Install Nginx with stream module:
sudo apt install nginx-full
The output of the command
nginx -V
should contain with-stream=dynamic.
And add the following to /etc/nginx/nginx.conf:
stream {
    server {
        listen 12345;                    # The port that Nginx listens to
        proxy_pass 192.168.0.121:389;    # Where to forward connections
    }
}
Reload Nginx configuration:
sudo nginx -t
sudo service nginx reload
Ensure the port is open:
telnet localhost 12345

