tar -zxf nginx-*.tar.gz
cd nginx-*/
./configure && make && sudo make install

vi nginx.conf
worker_processes 1;
events {
worker_connections 1024;
}
http {
include mime.types;
default_type application/octet-stream;
sendfile on;
keepalive_timeout 3;
server {
listen 1.2.3.4:81;
server_name nginx;
location / {
proxy_pass http://1.2.3.4:80;
proxy_redirect off;
proxy_set_header Host $host;
proxy_set_header X-Real-IP $remote_addr;
proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
proxy_max_temp_file_size 0;
proxy_connect_timeout 20;
proxy_send_timeout 20;
proxy_read_timeout 90;
proxy_buffer_size 4k;
proxy_buffers 4 32k;
proxy_busy_buffers_size 64k;
proxy_temp_file_write_size 64k;
}
}
}

iptables -t nat -I PREROUTING ! -s 1.2.3.4 -d 1.2.3.4 -p tcp --dport 80 -j DNAT --to :81