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.
The definition of the File Transfer Protocol, unfortunately, does not
(currently) support name-based virtual hosts, as HTTP1.1 supports. All FTP
virtual hosts are based on unique IP address/port combinations, not on DNS
names. The similarity of ProFTPD's configuration file syntax to Apache's
sometimes leads users to assuming that
proftpd will handle these
the same way -- but more on this later. The bottom line is that ProFTPD
does not support name-based virtual hosts; not because they are not implemented,
but simply because the protocol itself does not support them.
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:
<Global>, and "server config".
<VirtualHost> context is used to define the configuration
for a particular virtual host, bound to an IP address. For example:
<VirtualHost 126.96.36.199> ... </VirtualHost>defines a configuration for a virtual server that
proftpdshould use whenever a remote client connects to the IP address 188.8.131.52. DNS names, too, can be used with the
<VirtualHost>configuration directive, and this is where some of the name-based vhost support confusion creeps in:
<VirtualHost ftp.mydomain.com> ... </VirtualHost>When
proftpdparses this context 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. Use of DNS names like this, while convenient, can easily lead to confusion when multiple DNS names resolve to the same IP address. If this happens,
proftpdwill use the first context in the configuration file when serving that address.
<Global> context is provided as a convenience. Imagine
that the administrator has many
proftpd.conf, and yet has a lot of the same configuration
for each virtual host, such as common
DefaultRoot settings, etc. Rather than including the
same configuration over and over, she could use the
<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" context. 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
<Global> context in
proftpd.conf file. Unlike Apache's
ProFTPD's configuration is designed such that one should be able to use
the simplest file as possible. In fact,
proftpd will start
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> context, any configuration
directives inside the "server config" context do not apply
outside of the context. Many administrators often assume that this is the
case. It is not. This is what the
However, one particular drawback to the "server config" context 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
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
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
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)
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
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's a matching vhost for that IP address, simple:
use that configuration. If not,
proftpd will then resort to
using the configuration that bears the
which says that the server configuration in which it appers should be used
in cases like this. If there is no
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 184.108.40.206 220.127.116.11> ... </VirtualHost>
If, however, you want to specific the address to which the configuration
of the "server config" context, use
There is one last configuration directive about which an administrator should
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
proftpddaemon will listen to every address on the host machine.
Frequently Asked Questions
Question: Why do I see the following when I start
- warning: "Virtual Server" address/port (18.104.22.168:21) 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
<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
using DNS names, rather than IP addresses. And the "server config" section
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
Port 0as mentioned above. You can also use the
DefaultAddressdirective 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
<VirtualHost> section) listen for multiple IP
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 22.214.171.124 ftp.example.com> ... </VirtualHost>And for the "server config" context, you would use the
DefaultAddressdirective, which can also handle multiple IP addresses/DNS names:
DefaultAddress 126.96.36.199 ftp.example.com