nginx - connect() failed upstream under load testing

Already answered here: https://stackoverflow.com/questions/14144396/nginx-proxy-connect-to-ip80-failed-99-cannot-assign-requested-address

The message suggests you've run out of local sockets/ports.

Try to increase networking limits:

echo "10240 65535" > /proc/sys/net/ipv4/ip_local_port_range
sysctl net.ipv4.tcp_timestamps=1
sysctl net.ipv4.tcp_tw_recycle=0
sysctl net.ipv4.tcp_tw_reuse=1
sysctl net.ipv4.tcp_max_tw_buckets=10000

Alternatively you may try unix sockets to see if it helps.

Overview of Network Sockets When a connection is established over TCP, a socket is created on both the local and the remote host. The remote IP address and port belong to the server side of the connection, and must be determined by the client before it can even initiate the connection. In most cases, the client automatically chooses which local IP address to use for the connection, but sometimes it is chosen by the software establishing the connection. Finally, the local port is randomly selected from a defined range made available by the operating system.The port is associated with the client only for the duration of the connection, and so is referred to as ephemeral. When the connection is terminated, the ephemeral port is available to be reused.

Solution Enabling Keepalive Connections

Use the keepalive directive to enable keepalive connections from NGINX to upstream servers, defining the maximum number of idle keepalive connections to upstream servers that are preserved in the cache of each worker process. When this number is exceeded, the least recently used connections are closed. Without keepalives you are adding more overhead and being inefficient with both connections and ephemeral ports.

http {
    upstream backend {
        server 10.0.0.100:1234;
        server 10.0.0.101:1234;
 }

    server {
        # ...
        location / {
            # ...
            proxy_pass http://backend;
            proxy_bind $split_ip;
            proxy_set_header X-Forwarded-For $remote_addr;
        }
    }

    split_clients "$remote_addr$remote_port" $split_ip {
        10%  10.0.0.210;
        10%  10.0.0.211;
        10%  10.0.0.212;
        10%  10.0.0.213;
        10%  10.0.0.214;
        10%  10.0.0.215;
        10%  10.0.0.216;
        10%  10.0.0.217;
        10%  10.0.0.218;
        *    10.0.0.219;
    }
}

more : https://www.nginx.com/blog/overcoming-ephemeral-port-exhaustion-nginx-plus/