let's say i have 2 FTP server (FTP_1 and FTP_2) in a private network.
what should be the setup in the firewall for me to forward the FTP request coming from the outside network and how the forwarding would know that the request is intended to FTP_1 and not to FTP_2 or vice versa?
I think I understand what you're getting at here. If you want external clients to access FTP1 you would just set up your NAT/Port Forwarding to forward any attempts on the FTP ports (20 and 21) to the IP address of your FTP1 server. All external FTP accesses on the traditional ports would then be handled by FTP1. If you wanted external clients to be able to use FTP2 as well, you'd need to forward two different ports to the FTP2 server IP address and let people know which ports to go to should they want to access FTP2 instead. If you wanted FTP1 and FTP2 to both be accessible from the outside then the best way might be some sort of load-balancing arrangement where the two servers share/host a common IP address. That way your port forwarding would simply be as in the first case above
what i am planning to do is to setup an offsite backup and to run it automatically at night when the office is off. the data to be backed-up is not that big, it is only the modified files during the day (incremental).
is ftp a good option? :idea: any suggestions? thanks
Functionally yes; but from the security angle are you sure you want to do this? Have a careful think about the implications. What would be the impact of someone else grabbing a copy of your data? If you want the transfer encrypted perhaps you could look at SSH file copy instead. And make sure you use a good implementation of FTP (many have vulnerabilities) and that you tie down the access permissions and user accounts properly.