What is a virtual server?
A virtual server, also known as a virtual host (or vhost 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.
Until very recently, the definition of FTP did not allow for name-based
virtual hosts, such as supported by HTTP/1.1. That changed with
RFC 7151, which defined a
HOST FTP command. ProFTPD virtual hosts are IP-based and
name-based.
In some documents, one might see reference to both "daemon" and
"server". Sometimes these words are used interchangeably; however,
there is a slight difference. A "daemon" is a long-lived process
on a host machine, and a "server" is a process that provides some
service, typically to remote clients. A process, such as a standalone
proftpd process, can be considered both a "daemon"
and a "server". With this in mind, then, a "virtual server"
is not a separate process itself; it just looks like one to the remote clients.
Hence the "virtual".
There are three "server" contexts (sometimes also called
sections) in the proftpd.conf configuration file:
<VirtualHost>, <Global>,
and "server config".
The <VirtualHost> context is used to define the configuration
for a particular virtual host, bound to an IP address. For example:
<VirtualHost 1.2.3.4>
...
</VirtualHost>
defines a configuration for a virtual server that proftpd should
use whenever a remote client connects to the IP address 1.2.3.4. DNS names,
too, can be used with the <VirtualHost> configuration
directive:
<VirtualHost ftp.mydomain.com>
...
</VirtualHost>
When proftpd 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 automatically create a ServerAlias directive using that DNS name.
This allows for multiple <VirtualHost> sections using the
same DNS name to be defined in your proftpd.conf.
If you want the same vhost to be used for multiple different names at the
same time (e.g. because you have multiple DNS names for the same
server), you would use the ServerAlias directive to list those
other names, like so:
<VirtualHost ftp.mydomain.com>
# Use this vhost for other names, too
ServerAlias ftp.mydomain.org ftp2.mydomain.com ftp.otherdomain.org
...
</VirtualHost>
The <Global> section is provided as a convenience. Imagine
that the administrator has many <VirtualHost> sections
in her proftpd.conf, and yet has a lot of the same configuration
for each virtual host, such as common <Directory> sections,
DefaultRoot settings, etc. Rather than including the
same configuration over and over, she could use the <Global>
section:
<Global>
...
</Global>
Anything inside of a <Global> section is applied to every
server configuration in the file, to every <VirtualHost> as
well as the default "server config" server.
Which brings us to the "server config" section. The name is
ill-suited, and is really borrowed directly from Apache's naming conventions.
The "server config" context refers to anything not in a
<VirtualHost> or <Global> section in
the proftpd.conf file. Unlike Apache's httpd.conf,
ProFTPD's configuration is designed such that one should be able to use
the simplest file as possible. In fact, proftpd will start
if the proftpd.conf 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 is possible. With this in mind,
there is always at least one server configuration present: the default
server context, and it is this context that is known as the
"server config". Just like the <VirtualHost>
section, any configuration directives inside the "server config"
section do not apply outside of the context. Many administrators often
assume that this is the case. It is not. This is what the
<Global> section is for.
However, one particular drawback to the "server config" section was
that it did not provide a way to specify to which IP address that configuration
pertained. By default, when proftpd parses the
proftpd.conf file, it will use the gethostname()
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 DefaultAddress configuration directive provides: the
ability to specify to which IP address the "server config" vhost
should listen.
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
Port directive to
change the port. As might be mentioned elsewhere, if you have many different
<VirtualHost> sections using the same address but
different ports, you'll want to make sure that you leave each
Port-1 number empty. RFC 959 specifies that the source port for an active
data transfer (read here)
must be L-1, where L is the port on which your server
listens. Also, as mentioned in the Port documentation, using:
Port 0in any server context will effectively "disable" that server. This is sometimes used to disable the "server config" configuration.
There is another configuration directive that comes into play in all of this
: DefaultServer. Here is why: when a client contacts proftpd, 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,
proftpd will then resort to using the configuration that bears the
DefaultServer directive, which says that the server configuration
in which it appears should be used in cases like this. If there is no
DefaultServer directive in the proftpd.conf file,
and no matching configuration can be found, then the client will see a message
such as "no server available to service your request". The
DefaultServer can be used to say that a
<VirtualHost> should be the default, and not necessarily the
"server config" context, as is common.
If you would like the same virtual host configuration to be used for
multiple different IP addresses (or DNS names), the
<VirtualHost> supports this:
<VirtualHost 1.2.3.4 5.6.7.8>
...
</VirtualHost>
If, however, you want to specific the address to which the configuration
of the "server config" context, use DefaultAddress
(mentioned above).
There is one last configuration directive about which an administrator should
know: SocketBindTight. By default, the
proftpd daemon will listen on all addresses, port 21, for the
connection requests of remote clients. Sometimes, the administrator may
wish to have the proftpd daemon listen only on the IP
addresses for which it has been configured, and not every address.
To accomplish this, simply use the SocketBindTight configuration
directive:
SocketBindTight onThis configures the daemon to "bind tightly" only to those IP addresses to which it has been configured to listen, rather than every address. By default, the
proftpd daemon will listen to every address on
the host machine.
Question: Why do I see the following when I start proftpd?
- warning: "Virtual Server" address/port (1.2.3.4:21) already in use by "Main Server"Answer: This happens when a
<VirtualHost> section is "hidden" behind the default server in the "server config"
section (i.e. the context that is outside of all <VirtualHost> and <Global> sections). It is "hidden" because
both the <VirtualHost> section and the "server config"
section are using the same IP address and port.
It is quite common to configure <VirtualHost> sections
using DNS names, rather than IP addresses. And the "server config" section
in the proftpd.conf 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.
The quick-and-easy fix is to place the following your "server config" section
in your proftpd.conf:
Port 0as mentioned above. You can also use the
DefaultAddress
directive in the "server config" section to explicitly tell the "server config"
section to use a different IP address/DNS name.
Question: How can I have my "server config" section
(or a <VirtualHost> section) listen for multiple IP
addresses/DNS names?
Answer: In version 1.3.0rc1 and later, the
<VirtualHost> configuration was enhanced so that it could
handle multiple IP addresses/DNS names, e.g.:
<VirtualHost 1.2.3.4 ftp.example.com>
...
</VirtualHost>
And for the "server config" context, you would use the
DefaultAddress directive, which can also handle multiple IP
addresses/DNS names:
DefaultAddress 1.2.3.4 ftp.example.com