use the ngx_http_sub_module, you need to ensure that it is enabled in your Nginx configuration. By default, this module is included in most Nginx installations, so you just need to enable it by adding the following line to your Nginx configuration file:

http {
    ...
    sub_filter_types *;
    sub_filter_once off;
    ...
}

The sub_filter_types * directive ensures that the module is applied to all response types. The sub_filter_once off directive allows multiple replacements to be made in a single response. Make sure to reload or restart Nginx after making these changes.

Using the ngx_http_sub_module

Once the ngx_http_sub_module is enabled, you can start using it to filter and modify responses. The module provides the sub_filter directive, which is used to define the patterns to search for and the replacement content. Here's an example:

location / {
    ...
    sub_filter 'example' 'replacement';
    sub_filter_once off;
    ...
}

In this example, any occurrence of the word 'example' in the response will be replaced with 'replacement'. You can specify multiple sub_filter directives to perform multiple replacements. The sub_filter_once off directive allows these replacements to be made multiple times in a single response.

Advanced Usage

The ngx_http_sub_module also provides additional directives for more advanced filtering and modification:

• sub_filter_last_modified on;: This directive enables the module to modify the Last-Modified header of the response.
• sub_filter_once off;: This directive allows multiple replacements to be made in a single response.
• sub_filter_types: This directive specifies the response types to which the module should be applied. By default, it is set to *, which means all types.

These directives provide flexibility and control over how the ngx_http_sub_module operates, allowing you to tailor it to your specific needs.

Summary

The ngx_http_sub_module is a powerful module in Nginx that enables response filtering and modification. By using the sub_filter directive, you can search for specific patterns in the response and replace them with different content. This module is useful for various purposes, such as rewriting URLs, modifying HTML content, or adding headers to the response.

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