Inorder to enable passvie ftp in linux do the following steps:
1) Edit ftp configuration file ( /etc/proftpd.conf ) and append your desired passive ftp port range as follows:
PassivePorts 46000 47000
2) Open above port range in your IPTables, your IPTables should have a rule like this:
ACCEPT tcp -- anywhere anywhere tcp dpts:46000:47000 state NEW
3) Make sure that this port range is also open in your external firewall.
Now you should connect to your server in passive mode.