Newbie Questions: How to set the proxy to download files by ftp protocol?

Hi
I want to download a log file from the remote server, and I'm outside of
the firewall. so I need to access the remote server by proxy. But I
failed to figure out what's the correct way to set the proxy.

I had the followint code snippet:

  ENV['ftp_proxy'] = "192.168.254.129:4480"
  Net::FTP.open('132.201.245.80') do |ftp|
    ftp.login("usename", "password")

···

##
  end

It seems that the envinroment variable "ftp_proxy" has no effect.

Any clue for this problem will be appreciated.

Thanks
Eric