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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:


Edit /etc/vsftpd/vsftpd.conf and add the following lines:


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

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:

Next edit /etc/sysconfig/iptables and add a rule to allow TCP port 21. The new line is marked in red:

Now restart the iptables service:

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

and you’ll get something like this:

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.

On a AWS EC2 server with vsftpd I had to add “pasv_address=x.x.x.x” to the /etc/vsftpd/vsftpd.conf file where x.x.x.x was the public (elastic) address of the server. On an AWS EC2 server with Plesk and proftpd I had to add “MasqueradeAddress x.x.x.x” to a new file at /etc/proftpd.d/1-pasv_addr.conf.