ProFTPD and iptables firewall configuration

I am using Redhat Linux and have ProFTPD running.

I have an iptables firewall that allows connections from specific ips to the default FTP port.

I want to allow a user access to a specific directory utilizing a username and password. I have created the user with no shell access. I can ftp locally into that directory or remotely from a specific ip.

However, the user that I want to give access to uses DHCP and consequently, the firewall wouldn't always know what the ip was. Also, the user wants to access the directory from a variety of locations.

So, I know that I can create user access in the ProFTPD config file, but how can I solve this firewall issue?

You may want to look at port knocking.