Basic NAT information
NAT (Network Address Translation) is a system that acts like a proxy, but on a "packet" level. When a computer on your local network connects to a computer on the Internet, the NAT replaces the "from" information of packets with its own address, making your local network invisible to the Internet. Many firewalls perform NAT duties as well, so the following information is valid in firewalled environments as well.

For server systems, NAT can improve security and enable multiple servers to be accessed using a single IP address. This is done by allowing certain ports forwarded "inward" to the local network. However, the part of the FTP protocol known as "passive" data transfers is not by default compatible with NAT solutions. But NAT functionality is possible with ProFTPD versions 1.2rc2 and later.

Note: for details on NAT configuration for Linux, read the Linux IP-masq HOWTO at:

  http://www.linuxdoc.org/HOWTO/IP-Masquerade-HOWTO.html

Configuring ProFTPD behind NAT
First configure your installed proftpd so that it works correctly from inside the NAT. There are example configuration files included with the source. Then add the MasqueradeAddress directive to your proftpd.conf file to define the public name or IP address of the NAT. For example:

  MasqueradeAddress	ftp.mydomain.com  # using a DNS name
  MasqueradeAddress	123.45.67.89      # using an IP address

However, one big problem still exists. The passive FTP connections will use ports from 1024 and up, which means that you must forward all ports 1024-65535 from the NAT to the FTP server! And you have to allow many (possibly) dangerous ports in your firewalling rules! Not a good situation. For a good description of active versus passive FTP data transfers, see:

  http://slacksite.com/other/ftp.html

  PassivePorts 60000 65535	# These ports should be safe...

Now start the FTP daemon and you should see something like:

  123.45.67.89 - Masquerading as '123.45.67.89' (123.45.67.89)

A Linux Example
This example is for Linux kernel version 2.2.x with ipchains and ipmasqadm. The examples below assume that your FTP server has local address 192.168.1.2.

First we need to enable NAT for our FTP server. As root:

  echo "1">/proc/sys/net/ipv4/ip_forward
  ipchains -P forward DENY
  ipchains -I forward -s 192.168.1.2 -j MASQ

  insmod ip_masq_autofw
  ipmasqadm autofw -A -r tcp 20 21 -h 192.168.1.2

  ipmasqadm autofw -A -r tcp 60000 65535 -h 192.168.1.2

Double Checking
Setting up proftpd that allows passive data transfers srequires that a range of ports be forwarded from the NAT to the local network. This could be a security hazard, but since you can specify what port range to use, you are still able to setup relatively tight firewalling rules. To be sure that you have no other processes listening on the ports you have specified for passive transfers, use a port scanner such as nmap:

  nmap -sT -I -p 60000-65535 localhost

  All 5536 scanned ports on localhost (127.0.0.1) are: closed

Frequently Asked Questions

Question: How do I know if my MasqueradeAddress and PassivePorts configuration is working?
Answer: When performing a passive data transfer, an FTP client sends the PASV command to the FTP server. The server responds with the address and port to which the client should connect. For example:

  227 Entering Passive Mode (127,0,0,1,19,6).

  a1.a2.a3.a4

  p1 * 256 + p2

Question: Can I configure proftpd so that it refuses to handle passive transfers?
Answer: If you are using a version of proftpd older than 1.2.10rc1, no. In 1.2.10rc1, support for placing limits on the PASV and PORT (and their IPv6 equivalents EPSV and EPRT) was added, so that you could do the following:

  <Limit EPSV PASV>
    DenyAll
  </Limit>

Question: How can I make proftpd use a different MasqueradeAddress based on the address of the connecting client?
Answer: This question usually arises in the case where FTP clients connecting from inside the LAN see the same MasqueradeAddress as external clients, which causes problems. That MasqueradeAddress may be necessary in order to allow external FTP clients to do passive data transfers. The internal clients do not need it. To handle this, create a <VirtualHost> section in your proftpd.conf to handle the LAN address of the FTP server, the address that the internal clients are contacting. In this <VirtualHost> section, make sure there is no MasqueradeAddress directive. This way, the external FTP clients "see" the configuration with the MasqueradeAddress they need, and the internal FTP clients "see" a different configuration, one with no MasqueradeAddress.

For those that need to see a concrete example configuration of this:

  ServerName "Some Server Name"
  MasqueradeAddress my.domain.com
  PassivePorts 60000 65535

  # Note that your LAN address should be used here
  <VirtualHost 192.168.0.10>
    ServerName "Some Other Server Name"

    # Note that there is no MasqueradeAddress directive
    # used in this section!
  </VirtualHost>