How to Use limit_conn in Nginx to Control Connections

Use the limit_conn_zone directive to define a shared memory zone for tracking connections, then apply limit_conn inside a server or location block to limit simultaneous connections per key, such as per IP address. This helps prevent overload by restricting how many connections a client can open at once.

Syntax

The limit_conn_zone directive defines a shared memory zone and the key to track connections (e.g., $binary_remote_addr for client IP). The limit_conn directive sets the maximum number of simultaneous connections allowed per key in a context like http, server, or location.

• limit_conn_zone key zone=name:size; - defines tracking zone
• limit_conn zone_name number; - limits connections per key

nginx

limit_conn_zone $binary_remote_addr zone=addr:10m;

server {
    listen 80;
    limit_conn addr 10;
    location / {
        # your config
    }
}

Example

This example limits each client IP to 5 simultaneous connections. It defines a 10MB shared memory zone named addr to track connections by IP, then applies the limit inside the server block.

nginx

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

    server {
        listen 80;

        limit_conn addr 5;

        location / {
            root /usr/share/nginx/html;
            index index.html;
        }
    }
}

Output

When a client opens more than 5 simultaneous connections, Nginx returns a 503 Service Unavailable error for the extra connections.

Common Pitfalls

• Not defining limit_conn_zone before using limit_conn causes configuration errors.
• Using an insufficient zone size can cause inaccurate tracking; 10MB is usually enough for many clients.
• Applying limit_conn in the wrong context (like outside http or server) will not work.
• Confusing limit_conn with limit_req which limits request rate, not connections.

nginx

http {
    # Wrong: missing limit_conn_zone

    server {
        listen 80;
        limit_conn addr 5;  # Error: addr zone not defined
    }
}

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

    server {
        listen 80;
        limit_conn addr 5;
    }
}

Quick Reference

Directive	Purpose	Context	Example
limit_conn_zone	Defines shared memory zone and key for tracking connections	http	limit_conn_zone $binary_remote_addr zone=addr:10m;
limit_conn	Limits number of simultaneous connections per key	http, server, location	limit_conn addr 5;

Key Takeaways

Always define limit_conn_zone before using limit_conn to track connections by key.

Use $binary_remote_addr as the key to limit connections per client IP efficiently.

Set an appropriate zone size (e.g., 10m) to store connection states reliably.

Apply limit_conn inside server or location blocks to enforce connection limits.

Exceeding the limit returns a 503 error to clients opening too many connections.