nginx.conf

# Add to nginx.conf http section
map $http_upgrade $connection_upgrade {
    default upgrade;
    ''      close;
}

site.conf

upstream some_upsteram_com {
    server 127.0.0.1:3000;
    keepalive 15;
}

server {
	listen 80;

	root /home/user/www/public;
	index index.html index.htm;

	access_log  /home/user/logs/site-access.log;
	error_log  /home/user/logs/site-error.log;

	server_name site.com www.site.com;

	large_client_header_buffers 8 32k;

	location / {
	    try_files $uri @nodejs;
	}


# Important! Serve client socket.io file as normal static file, e.g. /js/libs/socket.io/socket.io.min.js
	location /socket.io/ {
	    proxy_pass http://some_upsteram_com;
            proxy_http_version 1.1;
            proxy_set_header Upgrade $http_upgrade;
            proxy_set_header Connection $connection_upgrade;
	    proxy_redirect off;

	    proxy_buffers 8 32k;
	    proxy_buffer_size 64k;

	    proxy_set_header X-Real-IP $remote_addr;
            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
	    proxy_set_header Host $http_host;
    	    proxy_set_header X-NginX-Proxy true;
	}

	location ~* \.(jpg|jpeg|png|gif|ico|css|js)$ {
	    expires 30d;
	}

	location @nodejs {
	    proxy_pass http://some_upsteram_com;
	}

	location ~ /\. {
		deny all;
	}
}