Connection limiting (limit_conn) in Nginx - Commands & Configuration

Introduction

Sometimes a web server gets too many users at once, which can slow it down or crash it. Connection limiting helps control how many users connect at the same time to keep the server stable and fast.

When you want to stop one user from opening too many connections and slowing down the server.

When your website is getting a sudden spike of visitors and you want to keep it running smoothly.

When you want to protect your server from simple attacks that open many connections quickly.

When you run multiple websites on one server and want to limit connections per site.

When you want to make sure your server resources are shared fairly among users.

Config File - nginx.conf

nginx.conf

http {
    limit_conn_zone $binary_remote_addr zone=addr:10m;

    server {
        listen 80;
        server_name example.com;

        location / {
            limit_conn addr 5;
            root /var/www/html;
            index index.html;
        }
    }
}

limit_conn_zone defines a shared memory zone named 'addr' to track connections by client IP address.

limit_conn addr 5; limits each IP address to 5 simultaneous connections.

This setup helps prevent any single user from opening too many connections at once.

Commands

Check the nginx configuration file for syntax errors before applying changes.

Terminal

sudo nginx -t

Expected Output

nginx: the configuration file /etc/nginx/nginx.conf syntax is ok

nginx: configuration file /etc/nginx/nginx.conf test is successful

Reload nginx to apply the new connection limiting settings without stopping the server.

Terminal

sudo systemctl reload nginx

Expected Output

No output (command runs silently)

Test the server response to confirm nginx is running and serving pages after applying limits.

Terminal

curl -I http://localhost

Expected Output

HTTP/1.1 200 OK

Server: nginx

Date: Thu, 01 Jun 2023 12:00:00 GMT

Content-Type: text/html

Content-Length: 612

Connection: keep-alive

Key Concept

If you remember nothing else from this pattern, remember: limit_conn_zone tracks clients and limit_conn sets how many connections each client can open at once.

Common Mistakes

Not defining limit_conn_zone before using limit_conn.

Nginx needs the zone to track connections; without it, the limit_conn directive will cause errors.

Always define limit_conn_zone with a unique zone name and enough memory before using limit_conn.

Setting the limit_conn value too low, like 1, causing users to get blocked unnecessarily.

Users often open multiple connections for images, scripts, or tabs; too low a limit blocks normal use.

Set a reasonable limit like 5 or more depending on your site's needs.

Forgetting to reload nginx after changing the config.

Changes won't take effect until nginx reloads the configuration.

Run 'sudo systemctl reload nginx' after editing the config.

Summary

Define a shared memory zone with limit_conn_zone to track client connections.

Use limit_conn inside server or location blocks to set max simultaneous connections per client.

Test the configuration syntax with 'nginx -t' and reload nginx to apply changes.