limit_conn

Sets the shared memory zone and the maximum allowed number of connections for a given key value. When this limit is exceeded, the server will return the 503 (Service Temporarily Unavailable) error in reply to a request. For example, the directives

limit_conn_zone $binary_remote_addr zone=addr:10m;

server {
    location /download/ {
        limit_conn addr 1;
    }

allow only one connection per an IP address at a time.

In HTTP/2 and SPDY, each concurrent request is considered a separate connection.

There could be several limit_conn directives. For example, the following configuration will limit the number of connections to the server per a client IP and, at the same time, the total number of connections to the virtual server:

limit_conn_zone $binary_remote_addr zone=perip:10m;
limit_conn_zone $server_name zone=perserver:10m;

server {
    ...
    limit_conn perip 10;
    limit_conn perserver 100;
}

These directives are inherited from the previous level if and only if there are no limit_conn directives on the current level.