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:
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pasv_min_port=49152 pasv_max_port=65534 |
proftpd
Edit /etc/proftpd.conf and add to the Global section:
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...... PassivePorts 49152 65534 |
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:
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IPTABLES_MODULES="ip_conntrack_ftp" |
Next edit /etc/sysconfig/iptables and add a rule to allow TCP port 21. The new line is marked in red:
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*filter :INPUT ACCEPT [0:0] :FORWARD ACCEPT [0:0] :OUTPUT ACCEPT [0:0] -A INPUT -m state --state ESTABLISHED,RELATED -j ACCEPT -A INPUT -p icmp -j ACCEPT -A INPUT -i lo -j ACCEPT -A INPUT -m state --state NEW -m tcp -p tcp --dport 21 -j ACCEPT -A INPUT -m state --state NEW -m tcp -p tcp --dport 22 -j ACCEPT -A INPUT -m state --state NEW -m tcp -p tcp --dport 80 -j ACCEPT -A INPUT -j REJECT --reject-with icmp-host-prohibited -A FORWARD -j REJECT --reject-with icmp-host-prohibited COMMIT |
Now restart the iptables service:
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/sbin/service iptables restart |
You can verify that the correct port range has been registered with lsmod like this:
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lsmod | grep conntrack_ftp |
and you’ll get something like this:
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ip_conntrack_ftp <strong>41361</strong> 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.
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.