Passive Mode FTP with iptables

There’s lots of advice on the net about how to setup a server with iptables to allow passive mode FTP. Below is the approach that we’ve found to be most effective.
Start by configuring your FTP daemon to use a fixed range of ports. We use 41361 to 65534 which is the IANA registered ephemeral port range. The exact config depends on what FTP software you’re using:
vsftpd
Edit /etc/vsftpd/vsftpd.conf and add the following lines:

pasv_min_port=49152 
pasv_max_port=65534

proftpd
Edit /etc/proftpd.conf and add to the Global section:

</Global>
......
PassivePorts 49152 65534
</Global>

Now restart your FTP service so the changes take effect.
Next you’ll need to configure the ip_conntrack_ftp iptables module to load. On Redhat/CentOS just edit /etc/sysconfig/iptables-config and add “ip_conntrack_ftp” to the IPTABLES_MODULES like this:

IPTABLES_MODULES="ip_conntrack_ftp"

Now restart the iptables service:

/sbin/service iptables restart

You can verify that the correct port range has been registered with lsmod like this:

lsmod | grep conntrack_ftp

and you’ll get something like this:

ip_conntrack_ftp       41361  0
ip_conntrack           91621  2 ip_conntrack_ftp,xt_state

And that’s all it takes to get passive mode ftp working behind iptables.
One extra note: If your server is NATed behind a physical firewall then you’ll probable need to load the “ip_nat_ftp” iptables module.