Q: Does FTPClient support FTP connections through an FTP proxy server?
 
A: Paul Buchanan answers this question here: http://marc.theaimsgroup.com/?l=jakarta-commons-user&m=107877944806547&w=2
 
The summary is:
 
Since the Net project uses java.net.Socket under the covers, you have to set up the Socket settings. If the proxy you are using is a SOCKS4 or SOCKS5 proxy, you can set the following System Properties:
 
      System.getProperties().put( "socksProxyPort", "1080" );
      System.getProperties().put( "socksProxyHost" ,"proxy.host.address" );
 
From what I can tell, 1080 is the default port for SOCKS proxies. The following site documents many of the networking System Properties available:
http://java.sun.com/j2se/1.4.2/docs/guide/net/properties.html