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Edit File: NAT.html

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<head>
<title>ProFTPD mini-HOWTO - Firewalls, Routers, and NAT</title>
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<hr>
<center><h2>Firewalls, Routers, and NAT</h2></center>
<hr>

Basic NAT information 
NAT (Network Address Translation) is a system that acts
like a proxy, but on a &quot;packet&quot; level. When a computer on your
local network connects to a computer on the Internet, the NAT replaces the
&quot;from&quot; 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 &quot;inward&quot; to the local network. However, the part of the
FTP protocol known as &quot;passive&quot; 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:
<pre>
 <a href="http://tldp.org/HOWTO/IP-Masquerade-HOWTO/">tldp.org/HOWTO/IP-Masquerade-HOWTO/</a>
</pre>
or search for information concerning your OS of choice.

Configuring ProFTPD behind NAT 
First configure your installed <code>proftpd</code> so that it works correctly
from inside the NAT. There are example configuration files included with the
source. Then add the <a href="http://www.proftpd.org/docs/directives/linked/config_ref_MasqueradeAddress.html"><code>MasqueradeAddress</code></a> directive
to your <code>proftpd.conf</code> file to define the public name or IP address
of the NAT. For example:
<pre>
 MasqueradeAddress	ftp.mydomain.com # using a DNS name
 MasqueradeAddress	123.45.67.89 # using an IP address
</pre>
Now your <code>proftpd</code> will hide its local address and instead use the
public address of your NAT.

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:
<pre>
 <a href="http://slacksite.com/other/ftp.html">http://slacksite.com/other/ftp.html</a>
</pre>
To resolve this, simply use the <a href="http://www.proftpd.org/docs/directives/linked/config_ref_PassivePorts.html"><code>PassivePorts</code></a> directive
in your <code>proftpd.conf</code> to control what ports <code>proftpd</code>
will use for its passive data transfers:
<pre>
 PassivePorts 60000 65535	# These ports should be safe...
</pre>
Note that if the configured range of ports is too small, connecting clients
may experience delays or be completely unable to operate when they
request passive data transfers. When the daemon cannot use one of the
ports in the configured range, it will fall back to using a kernel-assigned
port, and log a message reporting the issue. The clients' ability to
use this non-configured port will then depend on any NAT, router, or firewall
configuration.

Now start the FTP daemon and you should see something like:
<pre>
 123.45.67.89 - Masquerading as '123.45.67.89' (123.45.67.89)
</pre>
in the log files.

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

First we need to enable NAT for our FTP server. As <code>root</code>:
<pre>
 echo "1"&gt;/proc/sys/net/ipv4/ip_forward
 ipchains -P forward DENY
 ipchains -I forward -s 192.168.1.2 -j MASQ
</pre>
Now we load the <code>autofw</code> kernel module and forward ports 20 and 21
to the FTP server:
<pre>
 insmod ip_masq_autofw
 ipmasqadm autofw -A -r tcp 20 21 -h 192.168.1.2
</pre>
Then we forward ports for passive FTP transfers. In our
<code>proftpd.conf</code> file we restricted passive transfers to ports
60000-65535, so that is what we use here as well:
<pre>
 ipmasqadm autofw -A -r tcp 60000 65535 -h 192.168.1.2
</pre>

If instead your Linux system uses IP Filters, then you might do something
like the following. First, update your <code>ipf.conf</code> with:
<pre>
 # Allow passive FTP transfers from ports 49152 to 65534, the IANA-registered
 # ephemeral port range.
 pass in quick proto tcp from any to any port 49151 &gt;&lt; 65535 flags S keep state
</pre>
Then make sure that the changes take effect by using:
<pre>
 ipf -Fa -f /path/to/ipf.conf
</pre>

Double Checking 
Setting up <code>proftpd</code> 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 <code>nmap</code>:
<pre>
 nmap -sT -I -p 60000-65535 localhost
</pre>
If the result says something like:
<pre>
 All 5536 scanned ports on localhost (127.0.0.1) are: closed
</pre>
then you should be safe.

<a name="FAQ">
Frequently Asked Questions

Question: How do I know if my
<code>MasqueradeAddress</code> and <code>PassivePorts</code> configuration is
working? 
Answer: When performing a passive data transfer, an
FTP client sends the <code>PASV</code> command to the FTP server. The server
responds with the address and port to which the client should connect. For
example:
<pre>
 227 Entering Passive Mode (127,0,0,1,19,6).
</pre>
The address and port are contained in the parentheses, formatted as
a1,a2,a3,a4,p1,p2, where the IP address is:
<pre>
 a1.a2.a3.a4
</pre>
and the port number is:
<pre>
 p1 * 256 + p2
</pre>
If the address seen in the server's response is not a public IP address
or the port is not in the port range configured by your
<code>PassivePorts</code>, double-check your <code>proftpd.conf</code>.
Non-public IP addresses are defined by <a href="http://www.faqs.org/rfcs/rfc1918.html">RFC 1918</a>, and include <code>10.x</code>,
<code>172.16.x</code> and <code>192.168.0.x</code>.

Question: I am using the <code>PassivePorts</code>
directive, but my FTP client still doesn't work. In the proftpd debug logging,
I see:
<pre>
 May 20 17:00:55 www.example.com proftpd[10078] wwww.example.com (::ffff:1.2.3.4[::ffff:1.2.3.4]): Refused PORT 192,168,1,2,193,116 (address mismatch)
</pre>
Answer: The <code>PORT</code> there means that the
FTP client is requesting an active data transfer; this means that
proftpd is being asked to actively connect to the client
(to the given address 192.168.1.2, port 49524).

The first problem is, as the log message indicates, that the IP address
given in the <code>PORT</code> command, 192.168.1.2, does not match
the IP address from which the client connected (i.e. 1.2.3.4). By
default, proftpd will refuse to create a data transfer connection to anywhere
except back to the requesting client's IP address; see the
<a href="../howto/FXP.html"><code>AllowForeignAddress</code></a> directive for
more information.

The next problem is that 192.168.1.2 IP address is not a publicly routable I
 address; specifically, it is part of an
<a href="http://www.faqs.org/rfcs/rfc1918.html">RFC 1918</a> address space.
This means that is not possible for proftpd to connect to that
address (unless proftpd is located in the same LAN).

The solution for these situations is to a) configure proftpd to use
both <code>PassivePorts</code> and <code>MasqueradeAddress</code>,
and b) to configure your FTP client to request passive data
transfers (via the <code>PASV</code> and <code>EPSV</code> commands), rather
than active data transfers.

Question: I am using <code>MasqueradeAddress</code>
in my config, using the external DNS name for my site, but <code>proftpd</code>
still returns the internal/LAN IP address in the <code>PASV</code> response.
Is it a bug? 
Answer: Most likely not. If you use something like:
<pre>
 MasqueradeAddress ftp.example.com
</pre>
and see the internal IP address in the <code>PASV</code> response, it
suggests that <code>proftpd</code>, when starting up, resolves that DNS name
from within your LAN, and gets that internal IP address. Rather than using
the DNS name, you should explicitly use the external IP address in your
<code>MasqueradeAddress</code> directive:
<pre>
 MasqueradeAddress 1.2.3.4
</pre>
where "1.2.3.4" is your real external IP address.

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

Question: How can I make <code>proftpd</code> use
a different <code>MasqueradeAddress</code> 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
<code>MasqueradeAddress</code> as external clients, which causes problems.
That <code>MasqueradeAddress</code> 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 <code>&lt;VirtualHost&gt;</code>
section in your <code>proftpd.conf</code> to handle the LAN address of
the FTP server, the address that the internal clients are contacting.
In this <code>&lt;VirtualHost&gt;</code> section, make sure there is no
<code>MasqueradeAddress</code> directive. This way, the external FTP
clients &quot;see&quot; the configuration with the
<code>MasqueradeAddress</code> they need, and the internal FTP clients
&quot;see&quot; a different configuration, one with no
<code>MasqueradeAddress</code>.

For those that need to see a concrete example configuration of this:
<pre>
 ServerName "Some Server Name"
 MasqueradeAddress my.domain.com
 PassivePorts 60000 65535

# Note that your LAN address should be used here
 &lt;VirtualHost 192.168.0.10&gt;
 ServerName "Some Other Server Name"

# Note that there is no MasqueradeAddress directive
 # used in this section!
 &lt;/VirtualHost&gt;
</pre>

<hr>
Contributor: Tobias Ekbom &lt;tobias at vallcom dotcom&gt; 
Last Updated: $Date: 2012-05-22 16:31:39 $
<hr>