I recently updated my Ubuntu box to Ubuntu 12.04 and I found out the ftp server is not working. From my ftp client, I was getting this error:

do_ypcall: clnt_call: RPC: Unable to send; errno = Network is unreachable

It turns out there is a bug with vsftpd when running NIS.

The workaround is to simply add this to your /etc/vsftpd.conf file:

isolate_network=NO

Fixed!