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:
...... 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:
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:
*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:
/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:
nf_conntrack_ftp 12913 0 nf_conntrack 79645 4 nf_conntrack_ftp,nf_conntrack_ipv4,nf_conntrack_ipv6,xt_state
And that’s all it takes to get passive mode ftp working behind iptables.
P.S: If your server is behind a physical firewall and you are behind NAT, then you’ll probable need to load the “ip_nat_ftp” iptables module.
