This is frox, a transparent ftp proxy by James Hollingshead.

Current features include:

  o rfc959 compliant (I think) transparent proxying of ftp
    connections.
  o active --> passive mode conversion for data connections.
  o Optional caching support either locally or through an external
    HTTP cache.
  o Optional virus scanning
  o Optional non-transparent proxy support by logging in with
    user@host:port.
  o Options to bind to a specific interface, chroot, and drop
    priveleges for security.

WWW: http://www.hollo.org/frox/