ProFTPd Server
From DD-WRT Wiki
DD-WRT, by default on either mini-usb or MEGA builds, includes an FTP daemon known as ProFTPd. While it does not allow for very many configurable options via the DD-WRT Web-GUI, there are some good things to know before you go hosting the contents of a drive to either the LAN or the outside WAN.
Of the few configurable options, the one that is most important is setting the permission parameters. By default the "Allow Write" is disabled. If you wish to upload information, media, etc. from either your home LAN, or a remote location, then you must enable this. The next step is to set the users and passwords into the available box below. It is wise to note that when this is done, your "root" username for super-user should be placed on the first line, followed by other user-names and passwords. The format is as following:
root P@ssw0rd guest P@ssw0rd username P@ssw0rd
...and so on.
To allow for WAN access, it is wise to set up an account for a Dynamic DNS server