ProFTPD mini-HOWTO - ServerType

## ServerType

The `ServerType` configuration directive for ProFTPD can cause confusion for those just starting with this server. What is the purpose for this directive? What are these "inetd" and "standalone" types, and why does one need to choose one or the other?

The purpose of this directive is to choose between the two operating modes for almost all Unix network servers: does the server listen on its port for client requests itself, or does the server let some other process do the listening, and call the server when needed? Traditionally, that "other process" has been `inetd`, a "super server" that listens on all interfaces, all ports on a Unix machine, and calls the appropriate server based on the port contacted. A more modern replacement for `inetd` is found in the `xinetd` server; this server functions much the same way. The other mode of operation is to have the server listen on the port(s) itself, and handle client requests accordingly. The latter mode is the standalone `ServerType`, the former is the inetd mode (which covers both the `inetd` and `xinetd` processes).

This directive is mandatory, and must be set to one mode or the other. The two modes are incompatible (two processes cannot be bound to the same interface/port combination simultaneously), and thus the `proftpd` must be told in which mode it is to operate.

Inetd Mode
In inetd mode, the `proftpd` server expects to be started by the `inetd` (or `xinetd`) servers. It is these servers, `inetd/xinetd`, that listen on the FTP port (usually 21) for connection requests, then start `proftpd` and pass the connection off. This mode is usually best suited for low traffic sites, for sites that do not handle many FTP sessions.

Example `/etc/inetd.conf` entry:

      ftp    stream  tcp     nowait  root    /usr/sbin/tcpd  /usr/sbin/proftpd

The `inetd.conf` man pages discuss these fields in greater detail. See the ProFTPD User's Guide for an example xinetd configuration.

Note: Solaris users may find that their `/etc/inetd.conf` file ships with an `ftp` entry that looks similar to the above, except that it has "tcp6" rather than "tcp". For `proftpd` to function properly in such cases, that "tcp6" will need to be changed to "tcp". Solaris uses the `tcp6` keyword to have its `inetd` pass IPv6 sockets to the called program; `proftpd` must have been configured with the `--enable-ipv6` option to handle such sockets.

Inetd Mode and Non-standard Ports
A note about using non-standard ports in your configuration via the `Port` configuration directive: making these work while in `inetd` mode and using `inetd` (as oppposed to `xinetd`) is problematic. The first column of an `/etc/inetd.conf` entry lists a protocol name. The port for that protocol is listed in the `/etc/services` file. For services that run on non-standard ports, however, `/etc/services` has no entry, and `inetd` is programmed so as to always reference that file. This means that if use of non-standard ports and use of `inetd` are required, the `/etc/services` file will need to be edited. Avoid this situation if possible.

Compared to `inetd`, `xinetd`'s configuration format is more flexible: it can be configured to use non-standard ports using the `port` attribute:

           port             determines  the  service  port.  If  this
                            attribute  is  specified  for  a  service
                            listed in /etc/services, it must be equal
                            to the port number listed in that file.

as described in the `xinetd.conf(5)` man page.

If your `proftpd` server is running in this mode, you do not need to worry about restarting any servers whenever changes are made to the `proftpd.conf` configuration file. Since `proftpd` is started for each new FTP session by `inetd/xinetd`, and part of that startup process includes reading the configuration file, any changes will be seen by any new FTP sessions once the changes are made.

If you attempt to start a `proftpd` server configured with a `ServerType` of standalone, and already have `inetd/xinetd` also configured to handle FTP connections, this kind of error message will appear in your `proftpd` logs:

      golem.castaglia.org - Failed binding to 127.0.0.1, port 21: Address already in use
      golem.castaglia.org - Check the ServerType directive to ensure you are configured correctly.

More information might be found by debugging your configuration.

Standalone Mode
In this mode, the `proftpd` listens for incoming FTP session requests itself, and forks off child processes to handle those requests. This mode is best suited for high traffic, popular sites; the overhead of having to parse the configuration file each time, as is done for `inetd`-handled sessions, is avoided in this mode. Also, there is no need to change any other configuration files other than the `proftpd.conf`, for ports, virtual servers, or anything else.

When running in this mode, the server will need to be restarted whenever changes are made to the configuration file. There is a page that describes how this can be done.

Many administrators are accustomed to using `tcpwrappers` to secure their network servers; indeed, this is a good practice to get into. However, the most common way this is done is through `inetd`. When running a `proftpd` server in standalone mode, then, it is not quite as straightforward; however, it is not hard, either. The `mod_wrap` module can be compiled into your `proftpd`. This module allows a standalone `proftpd` server to use the normal `/etc/hosts.allow`, `/etc/hosts.deny` files, in addition to other files (something that normal `tcpwrappers` configurations cannot do).

If you try to start a `proftpd` server configured with a `ServerType` of inetd from the command line (or from some shell wrapper script), this kind of error message will appear in your `proftpd` logs:

      golem.castaglia.org - Fatal: Socket operation on non-socket
      golem.castaglia.org - (Running from command line? Use `ServerType standalone' in config file!)

More information might be found by debugging your configuration.

Switching Modes
Changing from one `ServerType` mode to the other is a simple process, as long as the few steps involved are followed.

To change from inetd to standalone, make sure to remove any FTP configurations from `/etc/inetd.conf`, `/etc/xinetd.conf`, `/etc/xinetd.d/`, or wherever your superserver's configuration file(s) reside. Once this is done, make sure those changes are seen by restarting `inetd/xinetd`. Then, make sure

      ServerType standalone

is in your `proftpd.conf`. Start the `proftpd` server from the command line, to make sure all is working. You can then easily start server from an `init.d` script such as the one mentioned here.

To change from standalone to inetd, make sure your `proftpd` is stopped completely. Add a configuration entry for FTP into your `inetd/xinetd` configuration (mentioned above), then restart `inetd/xinetd` to have those configuration changes seen. Check your `proftpd.conf` to see that

      ServerType inetd

is there.

Last Updated: $Date: 2004/06/08 23:08:42 $