How do you limit concurrent connections per client?

Connection limiting protects against clients holding many sockets open, such as slow downloaders or abandoned connections. It does not directly limit how many requests a client sends over one connection.

limit_conn_zone $binary_remote_addr zone=addr:10m;
limit_conn_zone $server_name zone=per_server:10m;

server {
    limit_conn addr 10;
    limit_conn per_server 1000;
    limit_conn_status 429;

    location /downloads/ {
        limit_conn addr 2;
        limit_rate 2m;
    }
}

How it differs from rate limiting

• limit_conn caps simultaneous connections; limit_req caps requests over time
• HTTP/2 multiplexes many streams over one connection, so connection limits are weaker there
• limit_rate caps bandwidth per response, not request count
• Zones use shared memory, so the limit applies across workers
• A low limit can break browsers that open several connections to the same host