I came across a problem when trying to use the Ant FTP task on Ubuntu. Whenever I tried executing my build file I kept getting:

java.lang.ClassNotFoundException: org.apache.commons.net.ftp.FTPClientConfig

The simple fix is to make sure Ant can find the Apache commons-net Jar, either from under the Ant lib directory or in a custom location.

I installed Ant using Aptitude which places all of the shared Ant jars in /usr/share/ant/lib. I was confused because there was a symbolic link to ant-commons-net.jar which I thought should contain the missing classes. Turns out this Jar is actually the Ant FTP task library, you still need to download the commons-net Jar file.