How to Fix “ssl_session_cache Conflicts” in Nginx Easily

Introduction

If you are using Nginx as a web server and encounter an error like this when trying to start it:

nginx: [emerg] the size 52428800 of shared memory zone "SSL" conflicts with already declared size 10485760

This means there is a conflict in the ssl_session_cache configuration, specifically in the shared memory zone "SSL." This article will explain the cause of this issue and how to fix it.

Cause of the Problem

Nginx only allows a single ssl_session_cache declaration with a consistent size throughout all configurations. If multiple declarations exist with different sizes, Nginx will fail to start. For example:

In /etc/nginx/sites-enabled/ghost:

ssl_session_cache shared:SSL:50m;

In /etc/nginx/nginx.conf:

ssl_session_cache shared:SSL:10m;

Since the "SSL" zone is declared with different sizes, Nginx will throw an error and fail to start.

How to Fix It

Follow these steps to resolve the issue:

1. Check All Nginx Configurations

Run the following command to locate all ssl_session_cache declarations in Nginx:

grep -r "ssl_session_cache" /etc/nginx/

If you find multiple declarations with different sizes, proceed to the next step.

2. Standardize the ssl_session_cache Size

Choose one ssl_session_cache size to use consistently. For example, if you prefer 50m, make sure all configurations use it:

ssl_session_cache shared:SSL:50m;

Edit the files containing different sizes and adjust them to match.

3. Validate Nginx Configuration

After making changes, run the following command to check for errors:

sudo nginx -t

If the output shows syntax is okay, proceed to the next step.

4. Restart Nginx

Finally, restart Nginx to apply the changes:

sudo systemctl restart nginx

Now, Nginx should start without errors.

Conclusion

The "ssl_session_cache Conflicts" error in Nginx occurs when there are multiple ssl_session_cache declarations with different sizes. The solution is to find all existing declarations, unify their sizes, validate the configuration, and restart Nginx.

By following these steps, you can prevent configuration conflicts and ensure Nginx runs smoothly without issues.