How to Configure Nginx Sub Filter Module

The sub_filter module modifies response content by replacing text strings. It is useful for URL rewriting in proxied responses. This guide walks through the specific troubleshooting steps to diagnose and resolve sub_filter issues.

Before You Begin

Before you begin, be sure to have the following in place:

• A Linux server with the relevant software installed
• Access to the command line interface
• Appropriate permissions (root or sudo)

Quick Fix

Wrong

Manual text replacement in application code

Wrong: Manual replacement in backend

Right

sub_filter 'http://old.example.com' 'http://new.example.com';\nsub_filter_once off;\nsub_filter_types text/html text/css;

Right: Nginx sub_filter for response text substitution

Output

Sub filter configured:\n  Replace: http://old.example.com → http://new.example.com\n  Types: text/html, text/css\n  Mode: all occurrences

Prevention

To avoid future issues, follow these best practices:

• Use sub_filter for simple text replacement in responses
• Set sub_filter_once off to replace all occurrences
• Use sub_filter_types to restrict to specific MIME types
• sub_filter works on compressed responses after decompression
• Combine with proxy_pass for reverse proxy content rewriting

DodaTech Tools

For further assistance with any of the above issues, consider using DodaTech consulting services or DodaTech tutorials for more in-depth guidance.

Common Mistakes with sub filter

1. Non-exhaustive pattern matches that compile with warnings then crash at runtime
2. Misunderstanding that String is [Char] with poor performance for large text operations
3. Using foldl instead of foldl' causing stack overflow on large lists

These mistakes appear frequently in real-world NGINX code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ