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<center><h2><b>ProFTPD: Virtual Servers</b></h2></center>
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<p><a name="Definition"></a>
<b>What is a virtual server?</b><br>
A virtual server, also known as a virtual host (or <i>vhost</i> for short),
refers to the practice of serving more than one address/site on a single host
machine.  The fact that these multiple sites are being served by the same
physical machine is transparent to the end user.

<p>
Until very recently, the definition of FTP did not allow for <i>name</i>-based
virtual hosts, such as supported by HTTP/1.1.  That changed with
<a href="https://tools.ietf.org/html/rfc7151">RFC 7151</a>, which defined a
<code>HOST</code> FTP command.  ProFTPD virtual hosts are IP-based <i>and</i>
name-based.

<p>
In some documents, one might see reference to both &quot;daemon&quot; and
&quot;server&quot;.  Sometimes these words are used interchangeably; however,
there is a slight difference.  A &quot;daemon&quot; is a long-lived process
on a host machine, and a &quot;server&quot; is a process that provides some
service, typically to remote clients.  A process, such as a standalone
<code>proftpd</code> process, can be considered both a &quot;daemon&quot;
and a &quot;server&quot;.  With this in mind, then, a &quot;virtual server&quot;
is not a separate process itself; it just looks like one to the remote clients.
Hence the &quot;virtual&quot;.

<p><a name="Configuration"></a>
There are three &quot;server&quot; contexts (sometimes also called
<i>sections</i>) in the <code>proftpd.conf</code> configuration file:
<a href="../modules/mod_core.html#VirtualHost"><code>&lt;VirtualHost&gt;</code></a>, <a href="../modules/mod_core.html#Global"><code>&lt;Global&gt;</code></a>,
and &quot;server config&quot;.

<p>
The <code>&lt;VirtualHost&gt;</code> context is used to define the configuration
for a particular virtual host, bound to an IP address.  For example:
<pre>
  &lt;VirtualHost 1.2.3.4&gt;
    ...
  &lt;/VirtualHost&gt;
</pre>
defines a configuration for a virtual server that <code>proftpd</code> should
use whenever a remote client connects to the IP address 1.2.3.4.  DNS names,
too, can be used with the <code>&lt;VirtualHost&gt;</code> configuration
directive:
<pre>
  &lt;VirtualHost ftp.mydomain.com&gt;
    ...
  &lt;/VirtualHost&gt;
</pre>
When <code>proftpd</code> parses this section on startup, it will resolve the
given DNS name to its IP address and use that, just as if that IP address
had been used in the first place.  In addition, when DNS names are used like
this, ProFTPD will <i>automatically</i> create a <a href="../modules/mod_core.html#ServerAlias"><code>ServerAlias</code></a> directive using that DNS name.
This allows for multiple <code>&lt;VirtualHost&gt;</code> sections using the
same DNS name to be defined in your <code>proftpd.conf</code>.

<p>
If you want the same vhost to be used for multiple different names at the
same time (<i>e.g.</i> because you have multiple DNS names for the same
server), you would use the <code>ServerAlias</code> directive to list those
other names, like so:
<pre>
  &lt;VirtualHost ftp.mydomain.com&gt;
    # Use this vhost for other names, too
    ServerAlias ftp.mydomain.org ftp2.mydomain.com ftp.otherdomain.org
    ...
  &lt;/VirtualHost&gt;
</pre>

<p>
The <code>&lt;Global&gt;</code> section is provided as a convenience.  Imagine
that the administrator has many <code>&lt;VirtualHost&gt;</code> sections
in her <code>proftpd.conf</code>, and yet has a lot of the same configuration
for each virtual host, such as common <code>&lt;Directory&gt;</code> sections,
<code>DefaultRoot</code> settings, <i>etc</i>.  Rather than including the
same configuration over and over, she could use the <code>&lt;Global&gt;</code>
section:
<pre>
  &lt;Global&gt;
    ...
  &lt;/Global&gt;
</pre>
Anything inside of a <code>&lt;Global&gt;</code> section is applied to every
server configuration in the file, to every <code>&lt;VirtualHost&gt;</code> as
well as the default &quot;server config&quot; server.

<p>
Which brings us to the &quot;server config&quot; section.  The name is
ill-suited, and is really borrowed directly from Apache's naming conventions.
The &quot;server config&quot; context refers to anything <b>not</b> in a
<code>&lt;VirtualHost&gt;</code> or <code>&lt;Global&gt;</code> section in
the <code>proftpd.conf</code> file.  Unlike Apache's <code>httpd.conf</code>,
ProFTPD's configuration is designed such that one should be able to use
the simplest file as possible.  In fact, <code>proftpd</code> will start
if the <code>proftpd.conf</code> is completely empty; try it!  This will
cause the daemon to use all of the default settings, which in most cases
is not what is wanted, but it <i>is</i> possible.  With this in mind,
there is always at least <i>one</i> server configuration present: the default
server context, and it is this context that is known as the
&quot;server config&quot;.  Just like the <code>&lt;VirtualHost&gt;</code>
section, any configuration directives inside the &quot;server config&quot;
section <b>do not apply</b> outside of the context.  Many administrators often
<i>assume</i> that this is the case.  It is not.  This is what the
<code>&lt;Global&gt;</code> section is for.

<p>
However, one particular drawback to the &quot;server config&quot; section was
that it did not provide a way to specify to which IP address that configuration
pertained.  By default, when <code>proftpd</code> parses the
<code>proftpd.conf</code> file, it will use the <code>gethostname()</code>
function to determine the IP address to which the default server should listen.
On a single address, single interface system, this default is fine. It is one
a multiple address system that the default handling does not always work;
the administrator may wish to explicitly specify to which address the default
server should listen.  This is what the <a href="../modules/mod_core.html#DefaultAddress"><code>DefaultAddress</code></a> configuration directive provides: the
ability to specify to which IP address the &quot;server config&quot; vhost
should listen.

<p>
By default, every server will listen to port 21, the IANA standard port for
FTP.  If you want to have server react to a different port, use the
<a href="../modules/mod_core.html#Port"><code>Port</code></a> directive to
change the port.  As might be mentioned elsewhere, if you have many different
<code>&lt;VirtualHost&gt;</code> sections using the <i>same address</i> but
different <i>ports</i>, you'll want to make sure that you leave each
<code>Port</code>-1 number empty.  <a href="http://www.faqs.org/rfcs/rfc959.html">RFC 959</a> specifies that the source port for an active
data transfer (read <a href="http://slacksite.com/other/ftp.html">here</a>)
must be <code>L-1</code>, where <code>L</code> is the port on which your server
listens.  Also, as mentioned in the <code>Port</code> documentation, using:
<pre>
  Port 0
</pre>
in any server context will effectively &quot;disable&quot; that server.  This
is sometimes used to disable the &quot;server config&quot; configuration.

<p>
There is another configuration directive that comes into play in all of this
: <a href="../modules/mod_core.html#DefaultServer"><code>DefaultServer</code></a>.  Here is why: when a client contacts <code>proftpd</code>, the server has to
determine which configuration to use for handling the client.  To do this, it
searches its list of configured vhosts, searching for a vhost whose IP address
matches the IP address that the client contacted.  If there is a matching vhost
for that IP address, simple: use that configuration.  If not,
<code>proftpd</code> will then resort to using the configuration that bears the
<code>DefaultServer</code> directive, which says that the server configuration
in which it appears should be used in cases like this.  If there is no
<code>DefaultServer</code> directive in the <code>proftpd.conf</code> file,
and no matching configuration can be found, then the client will see a message
such as &quot;no server available to service your request&quot;.  The
<code>DefaultServer</code> can be used to say that a
<code>&lt;VirtualHost&gt;</code> should be the default, and not necessarily the
&quot;server config&quot; context, as is common.

<p>
If you would like the same virtual host configuration to be used for
multiple different IP addresses (or DNS names), the
<code>&lt;VirtualHost&gt;</code> supports this:
<pre>
  &lt;VirtualHost 1.2.3.4 5.6.7.8&gt;
    ...
  &lt;/VirtualHost&gt;
</pre>

<p>
If, however, you want to specific the address to which the configuration
of the &quot;server config&quot; context, use <code>DefaultAddress</code>
(mentioned above).

<p>
There is one last configuration directive about which an administrator should
know: <a href="../modules/mod_core.html#SocketBindTight"><code>SocketBindTight</code></a>.  By default, the
<code>proftpd</code> daemon will listen on all addresses, port 21, for the
connection requests of remote clients.  Sometimes, the administrator may
wish to have the <code>proftpd</code> daemon listen <b>only</b> on the IP
addresses for which it has been configured, and not <i>every</i> address.
To accomplish this, simply use the <code>SocketBindTight</code> configuration
directive:
<pre>
  SocketBindTight on
</pre>
This configures the daemon to &quot;bind tightly&quot; only to those IP
addresses to which it has been configured to listen, rather than every address.
By default, the <code>proftpd</code> daemon will listen to every address on
the host machine.

<p><a name="FAQ"></a>
<b>Frequently Asked Questions</b><br>

<p>
<font color=red>Question</font>: Why do I see the following when I start <code>proftpd</code>?<br>
<pre>
- warning: "Virtual Server" address/port (1.2.3.4:21) already in use by "Main Server"
</pre>
<font color=blue>Answer</font>: This happens when a <code>&lt;VirtualHost&gt;</code> section is "hidden" behind the default server in the "server config"
section (<i>i.e.</i> the context that is outside of all <code>&lt;VirtualHost&gt;</code> and <code>&lt;Global&gt;</code> sections).  It is "hidden" because
both the <code>&lt;VirtualHost&gt;</code> section and the "server config"
section are using the same IP address and port.

<p>
It is quite common to configure <code>&lt;VirtualHost&gt;</code> sections
using DNS names, rather than IP addresses.  And the "server config" section
in the <code>proftpd.conf</code> file, by default, uses the IP address of
the machine's hostname.  This makes it quite easy to inadvertently have
multiple sections trying to use the same IP address and port.

<p>
The quick-and-easy fix is to place the following your "server config" section
in your <code>proftpd.conf</code>:
<pre>
  Port 0
</pre>
as mentioned above.  You can also use the <code>DefaultAddress</code>
directive in the "server config" section to explicitly tell the "server config"
section to use a different IP address/DNS name.

<p>
<font color=red>Question</font>: How can I have my "server config" section
(or a <code>&lt;VirtualHost&gt;</code> section) listen for multiple IP
addresses/DNS names?<br>
<font color=blue>Answer</font>: In version 1.3.0rc1 and later, the
<code>&lt;VirtualHost&gt;</code> configuration was enhanced so that it could
handle multiple IP addresses/DNS names, <i>e.g.</i>:
<pre>
  &lt;VirtualHost 1.2.3.4 ftp.example.com&gt;
    ...
  &lt;/VirtualHost&gt;
</pre>
And for the "server config" context, you would use the
<code>DefaultAddress</code> directive, which can also handle multiple IP
addresses/DNS names:
<pre>
  DefaultAddress 1.2.3.4 ftp.example.com
</pre>

<p>
<hr>
<font size=2><b><i>
&copy; Copyright 2017 The ProFTPD Project<br>
 All Rights Reserved<br>
</i></b></font>
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